,,transcriber',s ,,note ,italic text is denoted by underscores4 ,obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources4 ,more detail can be found at the end of the book4 ,,practical ,,talks ,,by ,,an ,,astronomer ,illustration3 ,the ,moon4 ,first ,quarter4 ,photographed by ,loewy and ,puiseux1 ,february #ac1 #ahid4 ,,practical ,,talks ,,by ,,an ,,astronomer ,,by ,,harold ,,jacoby ,,adjunct ,,professor ,,of ,,astronomy ,,in ,,columbia ,,university ,,illustrated ,,new ,,york ,,charles ,,scribner',s ,,sons #aijb ,,copyright1 #aijb1 ,,by ,,charles ,,scribner',s ,,sons ,published1 ,april1 #aijb ,,trow ,,directory ,,printing ,,and ,,bookbinding ,,company ,,new ,,york ,,preface ,the present volume has not been designed as a systematic treatise on astronomy4 ,there are many excellent books of that kind1 suitable for serious students as well as the general reader2 but they are necessarily somewhat dry and unattractive1 because they must aim at completeness4 ,completeness means detail1 and detail means dryness4 ,but the science of astronomy contains subjects that admit of detached treatment2 and as many of these are precisely the ones of greatest general interest1 it has seemed well to select several1 and describe them in language free from technicalities4 ,it is hoped that the book will thus prove useful to persons who do not wish to give the time required for a study of astronomy as a whole1 but who may take pleasure in devoting a half-hour now and then to a detached essay on some special topic4 ,preparation of the book in this form has made it suitable for prior publication in periodicals2 and the several essays have in fact all been printed before4 ,but the intention of collecting them into a book was kept in mind from the first2 and while no attempt has been made at consecutiveness1 it is hoped that nothing of merely ephemeral value has been included4 ,,contents ,,page ,,navigation ,,at ,,sea #a ,,the ,,pleiades #aj ,,the ,,pole-,,star #ah ,,nebul #bg ,,temporary ,,stars #cg ,,galileo #dg ,,the ,,planet ,,of #ahih #eh ,,how ,,to ,,make ,a ,,sun-,,dial #fi ,,photography ,,in ,,astronomy #ha ,,time ,,standards ,,of ,,the ,,world #aaa ,,motions ,,of ,,the ,,earth',s ,,pole #aca ,,saturn',s ,,rings #adj ,,the ,,heliometer #aeb ,,occultations #afa ,,mounting ,,great ,,telescopes #agj ,,the ,,astronomer',s ,,pole #ahd ,,the ,,moon ,,hoax #aii ,,the ,,sun',s ,,destination #baj ,,illustrations ,,the ,,moon4 ,,first ,,quarter ,frontispiece ,photographed by ,loewy and ,puiseux1 ,february #ac1 #ahid4 ,,facing ,,page ,,spiral ,,nebula ,,in ,,constellation ,,leo #bf ,photographed by ,keeler1 ,february #bd1 #aijj4 ,,nebula ,,in ,,andromeda #bh ,photographed by ,barnard1 ,november #ba1 #ahib4 ,,the ,,dumb-,,bell ,,nebula #cd ,photographed by ,keeler1 ,july #ca1 #ahii4 ,,star-,,field ,,in ,,constellation ,,monoceros #hd ,photographed by ,barnard1 ,february #a1 #ahid4 ,,solar ,,corona4 ,,total ,,eclipse #ajh ,photographed by ,campbell1 ,january #bb1 #ahih2 ,jeur1 ,india4 ,,forty-,,inch ,,telescope1 ,,yerkes ,,observatory #agj ,,yerkes ,,observatory1 ,,university ,,of ,,chicago #agf ,,practical ,,talks ,,by ,,an ,,astronomer ,,navigation ,,at ,,sea ,a short time ago the writer had occasion to rummage among the archives of the ,royal ,astronomical ,society in ,london1 to consult1 if possible1 the original manuscripts left by one ,stephen ,groombridge1 an ,english astronomer of the good old days1 who died in #ahcb4 ,it was known that they had been filed away about a generation ago1 by the late ,sir ,george ,airy1 who was ,astronomer ,royal of ,england between the years #ahce and #ahha4 ,after a long search1 a large and dusty box was found and opened4 ,it was filled with documents1 of which the topmost was in ,sir ,george's own handwriting1 and began substantially as follows3 ,list of articles within this box4 ,no4 #a1 ,this list1 ,no4 #b1 etc41 etc4 ,astronomical precision can no further go3 he had listed even the list itself4 ,truly1 ,airy was rightly styled prince of precisians4 ,a worthy ,astronomer ,royal was he1 to act under the royal warrant of ,charles ,,ii41 who established the office in #afge4 ,down to this present day that warrant still makes it the duty of ,his ,majesty's ,astronomer to apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens and the places of the fixed stars1 in order to find out the so much desired longitude at sea1 for the perfecting the art of navigation4 ,the so much desired longitude at sea is1 indeed1 a vastly important thing to a maritime nation like ,england4 ,and only in comparatively recent years has it become possible and easy for vessels to be navigated with safety and convenience upon long voyages4 ,the writer was well acquainted with an old sea-captain of ,new ,york1 who had commanded one of the earliest transatlantic steamers1 and who died only a few years ago4 ,he had a goodly store of ocean yarn1 fit and ready for the spinning1 if he could but find someone who1 like himself1 had known and loved the ocean4 ,in his early sea-going days1 only the wealthiest of captains owned chronometers4 ,this instrument is now considered indispensable in navigation1 but at that time it was a new invention1 very rare and costly4 ,upon a certain voyage from ,england to ,rio ,janeiro1 in ,south ,america1 the old captain could remember the following odd method of navigation3 ,the ship was steered by compass to the southward and westward1 more or less1 until the skipper's antique quadrant showed that they had about reached the latitude of ,rio4 ,then they swung her on a course due west by compass1 and away she went for ,rio1 relying on the lookout man forward to keep the ship from running ashore4 ,for after a certain lapse of time1 being ignorant of the longitude1 they could not know whether they would raise the land within an hour or in six weeks4 ,we are glad of an opportunity to put this story on record1 for the time is not far distant when there will be no man left among the living who can remember how ships were taken across the seas in the good old days before chronometers4 ,anyone who has ever been a passenger on a great transatlantic liner of to-day knows what an important1 imposing personage is the brass-bound skipper4 ,a very different creature is he on the deck of his ship from the modest seafaring man we meet on land1 clad for the time being in his shore-going togs4 ,but the captain's dignity is not all brass buttons and gold braid4 ,he has behind him the powerful support of a deep1 delightful mystery4 ,he it is who takes the sun at noon1 and finds out the ship's path at sea4 ,and in truth1 regarded merely as a scientific experiment1 the guiding of a vessel across the unmarked trackless ocean has few equals within the whole range of human knowledge4 ,it is our purpose here to explain quite briefly the manner in which this seeming impossibility is accomplished4 ,we shall not be able to go sufficiently into details to enable him who reads to run and navigate a magnificent steamer4 ,but we hope to diminish somewhat that small part of the captain's vast dignity which depends upon his mysterious operations with the sextant4 ,to begin1 then1 with the sextant itself4 ,it is nothing but an instrument with which we can measure how high up the sun is in the sky4 ,now1 everyone knows that the sun slowly climbs the sky in the morning1 reaches its greatest height at noon1 and then slowly sinks again in the afternoon4 ,the captain simply begins to watch the sun through the sextant shortly before noon1 and keeps at it until he discovers that the sun is just beginning to descend4 ,that is the instant of noon on the ship4 ,the captain quickly glances at the chronometer1 or calls out noon to an officer who is near that instrument4 ,and so the error of the chronometer becomes known then and there without any further astronomical calculations whatever4 ,navigators can also find the chronometer error by sextant observations when the sun is a long way from noon4 ,the methods of doing this are somewhat less simple than for the noon observation2 they belong to the details of navigation1 into which we cannot enter here4 ,incidentally1 the captain also notes with the sextant how high the sun was in the sky at the noon observation4 ,he has in his mysterious chart-room some printed astronomical tables1 which tell him in what terrestrial latitude the sun will have precisely that height on that particular day of the year4 ,thus the terrestrial latitude becomes known easily enough1 and if only the captain could get his longitude too1 he would know just where his ship was that day at noon4 ,we have seen that the sextant observations furnish the error of the chronometer according to ship's time4 ,in other words1 the captain is in possession of the correct local time in the place where the ship actually is4 ,now1 if he also had the correct time at that moment of some well-known place on shore1 he would know the difference in time between that place on shore and the ship4 ,but every traveller by land or sea is aware that there are always differences of time between different places on the earth4 ,if a watch be right on leaving ,new ,york1 for instance1 it will be much too fast on arriving at ,chicago or ,san ,francisco2 the farther you go the larger becomes the error of your watch4 ,in fact1 if you could find out how much your watch had gone into error1 you would in a sense know how far east or west you had travelled4 ,now the captain's chronometer is set to correct ,greenwich time on shore before the ship leaves port4 ,his observations having then told him how much this is wrong on that particular day1 and in that particular spot where the ship is1 he knows at once just how far he has travelled east or west from ,greenwich4 ,in other words1 he knows his longitude from ,greenwich1 for longitude is nothing more than distance from ,greenwich in an east-and-west direction1 just as latitude is only distance from the equator measured in a north-and-south direction4 ,greenwich observatory is usually selected as the beginning of things for measuring longitudes1 because it is almost the oldest of existing astronomical establishments1 and belongs to the most prominent maritime nation1 ,england4 ,one of the most interesting bits of astronomical history was enacted in connection with this matter of longitude4 ,from what has been said1 it is clear that the ship's longitude will be obtained correctly only if the chronometer has kept exact time since the departure of the ship from port4 ,even a very small error of the chronometer will throw out the longitude a good many miles1 and we can understand readily that it must be difficult in the extreme to construct a mechanical contrivance capable of keeping exact time when subjected to the rolling and pitching of a vessel at sea4 ,it was as recently as the year #agcf that the first instrument capable of keeping anything like accurate time at sea was successfully completed4 ,it was the work of an ,english watchmaker named ,john ,harrison1 and is one of the few great improvements in matters scientific which the world owes to a desire for winning a money prize4 ,it appears that in #agad a committee was appointed by the ,house of ,commons1 with no less a person than ,sir ,isaac ,newton himself as one of its members1 to consider the desirability of offering governmental encouragement for the invention of some means of finding the longitude at sea4 ,finally1 the ,british ,government offered a reward of #ej1jjj for an instrument which would find the longitude within sixty miles2 #ge1jjj1 if within forty miles1 and #ajj1jjj1 if within thirty miles4 ,harrison's chronometer was finished in #agcf1 but he did not receive the final payment of his prize until #agfd4 ,we shall not enter into a detailed account of the vexatious delays and official procedures to which he was forced to submit during those twenty-eight long years4 ,it is a matter of satisfaction that ,harrison lived to receive the money which he had earned4 ,he had the genius to plan and master intricate mechanical details1 but perhaps he lacked in some degree the ability of tongue and pen to bring them home to others4 ,this may be the reason he is so little known1 though it was his fortune to contribute so large and essential a part to the perfection of modern navigation4 ,let us hope this brief mention may serve to recall his memory from oblivion even for a fleeting moment2 that we may not have written in vain of that longitude to which his life was given4 ,,the ,,pleiades ,famed in legend2 sung by early minstrels of ,persia and ,hindustan2 --like a swarm of fire-flies tangled in a silver braid2 yonder distant misty little cloud of ,pleiades has always won and held the imagination of men4 ,but it was not only for the inspiration of poets1 for quickening fancy into song1 that the seven daughters of ,atlas were fixed upon the firmament4 ,the problems presented by this group of stars to the unobtrusive scientific investigator are among the most interesting known to astronomy4 ,their solution is still very incomplete1 but what we have already learned may be counted justly among the richest spoils brought back by science from the stored treasure-house of ,nature's secrets4 ,the true student of astronomy is animated by no mere vulgar curiosity to pry into things hidden4 ,if he seeks the concealed springs that move the complex visible mechanism of the heavens1 he does so because his imagination is roused by the grandeur of what he sees2 and deep down within him stirs the true love of the artist for his art4 ,for it is indeed a fine art1 that science of astronomy4 ,it can have been no mere chance that has massed the ,pleiades from among their fellow stars4 ,men of ordinary eyesight see but a half-dozen distinct objects in the cluster2 those of acuter vision can count fourteen2 but it is not until we apply the space-penetrating power of the telescope that we realize the extraordinary scale upon which the system of the ,pleiades is constructed4 ,with the ,paris instrument ,wolf in #ahgf catalogued #fbe stars in the group2 and the searching photographic survey of ,henry in #ahhg revealed no less than #b1cbf distinct stars within and near the filmy gauze of nebulous matter always so conspicuous a feature of the ,pleiades4 ,the means at our disposal for the study of stellar distances are but feeble4 ,only in the case of a very small number of stars have we been able to obtain even so much as an approximate estimate of distance4 ,the most powerful observational machinery1 though directed by the tried skill of experience1 has not sufficed to sound the profounder depths of space4 ,the ,pleiad stars are among those for which no measurement of distance has yet been made1 so that we do not know whether they are all equally far away from us4 ,we see them projected on the dark background of the celestial vault2 but we cannot tell from actual measurement whether they are all situated near the same point in space4 ,it may be that some are immeasurably closer to us than are the great mass of their companions2 possibly we look through the cluster at others far behind it1 clinging1 as it were1 to the very fringe of the visible universe4 ,farther on we shall find evidence that something like this really is the case4 ,but under no circumstances is it reasonable to suppose that the whole body of stars can be strung out at all sorts of distances near a straight line pointing in the direction of the visible cluster4 ,such a distribution may perhaps remain among the possibilities1 so long as we cannot measure directly the actual distances of the individual stars4 ,but science never accepts a mere possibility against which we can marshal strong circumstantial evidence4 ,we may conclude on general principles that the gathering of these many objects into a single close assemblage denotes community of origin and interests4 ,the ,pleiades then really belong to one another4 ,what is the nature of their mutual tie8 ,what is their mystery1 and can we solve it8 ,the most obvious theory is1 of course1 suggested by what we know to be true within our own solar system4 ,we owe to ,newton the beautiful conception of gravitation1 that unique law by means of which astronomers have been enabled to reduce to perfect order the seeming tangle of planetary evolutions4 ,the law really amounts1 in effect1 to this3 ,all objects suspended within the vacancy of space attract or pull one another4 ,how they can do this without a visible connecting link between them is a mystery which may always remain unsolved4 ,but mystery as it is1 we must accept it as an ascertained fact4 ,it is this pull of gravitation that holds together the sun and planets1 forcing them all to follow out their due and proper paths1 and so to continue throughout an unbroken cycle until the great survivor1 ,time1 shall be no more4 ,this same gravitational attraction must be at work among the ,pleiades4 ,they1 too1 like ourselves1 must have bounds and orbits set and interwoven1 revolutions and gyrations far more complex than the solar system knows4 ,the visual discovery of such motion of rotation among the ,pleiades may be called one of the pressing problems of astronomy to-day4 ,we feel sure that the time is ripe1 and that the discovery is actually being made at the present moment3 for a generation of men is not too great a period to call a moment1 when we have to deal with cosmic time4 ,it is indeed the lack of observations extending through sufficient centuries that stays our hand from grasping the coveted result4 ,the ,pleiades are so far from us that we cannot be sure of changes among them4 ,magnitudes are always relative4 ,it matters not how large the actual movements may be2 if they are extremely small in comparison with our distance1 they must shrink to nothingness in our eyes4 ,trembling on the verge of invisibility1 elusive1 they are in that borderland where science as yet but feels her way1 though certain that the way is there4 ,the foundations of exact modern knowledge of the group were laid by ,bessel about #ahdj4 ,with the modesty characteristic of the great1 he says quite simply that he has made a number of measures of the ,pleiades1 thinking that the time may come when astronomers will be able to find some evidence of motion4 ,in this unassuming way he prefaces what is still the classic model of precision and thoroughness in work of this kind4 ,bessel cleared the ground for a study of inter-stellar motion within the close star-clusters2 and it is probable that only by such study may we hope to demonstrate the universality of the law of gravitation in cosmic space4 ,bessel's acuteness in forecasting the direction of coming research was amply verified by the work of ,elkin in #ahhe at ,yale ,college4 ,provided with a more modern instrument1 but similar to ,bessel's1 ,elkin was able to repeat his observations with a slight increase of precision4 ,motions in the interval of forty-five years1 sufficiently great to hint at coming possibilities1 were shown conclusively to exist4 ,six stars at all events have been fairly excluded from the group on account of their peculiar motions shown by ,elkin's research4 ,it is possible that they are merely seen in the background through the interstices of the cluster itself1 or they may be suspended between us and the ,pleiades1 in either case having no real connection with the group4 ,finally1 these observations make it reasonably certain that many of the remaining mass of stars really constitute a unit aggregation in space4 ,astronomers of a coming generation will again repeat the ,besselian work4 ,at present we have been able to use his method only for the separation from the true ,pleiades of chance stars that happen to lie in the same direction4 ,let us hope that man shall exist long enough upon this earth to see the clustered stars themselves begin and carry out such gyrations as gravitation imposes4 ,these will doubtless be of a kind not even suggested by the lesser complexities of our solar system4 ,for the most wonderful thing of all about the ,pleiades seems to point to an intricacy of structure whose details may be destined to shake the confidence of the profoundest mathematician4 ,there is an extraordinary nebulous condensation that seems to pervade the entire space occupied by the stellar constituents of the group4 ,the stars are swimming in a veritable sea of luminous cloud4 ,there are filmy tenuous places1 and again condensing whirls of material doubtless still in the gaseous or plastic stage4 ,most noticeable of all are certain almost straight lines of nebula that connect series of stars4 ,in one case1 shown upon a photograph made by ,henry at ,paris1 six stars are strung out upon such a hazy line4 ,we might give play to fancy1 and see in this the result of some vast eruption of gaseous matter that has already begun to solidify here and there into stellar nuclei4 ,but sound science gives not too great freedom to mere speculative theories4 ,her duty has been found in quiet research1 and her greatest rewards have flowed from imaginative speculation1 only when tempered by pure reason4 ,,the ,,pole-,,star ,one of the most brilliant observations of the last few years is ,campbell's recent discovery of the triple character of this star4 ,centuries and centuries ago1 when astronomy1 that venerable ancient among the sciences1 was but an infant1 the pole-star must have been considered the very oldest of observed heavenly bodies4 ,in the beginning it was the only sure guide of the navigator at night1 just as to this day it is the foundation-stone for all observational stellar astronomy of precision4 ,there has never been a time in the history of astronomy when the pole-star might not have been called the most frequently measured object in the sky of night4 ,so it is indeed strange that we should find out something altogether new about it after all these ages of study4 ,but the importance of the discovery rests upon a surer foundation than this4 ,the method by which it has been made is almost a new one in the science4 ,a generation ago1 men thought the perfect science1 for so we love to call astronomy1 could advance only by increasing a little the exact precision of observation4 ,the citadel of perfect truth might be more closely invested2 the forces of science might push forward step by step2 the machinery of research might be strengthened1 but that a new engine of investigation would be discovered capable of penetrating where no telescope can ever reach1 this1 indeed1 seemed far beyond the liveliest hope of science4 ,even the discoverer of the spectroscope could never have dreamed of its possibilities1 could never have foreseen its successes1 its triumphs4 ,the very name of this instrument suggests mystery to the popular mind4 ,it is set down at once among the things too difficult1 too intricate1 too abstruse to understand4 ,yet in its essentials there is nothing about the spectroscope that cannot be made clear in a few words4 ,even the modern undulatory theory of light itself is terrible only in the length of its name4 ,anyone who has seen the waves of ocean roll1 roll1 and ever again roll in upon the shore1 can form a very good notion of how light moves4 ',tis just such a series of rolling waves2 started perhaps from some brilliant constellation far out upon the confining bounds of the visible universe1 or perhaps coming from a humble light upon the student's table2 yet it is never anything but a succession of rolling waves4 ,only1 unlike the waves of the sea1 light waves are all excessively small4 ,we should call one whose length was a twenty-thousandth of an inch a big one6 ,now the human eye possesses the property of receiving and understanding these little waves4 ,the process is an unconscious one4 ,let but a set of these tiny waves roll up1 as it were1 out of the vast ocean of space and impinge upon the eye1 and all the phenomena of light and color become what we call visible4 ,we see the light4 ,and how does all this find an application in astronomy8 ,not to enter too much into technical details1 we may say that the spectroscope is an instrument which enables us to measure the length of these light waves1 though their length is so exceedingly small4 ,the day has indeed gone by when that which poets love to call the ,book of ,nature was printed in type that could be read by the eye unaided4 ,telescope1 microscope1 and spectroscope are essential now to him who would penetrate any of ,nature's secrets4 ,but measurements with a telescope1 like eye observations1 are limited strictly to determining the directions in which we see the heavenly bodies4 ,ever since the beginning of things1 when old ,hipparchus and ,ulugh ,beg made the first rude but successful attempts to catalogue the stars1 the eye and telescope have been able to measure only such directions4 ,we aim the telescope at a star1 and record the direction in which it was pointed4 ,distances in astronomy can never be measured directly4 ,all that we know of them has been obtained by calculations based upon the ,newtonian law of gravitation and observations of directions4 ,now the spectroscope seems to offer a sort of exception to this rule4 ,suppose we can measure the wave-lengths of the light sent us from a star4 ,suppose again that the star is itself moving swiftly toward us through space1 while continually setting in motion the waves of light that are ultimately to reach the waiting astronomer4 ,evidently the light waves will be crowded together somewhat on account of the star's motion4 ,more waves per second will reach us than would be received from a star at rest4 ,it is as though the light waves were compressed or shortened a little4 ,and if the star is leaving us1 instead of coming nearer1 opposite effects will occur4 ,we have then but to compare spectroscopically starlight with some artificial source of light in the observatory in order to find out whether the star is approaching us or receding from us4 ,and by a simple process of calculation this stellar motion can be obtained in miles per second4 ,thus we can now actually measure directly1 in a certain sense1 linear speed in stellar space1 though we are still without the means of getting directly at stellar distances4 ,but the most wonderful thing of all about these spectroscopic measures is the fact that it makes no difference whatever how far away is the star under observation4 ,what we learn through the spectroscope comes from a study of the waves themselves1 and it is of no consequence how far they have travelled1 or how long they have been a-coming4 ,for it must not be supposed that these waves consume no time in passing from a distant star to our own solar system4 ,it is true that they move exceeding fast2 certainly #ahj1jjj miles per second may be called rapid motion4 ,but if this cosmic velocity of light is tremendous1 so also are cosmic distances correspondingly vast4 ,light needs to move quickly coming from a star1 for even at the rate of motion we have mentioned it requires many years to reach us from some of the more distant constellations4 ,it has been well said that an observer on some far-away star1 if endowed with the power to see at any distance1 however great1 might at this moment be looking on the ,crusaders proceeding from ,europe against the ,saracen at ,jerusalem4 ,for it is quite possible that not until now has the light which would make the earth visible had time to reach him4 ,yet distant as such an observer might be1 light from the star on which he stood could be measured in the spectroscope1 and would infallibly tell us whether the earth and star are approaching in space or gradually drawing farther asunder4 ,the pole-star is not one of the more distant stellar systems4 ,we do not know how far it is from us very exactly1 but certainly not less than forty or fifty years are necessary for its light to reach us4 ,the star might have gone out of existence twenty years ago1 and we not yet know of it1 for we would still be receiving the light which began its long journey to us about #ahej or #ahfj4 ,but no matter what may be its distance1 ,campbell found by careful observations1 made in the latter part of #ahif1 that the pole-star was then approaching the earth at the rate of about twelve miles per second4 ,so far there was nothing especially remarkable4 ,but in ,august and ,september of the present year twenty-six careful determinations were made1 and these showed that now the rate of approach varied between about five and nine miles per second4 ,more astonishing still1 there was a uniform period in the changes of velocity4 ,in about four days the rate of motion changed from about five to nine miles and back again4 ,and this variation kept on with great regularity4 ,every successive period of four days saw a complete cycle of velocity change forward and back between the same limits4 ,there can be but one reasonable explanation4 ,this star must be a double1 or binary star4 ,the two components1 under the influence of powerful mutual gravitational attraction1 must be revolving in a mighty orbit4 ,yet this vast orbit1 as a whole1 with the two great stars in it1 must be approaching our part of the universe all the time4 ,for the spectroscope shows the velocity of approach to increase and diminish1 indeed1 but it is always present4 ,here1 then1 is this great stellar system1 having a four-day revolution of its own1 and yet swinging rapidly through space in our direction4 ,nor is this all4 ,one of the component stars must be nearly or quite dark2 else its presence would infallibly be detected by our instruments4 ,and now we come to the most astonishing thing of all4 ,how comes it that the average rate of approach of the four-day system1 as a whole1 changed between #ahif and #ahii8 ,in #ahif only this velocity of the whole system was determined1 the four-day period remaining undiscovered until the more numerous observations of #ahii4 ,but even without considering the four-day period1 the changing velocity of the entire system offers one of those problems that exact science can treat only by the help of the imagination4 ,there must be some other great centre of attraction1 some cosmic giant1 holding the visible double pole-star under its control4 ,thus1 that which we see1 and call the pole-star1 is in reality threading its path about the third and greatest member of the system1 itself situated in space1 we know not where4 ,illustration3 ,spiral ,nebula in ,constellation ,leo4 ,photographed by ,keeler1 ,february #bd1 #aijj4 ,exposure1 three hours1 fifty minutes4 ,,nebul ,scattered about here and there among the stars are certain patches of faint luminosity called by astronomers ,nebul4 ,these little clouds of filmy light are among the most fascinating of all the kaleidoscopic phenomena of the heavens2 for it needs but a glance at one of them to give the impression that here before us is the stuff of which worlds are made4 ,all our knowledge of ,nature leads us to expect in her finished work the result of a series of gradual processes of development4 ,highly organized phenomena such as those existing in our solar system did not spring into perfection in an instant4 ,influential forces1 easy to imagine1 but difficult to define1 must have directed the slow1 sure transformation of elemental matter into sun and planets1 things and men4 ,therefore a study of those forces and of their probable action upon nebular material has always exerted a strong attraction upon the acutest thinkers among men of exact science4 ,our knowledge of the nebul is of two kinds--that which has been ascertained from observation as to their appearance1 size1 distribution1 and distance2 and that which is based upon hypotheses and theoretical reasoning about the condensation of stellar systems out of nebular masses4 ,it so happens that our observational material has received a very important addition quite recently through the application of photography to the delineation of nebul1 and this we shall describe farther on4 ,two nebul only are visible to the unaided eye4 ,the brighter of these is in the constellation ,andromeda2 it is of oval or elliptical shape1 and has a distinct central condensation or nucleus4 ,upon a photograph by ,roberts it appears to have several concentric rings surrounding the nebula proper1 and gives the general impression of a flat round disk foreshortened into an oval shape on account of the observer's position not being square to the surface of the disk4 ,very recent photographs of this nebula1 made with the three-foot reflecting telescope of the ,lick ,observatory1 bring out the fact that it is really spiral in form1 and that the outlying nebulous rings are only parts of the spires in a great cosmic whorl4 ,illustration3 ,nebula in ,andromeda4 ,lower object in the photograph is a ,comet4 ,photographed by ,barnard1 ,november #ba1 #ahib4 ,this ,andromeda nebula is the one in which the temporary star of #ahhe appeared4 ,it blazed up quite suddenly near the apparent centre of the nebula1 and continued in view for six months1 fading finally beyond the reach of our most powerful telescopes4 ,there can be little doubt that the star was actually in the nebula1 and not merely seen through it1 though in reality situated in the extreme outlying part of space at a distance immeasurably greater than that separating us from the nebula itself4 ,such an accidental superposition of nebula and star might even be due to sudden incandescence of a new star between us and the nebula4 ,in such a case we should see the star projected upon the surface of the nebula1 so that the superposition would be identical with that actually observed4 ,therefore1 while it is1 indeed1 possible that the star may have been either far behind the nebula or in front of it1 we must accept as more probable the supposition that there was a real connection between the two4 ,in that case there is little doubt that we have actually observed one of those cataclysms that mark successive steps of cosmic evolution4 ,we have no thoroughly satisfactory theory to account for such an explosive catastrophe within the body of the nebula itself4 ,the other naked-eye nebula is in the constellation ,orion4 ,in the telescope it is a more striking object1 perhaps1 than the ,andromeda nebula2 for it has no well-defined geometrical form1 but consists of an immense odd-shaped mass of light enclosing and surrounding a number of stars4 ,it is unquestionably of a very complicated structure1 and is1 therefore1 less easily studied and explained than the nebul of simpler form4 ,there is no doubt that the ,orion nebula is composed of luminous gas1 and is not merely a cluster of small stars too numerous and too near together to be separated from each other1 even in our most powerful telescopes4 ,it was1 indeed1 supposed1 until about forty years ago1 that all the nebul are simply irresolvable star-clusters2 but we now have indisputable evidence1 derived from the spectroscope1 that many nebul are composed of true gases1 similar to those with which we experiment in chemical laboratories4 ,this spectroscopic proof of the gaseous character of nebul is one of the most important discoveries contributed by that instrument to our small stock of facts concerning the structure of the sidereal universe4 ,coming now to the smaller nebul1 we find a great diversity of form and appearance4 ,some are ring-shaped1 perhaps having a less brilliant nebulosity within the ring4 ,many show a central condensation of disk-like appearance 7planetary nebul71 or have simply a star at the centre 7nebulous stars74 ,altogether about ten thousand such objects have been catalogued by successive generations of astronomers since the invention of the telescope1 and most of these have been reported as oval in form4 ,now we have already referred to the important addition to our knowledge of the nebul obtained by recent photographic observations2 and this addition consists in the discovery that most of these oval nebul are in reality spirals4 ,indeed1 it appears that the spiral type is the normal type1 and that nebul of irregular or other forms are exceptions to the general rule4 ,even the great ,andromeda nebula1 as we have seen1 is now recognized as a spiral4 ,the instrument with which its convolute structure was discovered is a three-foot reflecting telescope1 made by ,common of ,england1 and now mounted at the ,lick ,observatory1 in ,california4 ,the late ,professor ,keeler devoted much of his time to photographing nebul during the last year or two4 ,he was able to establish the important fact just mentioned1 that most nebul formerly thought to be mere ovals1 turn out to be spiral when brought under the more searching scrutiny of the photographic plate applied at the focus of a telescope of great size1 and with an exposure to the feeble nebular light extending through three or four consecutive hours4 ,many of the spirals have more than a single volute4 ,it is as though one were to attach a number of very flexible rods to an axle1 like spokes of a wheel without a rim and then revolve the axle rapidly4 ,the flexible rods would bend under the rapid rotation1 and form a series of spiral curves not unlike many of these nebul4 ,indeed1 it is impossible to escape the conviction that these great celestial whorls are whirling around an axis4 ,and it is most important in the study of the growth of worlds1 to recognize that the type specimen is a revolving spiral4 ,therefore1 the rotating flattened globe of incandescent matter postulated by ,laplace's nebular hypothesis would make of our solar system an exceptional world1 and not a type of stellar evolution in general4 ,keeler's photographs have taught us one thing more4 ,scarcely is there a single one of his negatives that does not show nebul previously uncatalogued4 ,it is estimated that if this process of photography could be extended so as to cover the entire sky1 the whole number of nebul would add up to the stupendous total of #abj1jjj2 and of these the great majority would be spiral4 ,when we approach the question of the distribution of nebul in different parts of the sky1 as shown by their catalogued positions1 we are met by a curious fact4 ,it appears that the region in the neighborhood of the ,milky ,way is especially poor in nebul1 whereas these objects seem to cluster in much larger numbers about those points in the sky that are farthest from the ,milky ,way4 ,but we know that the ,milky ,way is richer in stars than any other part of the sky1 since it is1 in fact1 made up of stellar bodies clustered so closely that it is wellnigh impossible to see between them in the denser portions4 ,now1 it cannot be the result of chance that the stars should tend to congregate in the ,milky ,way1 while the nebul tend to seek a position as far from it as possible4 ,whatever may be the cause1 we must conclude that the sidereal system1 as we see it1 is in general constructed upon a single plan1 and does not consist of a series of universes scattered at random throughout space4 ,if we are to suppose that nebul turn into stars as a result of condensation or any other change1 then it is not astonishing to find a minimum of nebul where there is a maximum of stars1 since the nebul will have been consumed1 as it were1 in the formation of the stars4 ,illustration3 ,the ,dumb-,bell ,nebula4 ,photographed by ,keeler1 ,july #ca1 #ahii4 ,exposure1 three hours4 ,it is never advisable to push philosophical speculation very far when supported by too slender a basis of fact4 ,but if we are to regard the visible universe as made up on the whole of a single system of bodies1 we may well ask one or two questions to be answered by speculative theory4 ,we have said the stars are not uniformly distributed in space4 ,their concentration in the ,milky ,way1 forming a narrow band dividing the sky into two very nearly equal parts1 must be due to their being actually massed in a thin disk or ring of space within which our solar system is also situated4 ,this thin disk projected upon the sky would then appear as the narrow star-band of the ,milky ,way4 ,now1 suppose this disk has an axis perpendicular to itself1 and let us imagine a rotation of the whole sidereal system about that axis4 ,then the fact that the visible nebul are congregated far from the ,milky ,way means that they are actually near the imaginary axis4 ,possibly the diminished velocity of motion near the axis may have something to do with the presence of the nebul there4 ,possibly the nebul themselves have axes perpendicular to the plane of the ,milky ,way4 ,if so1 we should see the spiral nebul near the ,milky ,way edgewise1 and those far from it without foreshortening4 ,thus1 the paucity of nebul near the ,milky ,way may be due in part to the increased difficulty of seeing them when looked at edgewise4 ,indeed1 there is no limit to the possibilities of hypothetical reasoning about the nebular structure of our universe2 unfortunately1 the whole question must be placed for the present among those intensely interesting cosmic problems awaiting elucidation1 let us hope1 in this new century4 ,,temporary ,,stars ,nothing can be more erroneous than to suppose that the stellar multitude has continued unchanged throughout all generations of men4 ,eternal fires poets have called the stars2 yet they burn like any little conflagration on the earth2 now flashing with energy1 brilliant1 incandescent1 and again sinking into the dull glow of smouldering half-burned ashes4 ,it is even probable that space contains many darkened orbs1 stars that may have risen in constellations to adorn the skies of prehistoric time--now cold1 unseen1 unknown4 ,so far from dealing with an unvarying universe1 it is safe to say that sidereal astronomy can advance only by the discovery of change4 ,observational science watches with untiring industry1 and night hides few celestial events from the ardent scrutiny of astronomers4 ,old theories are tested and newer ones often perfected by the detection of some slight and previously unsuspected alteration upon the face of the sky4 ,the interpretation of such changes is the most difficult task of science2 it has taxed the acutest intellects among men throughout all time4 ,if1 then1 changes can be seen among the stars1 what are we to think of the most important change of all1 the blazing into life of a new stellar system8 ,fifteen times since men began to write their records of the skies has the birth of a star been seen4 ,surely we may use this term when we speak of the sudden appearance of a brilliant luminary where nothing visible existed before4 ,but we shall see further on that scientific considerations make it highly probable that the phenomenon in question does not really involve the creation of new matter4 ,it is old material becoming suddenly luminous for some hidden reason4 ,in fact1 whenever a new object of great brilliancy has been discovered1 it has been found to lose its light again quite soon1 ending either in total extinction or at least in comparative darkness4 ,it is for this reason that the name temporary star has been applied to cases of this kind4 ,the first authenticated instance dates from the year #acd ,b4,c41 when a new star appeared in the constellation ,scorpio4 ,it was this star that led ,hipparchus to construct his stellar catalogue1 the first ever made4 ,it occurred to him1 of course1 that there could be but one way to make sure in the future that any given object discovered in the sky was new2 it was necessary to make a complete list of everything visible in his day4 ,later astronomers need then only compare ,hipparchus's catalogue with the heavens from time to time in order to find out whether anything unknown had appeared4 ,this work of ,hipparchus became the foundation of sidereal study1 and led to most important discoveries of various kinds4 ,but no records remain concerning his new star except the bare fact of its appearance in ,scorpio4 ,hipparchus's published works are all lost4 ,we do not even know the exact place of his birth1 and as for those two dates of entry and exit that history attaches to great names--we have them not4 ,yet he was easily the first astronomer of antiquity1 one of the first of all time2 and we know of him only from the writings of ,ptolemy1 who lived three hundred years after him4 ,more than five centuries elapsed before another temporary star was entered in the records of astronomy4 ,this happened in the year #chi ,a4,d41 when a star appeared in ,aquila2 and of this one also we know nothing further4 ,but about twelve centuries later1 in ,november1 #aegb1 a new and brilliant object was found in the constellation ,cassiopeia4 ,it is known as ,tycho's star1 since it was the means of winning for astronomy a man who will always take high rank in her annals1 ,tycho ,brahe1 of ,denmark4 ,when he first saw this star1 it was already very bright1 equalling even ,venus at her best2 and he continued a careful series of observations for sixteen months1 when it faded finally from his view4 ,the position of the new star was measured with reference to other stars in the constellation ,cassiopeia1 and the results of ,tycho's observations were finally published by him in the year #aegc4 ,it appears that much urging on the part of friends was necessary to induce him to consent to this publication1 not because of a modest reluctance to rush into print1 but for the reason that he considered it undignified for a nobleman of ,denmark to be the author of a book6 ,an important question in cosmic astronomy is opened by ,tycho's star4 ,did it really disappear from the heavens when he saw it no more1 or had its lustre simply been reduced below the visual power of the unaided eye8 ,unfortunately1 ,tycho's observations of the star's position in the constellation were necessarily crude4 ,he possessed no instruments of precision such as we now have at our disposal1 and so his work gives us only a rather rough approximation of the true place of the star4 ,a small circle might be imagined on the sky of a size comparable with the possible errors of ,tycho's observations4 ,we could then say with certainty that his star must have been situated somewhere within that little circle1 but it is impossible to know exactly where4 ,it happens that our modern telescopes reveal the existence of several faint stars within the space covered by such a circle4 ,any one of these would have been too small for ,tycho to see1 and1 therefore1 any one of them may be his once brilliant luminary reduced to a state of permanent or temporary semi-darkness4 ,these considerations are1 indeed1 of great importance in explaining the phenomena of temporary stars4 ,if ,tycho had been able to leave us a more exact determination of his star's place in the sky1 and even if our most powerful instruments could not show anything in that place to-day1 we might nevertheless theorize on the supposition that the object still exists1 but has reached a condition almost entirely dark4 ,indeed1 the latest theory classes temporary stars among those known as variable4 ,for many stars are known to undergo quite decided changes in brilliancy2 possibly inconstancy of light is the rule rather than the exception4 ,but while such changes1 when they exist1 are too small to be perceptible in most cases1 there is certainly a large number of observable variables1 subject to easily measurable alterations of light4 ,astronomers prefer to see in the phenomena of temporary stars simple cases of variation in which the increase of light is sudden1 and followed by a gradual diminution4 ,possibly there is then a long period of comparative or even complete darkness1 to be followed as before by a sudden blazing up and extinction4 ,no temporary star1 however1 has been observed to reappear in the same celestial place where once had glowed its sudden outburst4 ,but cases are not wanting where incandescence has been both preceded and followed by a continued existence1 visible though not brilliant4 ,for such cases as these it is necessary to come down to modern records4 ,we cannot be sure that some faint star has been temporarily brilliant1 unless we actually see the conflagration itself1 or are able to make the identity of the object's precise location in the sky before and after the event perfectly certain by the aid of modern instruments of precision4 ,but no one has ever seen the smouldering fires break out4 ,temporary stars have always been first noticed only after having been active for hours if not for days4 ,so we must perforce fall back on instrumental identification by determinations of the star's exact position upon the celestial vault4 ,some time between ,may #ajth and #abth in the year #ahff the ninth star in the list of known temporaries appeared4 ,it possessed very great light-giving power1 being surpassed in brilliancy by only about a score of stars in all the heavens4 ,it retained a maximum luminosity only three or four days1 and in less than two months had diminished to a point somewhere between the ninth and tenth magnitudes4 ,in other words1 from a conspicuous star1 visible to the naked eye1 it had passed beyond the power of anything less than a good telescope4 ,fortunately1 we had excellent star-catalogues before #ahff4 ,these were at once searched1 and it was possible to settle quite definitely that a star of about the ninth or tenth magnitude had really existed before #ahff at precisely the same point occupied by the new one4 ,needless to say1 observations were made of the new star itself1 and afterward compared with later observations of the faint one that still occupies its place4 ,these render quite certain the identity of the temporary bright star with the faint ones that preceded and followed it4 ,such results1 on the one hand1 offer an excellent vindication of the painstaking labor expended on the construction of star-catalogues1 and1 on the other1 serve to elucidate the mystery of temporary stars4 ,nothing can be more plausible than to explain by analogy those cases in which no previous or subsequent existence has been observed4 ,it is merely necessary to suppose that1 instead of varying from the ninth or tenth magnitude1 other temporary objects have begun and ended with the twentieth2 for the twentieth magnitude would be beyond the power of our best instruments4 ,nor is the star of #ahff an isolated instance4 ,ten years later1 in #ahgf1 a temporary star blazed up to about the second magnitude1 and returned to invisibility1 so far as the naked eye is concerned1 within a month1 having retained its greatest brilliancy only one or two days4 ,this star is still visible as a tiny point of light1 estimated to be of the fifteenth magnitude4 ,whether it existed prior to its sudden outburst can never be known1 because we do not possess catalogues including the generality of stars as faint as this one must have been4 ,but at all events1 the continued existence of the object helps to place the temporary stars in the class of variables4 ,the next star1 already mentioned under nebula1 was first seen in #ahhe4 ,it was in one respect the most remarkable of all1 for it appeared almost in the centre of the great nebula in the constellation ,andromeda4 ,it was never very bright1 reaching only the sixth magnitude or thereabouts1 was observed during a period of only six months1 and at the end of that time had faded beyond the reach of our most powerful glasses4 ,it is a most impressive fact that this event occurred within the nebula4 ,whatever may be the nature of the explosive catastrophe to which the temporary stars owe their origin1 we can now say with certainty that not even those vast elemental luminous clouds men call nebul are free from danger4 ,the last outburst on our records was first noticed ,february #bb1 #aija4 ,the star appeared in the constellation ,perseus1 and soon reached the first magnitude1 surpassing almost every other star in the sky4 ,it has been especially remarkable in that it has become surrounded by a nebulous mass in which are several bright condensations or nuclei2 and these seem to be in very rapid motion4 ,the star is still under observation 7,january1 #aijb74 ,,galileo ,among the figures that stand out sharply upon the dim background of old-time science1 there is none that excites a keener interest than ,galileo4 ,most people know him only as a distinguished man of learning2 one who carried on a vigorous controversy with the ,church on matters scientific4 ,it requires some little study1 some careful reading between the lines of astronomical history1 to gain acquaintance with the man himself4 ,he had a brilliant1 incisive wit2 was a genuine humorist2 knew well and loved the amusing side of things2 and could not often forego a sarcastic pleasantry1 or deny himself the pleasure of argument4 ,yet it is more than doubtful if he ever intended impertinence1 or gave willingly any cause of quarrel to the ,church4 ,his acute understanding must have seen that there exists no real conflict between science and religion2 for time1 in passing1 has made common knowledge of this truth1 as it has of many things once hidden4 ,when we consider events that occurred three centuries ago1 it is easy to replace excited argument with cool judgment2 to remember that those were days of violence and cruelty2 that public ignorance was of a density difficult to imagine to-day2 and that it was universally considered the duty of the ,church to assume an authoritative attitude upon many questions with which she is not now required to concern herself in the least4 ,charlatans1 unbalanced theorists1 purveyors of scientific marvels1 were all liable to be passed upon definitely by the ,church1 not in a spirit of impertinent interference1 but simply as part of her regular duties4 ,if the ,church's judgment in such matters was sometimes erroneous2 if her interference now and again was cruel1 the cause must be sought in the manners and customs of the time1 when persecution rioted in company with ignorance1 and violence was the law4 ,perhaps even to-day it would not be amiss to have a modern scientific board pass authoritatively upon novel discoveries and inventions1 so as to protect the public against impostors as the ,church tried to do of old4 ,galileo was born at ,pisa in #aefd1 and his long life lasted until #afdb1 the very year of ,newton's birth4 ,his most important scientific discoveries may be summed up in a few words2 he was the first to use a telescope for examining the heavenly bodies2 he discovered mountains on the moon2 the satellites of ,jupiter2 the peculiar appearance of ,saturn which ,huygens afterward explained as a ring surrounding the ball of the planet2 and1 finally1 he found black spots on the sun's disk4 ,these discoveries1 together with his remarkable researches in mechanical science1 constitute ,galileo's claim to immortality as an investigator4 ,but1 as we have said1 it is not our intention to consider his work as a series of scientific discoveries4 ,we shall take a more interesting point of view1 and deal with him rather as a human being who had contracted the habit of making scientific researches4 ,what must have been his feelings when he first found with his new telescope the satellites of ,jupiter8 ,they were seen on the night of ,january #g1 #afaj4 ,he had already viewed the planet through his earlier and less powerful glass1 and was aware that it possessed a round disk like the moon1 only smaller4 ,now he saw also three objects that he took to be little stars near the planet4 ,but on the following night1 as he says1 drawn by what fate ,i know not1 the tube was again turned upon the planet4 ,the three small stars had changed their positions1 and were now all situated to the west of ,jupiter1 whereas on the previous night two had been on the eastern side4 ,he could not explain this phenomenon1 but he recognized that there was something peculiar at work4 ,long afterward1 in one of his later works1 translated into quaint old ,english by ,salusbury1 he declared that one sole experiment sufficeth to batter to the ground a thousand probable ,arguments4 ,this was already the guiding principle of his scientific activity1 a principle of incomparable importance1 and generally credited to ,bacon4 ,needless to say1 ,jupiter was now examined every night4 ,the #ith was cloudy1 but on the #ajth he again saw his little stars1 their number now reduced to two4 ,he guessed that the third was behind the planet's disk4 ,the position of the two visible ones was altogether different from either of the previous observations4 ,on the #aath he became sure that what he saw was really a series of satellites accompanying ,jupiter on his journey through space1 and at the same time revolving around him4 ,on the #abth1 at #c ,a4,m41 he actually saw one of the small objects emerge from behind the planet2 and on the #acth he finally saw four satellites4 ,two hundred and eighty-two years were destined to pass away before any human eye should see a fifth4 ,it was ,barnard in #ahib who followed ,galileo4 ,to understand the effect of this discovery upon ,galileo requires a person who has himself watched the stars1 not1 as a dilettante1 seeking recreation or amusement1 but with that deep reverence that comes only to him who feels--nay1 knows--that in the moment of observation just passed he too has added his mite to the great fund of human knowledge4 ,galileo's mummied forefinger still points toward the stars from its little pedestal of wood in the ,museo at ,florence1 a sign to all men that he is unforgotten4 ,but ,galileo knew on that #aath of ,january1 #afaj1 that the memory of him would never fade2 that the very music of the spheres would thenceforward be attuned to a truer note1 if any would but hearken to the ,jovian harmony4 ,for he recognized at once that the visible revolution of these moons around ,jupiter1 while that planet was himself visibly travelling through space1 must deal its death-blow to the old ,ptolemaic system of the universe4 ,here was a great planet1 the centre of a system of satellites1 and yet not the centre of the universe4 ,surely1 then1 the earth1 too1 might be a mere planet like ,jupiter1 and not the supposed motionless centre of all things4 ,the satellite discovery was published in #afaj in a little book called ,sidereus ,nuncius1 usually translated ,the ,sidereal ,messenger4 ,it seems to us1 however1 that the word messenger is not strong enough2 surely in ,papal ,italy a nuncius was more than a mere messenger4 ,he was clothed with the very highest authority1 and we think it probable that ,galileo's choice of this word in the title of his book means that he claimed for himself similar authority in science4 ,at all events1 the book made him at once a great reputation and numerous enemies4 ,but it was not until #afaf that the ,holy ,office 7,inquisition7 issued an edict ordering ,galileo to abandon his opinion that the earth moved1 and at the same time placed ,copernicus's ,de ,revolutionibus and two other books advocating that doctrine on the ,index ,librorum ,prohibitorum1 or list of books forbidden by the ,church4 ,these volumes remained in subsequent editions of the ,index down to #ahba1 but they no longer appear in the edition in force to-day4 ,galileo's most characteristic work is entitled the ,dialogue on the ,two ,chief ,systems of the ,world4 ,it was not published until #afcb1 although the idea of the book was conceived many years earlier4 ,in it he gave full play to his extraordinary powers as a true humorist1 a fine lame among controversialists1 and a genuine man of science1 valuing naked truth above all other things4 ,as may be imagined1 it was no small matter to obtain the authorities' consent to this publication4 ,galileo was already known to hold heretical opinions1 and it was suspected that he had not laid them aside when commanded to do so by the edict of #afaf4 ,but perhaps ,galileo's introduction to the ,dialogue secured the censor's imprimatur2 it is even suspected that the ,roman authorities helped in the preparation of this introduction4 ,fortunately1 we have a delightful contemporary translation into ,english1 by ,thomas ,salusbury1 printed at ,london by ,leybourne in #affa4 ,we have already quoted from this translation1 and now add from the same work part of ,galileo's masterly preface to the ,dialogue3 ,judicious reader1 there was published some years since in ,rome a salutiferous ,edict1 that1 for the obviating of the dangerous ,scandals of the ,present ,age1 imposed a reasonable ,silence upon the ,pythagorean 7,copernican7 opinion of the ,mobility of the ,earth4 ,there want not such as unadvisedly affirm1 that the ,decree was not the production of a sober ,scrutiny1 but of an ill-formed passion2 and one may hear some mutter that ,consultors altogether ignorant of ,astronomical observations ought not to clipp the wings of speculative wits with rash prohibitions4 ,galileo first states his own views1 and then pretends that he will oppose them4 ,he goes on to say that he believes in the earth's immobility1 and takes the contrary only for a mathematical ,capriccio1 as he calls it2 something to be considered1 because possessing an academical interest1 but on no account having a real existence4 ,of course any one 7even a censor7 ought to be able to see that it is the ,capriccio1 and not its opposite1 that ,galileo really advocates4 ,three persons appear in the ,dialogue3 ,salviati1 who believes in the ,copernican system2 ,simplicio1 of suggestive name1 who thinks the earth cannot move2 and1 finally1 ,sagredus1 a neutral gentleman of humorous propensities1 who usually begins by opposing ,salviati1 but ends by being convinced4 ,he then helps to punish poor ,simplicio1 who is one of those persons apparently incapable of comprehending a reasonable argument4 ,here is an interesting specimen of the ,dialogue taken from ,salusbury's translation3 ,salviati refers to the argument1 then well known1 that the earth cannot rotate on its axis1 because of the impossibility of its moving long without wearinesse4 ,sagredus replies3 ,there are some kinds of animals which refresh themselves after wearinesse by rowling on the earth2 and that therefore there is no need to fear that the ,terrestrial ,globe should tire1 nay1 it may be reasonably affirmed that it enjoyeth a perpetual and most tranquil repose1 keeping itself in an eternal rowling4 ,salviati's comment on this sally is1 ,you are too tart and satyrical1 ,sagredus4 ,there is no doubt that the ,dialogue finished the ,ptolemaic theory1 and made that of ,copernicus the only possible one4 ,at all events1 it brought about the well-known attack upon ,galileo from the authorities of the ,holy ,office4 ,we shall not recount the often-told tale of his recantation4 ,he was convicted 7very rightly7 of being a ,copernican1 and was forced to abjure that doctrine4 ,galileo's life may be summed up as one of those through which the world has been made richer4 ,a clean-cutting analytic wit1 never becoming dull3 heated again and again in the fierce blaze of controversy1 it was allowed to cool only that it might acquire a finer temper1 to pierce with fatal certainty the smallest imperfections in the armor of his adversaries4 ,,the ,,planet ,,of #ahih ,the discovery of a new and important planet usually receives more immediate popular attention and applause than any other astronomical event4 ,philosophers are fond of referring to our solar system as a mere atom among the countless universes that seem to be suspended within the profound depths of space4 ,they are wont to point out that this solar system1 small and insignificant as a whole in comparison with many of the stellar worlds1 is1 nevertheless1 made up of a large number of constituent planets2 and these in turn are often accompanied with still smaller satellites1 or moons4 ,thus does ,nature provide worlds within worlds1 and it is not surprising that public attention should be at once attracted by any new member of our sun's own special family of planets4 ,the ancients were acquainted with only five of the bodies now counted as planets1 viz43 ,mercury1 ,venus1 ,mars1 ,jupiter1 and ,saturn4 ,the dates of their discovery are lost in antiquity4 ,to these ,uranus was added in #agha by a brilliant effort of the elder ,herschel4 ,we are told that intense popular excitement followed the announcement of ,herschel's first observation3 he was knighted and otherwise honored by the ,english ,king1 and was enabled to lay a secure foundation for the future distinguished astronomical reputation of his family4 ,herschel's discovery quickened the restless activity of astronomers4 ,persistent efforts were made to sift the heavens more and more closely1 with the strengthened hope of adding still further to our planetary knowledge4 ,an association of twenty-four enthusiastic ,german astronomers was formed for the express purpose of hunting planets4 ,but it fell to the lot of an ,italian1 ,piazzi1 of ,palermo1 to find the first of that series of small bodies now known as the asteroids or minor planets4 ,he made the discovery at the very beginning of our century1 ,january #a1 #ahja4 ,but news travelled slowly in those days1 and it was not until nearly ,april that the ,german observers heard from ,piazzi4 ,in the meantime1 he had himself been prevented by illness from continuing his observations4 ,unfortunately1 the planet had by this time moved so near the sun1 on account of its own motions and those of the earth1 that it could no longer be observed4 ,the bright light of the sun made observations of the new body impossible2 and it was feared that1 owing to lack of knowledge of the planet's orbit1 astronomers would be unable to trace it4 ,so there seemed1 indeed1 to be danger of an almost irreparable loss to science4 ,but in scientific1 as in other human emergencies1 someone always appears at the proper moment4 ,a very young mathematician at ,gttingen1 named ,gauss1 attacked the problem1 and was able to devise a method of predicting the future course of the planet on the sky1 using only the few observations made by ,piazzi himself4 ,up to that time no one had attempted to compute a planetary orbit1 unless he had at his disposal a series of observations extending throughout the whole period of the planet's revolution around the sun4 ,but the ,piazzi planet offered a new problem in astronomy4 ,it had become imperatively necessary to obtain an orbit from a few observations made at nearly the same date4 ,gauss's work was signally triumphant1 for the planet was actually found in the position predicted by him1 as soon as a change in the relative places of the planet and earth permitted suitable observations to be made4 ,but after all1 ,piazzi's planet belongs to a class of quite small bodies1 and is by no means as interesting as ,herschel's discovery1 ,uranus4 ,yet even this must be relegated to second rank among planetary discoveries4 ,on ,september #bc1 #ahdf1 the telescope of the ,berlin ,observatory was directed to a certain point on the sky for a very special reason4 ,galle1 the astronomer of ,berlin1 had received a letter from ,leverrier1 of ,paris1 telling him that if he would look in a certain direction he would detect a new and large planet4 ,leverrier's information was based upon a mathematical calculation4 ,seated in his study1 with no instruments but pen and paper1 he had slowly figured out the history of a world as yet unseen4 ,tiny discrepancies existed in the observed motions of ,herschel's planet ,uranus4 ,no man had explained their cause4 ,to ,leverrier's acute understanding they slowly shaped themselves into the possible effects of attraction emanating from some unknown planet exterior to ,uranus4 ,was it conceivable that these slight tremulous imperfections in the motion of a planet could be explained in this way8 ,leverrier was able to say confidently1 ,yes4 ,but we may rest assured that ,galle had but small hopes that upon his eye first1 of all the myriad eyes of men1 would fall a ray of the new planet's light4 ,careful and methodical1 he would neglect no chance of advancing his beloved science4 ,he would look4 ,only one who has himself often seen the morning's sunrise put an end to a night's observation of the stars can hope to appreciate what ,galle's feelings must have been when he saw the planet4 ,to his trained eye it was certainly recognizable at once4 ,and then the good news was sent on to ,paris4 ,we can imagine ,leverrier1 the cool calculator1 saying to himself3 ,of course he found it4 ,it was a mathematical certainty4 ,nevertheless1 his satisfaction must have been of the keenest4 ,no triumphs give a pleasure higher than those of the intellect4 ,let no one imagine that men who make researches in the domain of pure science are under-paid4 ,they find their reward in pleasure that is beyond any price4 ,the ,leverrier planet was found to be the last of the so-called major planets1 so far as we can say in the present state of science4 ,it received the name ,neptune4 ,observers have found no other member of the solar system comparable in size with such bodies as ,uranus and ,neptune4 ,more than one eager mathematician has tried to repeat ,leverrier's achievement1 but the supposed planet was not found4 ,it has been said that figures never lie2 yet such is the case only when the computations are correctly made4 ,people are prone to give to the work of careless or incompetent mathematicians the same degree of credence that is really due only to masters of the craft4 ,it requires the test of time to affix to any man's work the stamp of true genius4 ,while1 then1 we have found no more large planets1 quite a group of companions to ,piazzi's little one have been discovered4 ,they are all small1 probably never exceeding about #djj miles in diameter4 ,all travel around the sun in orbits that lie wholly within that of ,jupiter and are exterior to that of ,mars4 ,the introduction of astronomical photography has given a tremendous impetus to the discovery of these minor planets1 as they are called4 ,it is quite interesting to examine the photographic process by which such discoveries are made possible and even easy4 ,the matter will not be difficult to understand if we remember that all the planets are continually changing their places among the other stars4 ,for the planets travel around the sun at a comparatively small distance4 ,the great majority of the stars1 on the contrary1 are separated from the sun by an almost immeasurable space4 ,as a result1 they do not seem to move at all among themselves1 and so we call them fixed stars3 they may1 indeed1 be in motion1 but their great distance prevents our detecting it in a short period of time4 ,now1 stellar photographs are made in much the same way as ordinary portraits4 ,only1 instead of using a simple camera1 the astronomer exposes his photographic plate at the eye-end of a telescope4 ,the sensitive surface of the plate is substituted for the human eye4 ,we then find on the picture a little dot corresponding to every star within the photographed region of the sky4 ,but1 as everyone knows1 the turning of the earth on its axis makes the whole heavens1 including the sun1 moon1 and stars1 rise and set every day4 ,so the stars1 when we photograph them1 are sure to be either climbing up in the eastern sky or else slowly creeping down in the western4 ,and that makes astronomical photography very different from ordinary portrait work4 ,the stars correspond to the sitter1 but they don't sit still4 ,for this reason it is necessary to connect the telescope with a mechanical contrivance which makes it turn round like the hour-hand of an ordinary clock4 ,the arrangement is so adjusted that the telescope1 once aimed at the proper object in the sky1 will move so as to remain pointed exactly the same during the whole time of the photographic exposure4 ,thus1 while the light of any star is acting on the plate1 such action will be continuous at a single point4 ,consequently1 the finished picture will show the star as a little dot2 while without this arrangement1 the star would trail out into a line instead of a dot4 ,now we have seen that the planets are all moving slowly among the fixed stars4 ,so if we make a star photograph in a part of the sky where a planet happens to be1 the planet will make a short line on the plate2 whereas1 if the planet remained quite unmoved relatively to the stars it would give a dot like the star dots4 ,the presence of a line1 therefore1 at once indicates a planet4 ,this method of planet-hunting has proved most useful4 ,more than #djj small planets similar to ,piazzi's have been found1 though never another one like ,uranus and ,neptune4 ,as we have said1 all these little bodies lie between ,mars and ,jupiter4 ,they evidently belong to a group or family1 and many astronomers have been led to believe that they are but fragments of a former large planet4 ,in ,august1 #ahih1 however1 one was found by ,witt1 of ,berlin1 which will probably occupy a very prominent place in the annals of astronomy4 ,for this planet goes well within the orbit of ,mars1 and this will bring it at times very close to the earth4 ,in fact1 when the motions of the new planet and the earth combine to bring them to their positions of greatest proximity1 the new planet will approach us closer than any other celestial body except our own moon4 ,witt named his new planet ,eros4 ,its size1 though small1 may prove to be sufficient to bring it within the possibilities of naked-eye observation at the time of closest approach to the earth4 ,to astronomers the great importance of this new planet is due to the following circumstance3 ,for certain reasons too technical to be stated here in detail1 the distance from the earth to any planet can be determined with a degree of precision which is greatest for planets that are near us4 ,thus in time we shall learn the distance of ,eros more accurately than we know any other celestial distance4 ,from this1 by a process of calculation1 the solar distance from the earth is determinable4 ,but the distance from earth to sun is the fundamental astronomical unit of measure2 so that ,witt's discovery1 through its effect on the unit of measure1 will doubtless influence every part of the science of astronomy4 ,here we have once more a striking instance of the reward sure to overtake the diligent worker in science--a whole generation of men will doubtless pass away before we shall have exhausted the scientific advantages to be drawn from ,witt's remarkable observation of #ahih4 ,,how ,,to ,,make ,a ,,sun-,,dial,a ,long before clocks and watches had been invented1 people began to measure time with sun-dials4 ,nowadays1 when almost everyone has a watch in his pocket1 and can have a clock1 too1 on the mantel-piece of every room in the house1 the sun-dial has ceased to be needed in ordinary life4 ,but it is still just as interesting as ever to anyone who would like to have the means of getting time direct from the sun1 the great hour-hand or timekeeper of the sky4 ,any person who is handy with tools can make a sun-dial quite easily1 by following the directions given below4 ,in the first place1 you must know that the sun-dial gives the time by means of the sun's shadow4 ,if you stick a walking-cane up in the sand on a bright1 sunshiny day1 the cane has a long shadow that looks like a dark line on the ground4 ,now if you watch this shadow carefully1 you will see that it does not stay in the same place all day4 ,slowly but surely1 as the sun climbs up in the sky1 the shadow creeps around the cane4 ,you can see quite easily that if the cane were fastened in a board floor1 and if we could mark on the floor the places where the shadow was at different hours of the day1 we could make the shadow tell us the time just like the hour-hand of a clock4 ,a sun-dial is just such an arrangement as this1 and ,i will show you how to mark the shadow places exactly1 so as to tell the right time without any trouble whenever the sun shines4 ,if you were to watch very carefully such an arrangement as a cane standing in a board floor1 you would not find the creeping shadow in just the same place at the same time every day4 ,if you marked the place of the shadow at exactly ten o'clock by your watch some morning1 and then went back another day at ten1 you would not find the shadow on the old mark4 ,it would not get very far from it in a day or two1 but in a month or so it would be quite a distance away4 ,now1 of course1 a sun-dial would be of no use if it did not tell the time correctly every day2 and in fact1 it is not easy to make a dial when the shadow is cast by a stick standing straight up4 ,but we can get over this difficulty very well by letting the shadow be cast by a stick that leans over toward the floor just the right amount1 as ,i will explain in a moment4 ,of course1 we should not really use the floor for our sun-dial4 ,it is much better to mark out the hour-lines1 as they are called1 on a smooth piece of ordinary white board1 and then1 after the dial is finished1 it can be screwed down to a piazza floor or railing1 or it can be fastened on a window-sill4 ,it ought to be put in a place where the sun can get at it most of the time1 because1 of course1 you cannot use the sun-dial when the sun is not shining on it4 ,if the dial is set on a window-sill 7of a city house1 for instance7 you must choose a south window if you can1 so as to get the sun nearly all day4 ,if you have to take an east window1 you can use the dial in the morning only1 and in a west window only in the afternoon4 ,sometimes it is best not to try to fasten the dial to its support with screws1 but just to mark its place1 and then set it out whenever you want to use it4 ,for if the dial is made of wood1 and not painted1 it might be injured by rain or snow in bad weather if left out on a window-sill or piazza4 ,illustration3 ,fig4 #a4 ,it is not quite easy to fasten a little stick to a board so that it will lean over just right4 ,so it is better not to use a stick or a cane in the way ,i have described1 but instead to use a piece of board cut to just the right shape4 ,fig4 #a shows what a sun-dial should look like4 ,the lines to show the shadow's place at the different hours of the day will be marked on the board ,,abcd1 and this will be put flat on the window-sill or piazza floor4 ,the three-cornered piece of board abc is fastened to the bottom-board ,,abcd by screws going through ,,abcd from underneath4 ,the edge ab of the three-cornered board abc then takes the place of the leaning stick or cane1 and the time is marked by the shadow cast by the edge ab4 ,of course1 it is important that this edge should be straight and perfectly flat and even4 ,if you are handy with tools1 you can make it quite easily1 but if not1 you can mark the right shape on a piece of paper very carefully1 and take it to a carpenter1 who can cut the board according to the pattern you have marked on the paper4 ,illustration3 ,fig4 #b4 ,now ,i must tell you how to draw the shape of the three-cornered board abc4 ,fig4 #b shows how it is done4 ,the side ac should always be just five inches long4 ,the side bc is drawn at right angles to ac1 which you can do with an ordinary carpenter's square4 ,the length of bc depends on the place for which the dial is made4 ,the following table gives the length of bc for various places in the ,united ,states1 and1 after you have marked out the length of bc1 it is only necessary to complete the three-cornered piece by drawing the side ab from a to b4 ,,table ,,showing ,,the ,,length ,,of ,,the ,,side bc4 -------------------------- ,place4 b c ,inches4 -------------------------- ,albany #d-aa_/af ,baltimore #d-a_/af ,boston #d-a_/b ,buffalo #d-aa_/af ,charleston #c-a_/d ,chicago #d-a_/b ,cincinnati #d-a_/af ,cleveland #d-a_/b ,denver #d-c_/af ,detroit #d-a_/b ,indianapolis #d-a_/af ,kansas ,city #c-ae_/af ,louisville #c-ae_/af ,milwaukee #c-aa_/af ,new ,orleans #b-g_/h ,new ,york #d-c_/h ,omaha #d-c_/h ,philadelphia #d-c_/af ,pittsburg #d-c_/h ,portland1 ,me #d-ac_/af ,richmond #c-ae_/af ,rochester #d-aa_/af ,san ,diego #c-a_/d ,san ,francisco #c-ae_/af ,savannah #c-a_/h ,st4 ,louis #c-ae_/af ,st4 ,paul #e ,seattle #e-i_/af ,washington1 ,d4 ,c4 #d-a_/af -------------------------- ,if you wish to make a dial for a place not given in the table1 it will be near enough to use the distance bc as given for the place nearest to you4 ,but in selecting the nearest place from the table1 please remember to take that one of the cities mentioned which is nearest to you in a north-and-south direction4 ,it does not matter how far away the place is in an east-and-west direction4 ,so1 instead of taking the place that is nearest to you on the map in a straight line1 take the place to which you could travel by going principally east or west1 and very little north or south4 ,the figure drawn is about the right shape for ,new ,york4 ,the board used for the three-cornered piece should be about one-half inch thick4 ,but if you are making a window-sill dial1 you may prefer to have it smaller than ,i have described4 ,you can easily have it half as big by making all the sizes and lines in half-inches where the table calls for inches4 ,after you have marked out the dimensions for the three-cornered piece that is to throw the shadow1 you can prepare the dial itself1 with the lines that mark the place of the shadow for every hour of the day4 ,this you can do in the manner shown in ,fig4 #c4 ,just as in the case of the three-cornered piece1 you can draw the dial with a pencil directly on a smooth piece of white board1 about three-quarters of an inch thick1 or you can mark it out on a paper pattern and transfer it afterward to the board4 ,perhaps it will be as well to begin by drawing on paper1 as any mistakes can then be corrected before you commence to mark your wood4 ,illustration3 ,fig4 #c4 ,in the first place you must draw a couple of lines ,,mn and ,m,n1 eight inches long1 and just far enough apart to fit the edge of your three-cornered shadow-piece4 ,you will remember ,i told you to make that one-half inch thick1 so your two lines will also be one-half inch apart4 ,now draw the two lines ,,no and ,n,o square with ,,mn and ,m,n1 and make the distances ,,no and ,n,o just five inches each4 ,the lines ,,ok1 ,o,k1 and the other lines forming the outer border of the dial1 are then drawn just as shown1 ,,ok and ,o,k being just eight inches long1 the same as ,,mn and ,m,n4 ,the lower lines in the figure1 which are not very important1 are to complete the squares4 ,you must mark the lines ,,no and ,n,o with the figures ,,vi1 these being the lines reached by the shadow at six o'clock in the morning and evening4 ,the points where the ,,vii1 ,,viii1 and other hour-lines cut the lines ,,ok1 ,o,k1 ,,mk1 and ,m,k can be found from the table on page #gh4 ,in using the table you will notice that the line ,,ix falls sometimes on one side of the corner ,k1 and sometimes on the other4 ,thus for ,albany the line passes seven and seven-sixteenth inches from ,o1 while for ,charleston it passes four and three-eighth inches from ,m4 ,for ,baltimore it passes exactly through the corner ,k4 ,,table ,,showing ,,how ,,to ,,mark ,,the ,,hour-,,lines4 ---------------------------------------- ------------------------------ ,distance from ,o to the line ,distance from ,m to the marked line marked ,,place4 ---------------------------------------- --------- ,,vii4 ,,viii4 ,,ix4 ,,ix4 ,x4 ,,xi4 ---------------------------------------- -------------------------- ,inches4 ,inches4 ,inches4 ,inches4,inches4 ,inches4 ,albany #a-ae_/af #d-c_/af #g-g_/af #c-a_/af #a-g_/af ,baltimore #b-a_/h #d-aa_/af #h #b-g_/h #a-g_/af ,boston #b #d-e_/af #g-g_/af #c-a_/af #a-g_/af ,buffalo #a-ae_/af #d-c_/af #g-g_/af #c-a_/af #a-g_/af ,charleston #b-g_/af #e-c_/h #d-c_/h #b-a_/b #a-a_/h ,chicago #b #d-e_/af #g-g_/af #c-a_/af #a-g_/af ,cincinnati #b-a_/h #d-aa_/af #h #b-g_/h #a-g_/af ,cleveland #b #d-e_/af #g-g_/af -- #c-a_/af #a-g_/af ,denver #b-a_/h #d-a_/b #g-aa_/af #b-g_/h #a-g_/af ,detroit #b #d-e_/af #g-g_/af #c-a_/af #a-g_/af ,indianapolis #b-a_/h #d-aa_/af #h #b-g_/h #a-g_/af ,kansas ,city #b-a_/d #d-aa_/af #h #b-g_/h #a-e_/af ,louisville #b-a_/d #d-aa_/af #h #b-g_/h #a-e_/af ,milwaukee #a-ae_/af #d-c_/af #g-g_/af #c-a_/af #a-g_/af ,new ,orleans #b-aa_/af #e-c_/d #d-a_/af #b-e_/af #a-a_/h ,new ,york #b #d-e_/af #g-aa_/af #c-a_/af #a-g_/af ,omaha #b #d-e_/af #g-aa_/af #c-a_/af #a-g_/af ,philadelphia #b-a_/h #d-a_/b #g-aa_/af #b-g_/h #a-g_/af ,pittsburg #b #d-e_/af #g-aa_/af #c-a_/af #a-g_/af ,portland1 ,me #a-ae_/af #d-c_/af #g-a_/h #c-c_/af #a-a_/b ,richmond #b-a_/d #d-aa_/af #h #b-g_/h #a-e_/af ,rochester #a-ae_/af #d-c_/af #g-g_/af #c-a_/af #a-g_/af ,san ,diego #b-g_/af #e-c_/h #d-c_/h #b-a_/b #a-a_/h ,san ,francisco #b-a_/d #d-aa_/af #h #b-g_/h #a-e_/af ,savannah #b-i_/af #e-i_/af #d-a_/d #b-a_/b #a-a_/h ,st4 ,louis #b-a_/d #d-aa_/af #h #b-g_/h #a-e_/af ,st4 ,paul #a-ae_/af #d-a_/af #g-a_/h #c-c_/af #a-a_/b ,seattle #a-ac_/af #c-ae_/af #f-e_/h #c-c_/h #a-a_/b ,washington1 ,d4 ,c4 #b-a_/h #d-aa_/af #h #b-g_/h #a-g_/af ---------------------------------------- -------------------------- ,the distance for the line marked ,v from ,o is just the same as the distance from ,o to ,,vii4 ,similarly1 ,,iv corresponds to ,,viii1 ,,iii to ,,ix1 ,,ii to ,x1 and ,i to ,,xi4 ,the number ,,xii is marked at ,,mm as shown4 ,if you desire to add lines 7not shown in ,fig4 #c to avoid confusion7 for hours earlier than six in the morning1 it is merely necessary to mark off a distance on the line ,,ko1 below the point ,o1 and equal to the distance from ,o to ,,vii4 ,this will give the point where the #e ,a4,m4 shadow line drawn from ,n cuts the line ,,ko4 ,a corresponding line for #g ,p4,m4 can be drawn from ,n on the other side of the figure4 ,after you have marked out the dial very carefully1 you must fasten the three-cornered shadow-piece to it in such a way that the whole instrument will look like ,fig4 #a4 ,the edge ac 7,fig4 #b7 goes on ,,nm 7,fig4 #c74 ,the point a 7,fig4 #b7 must come exactly on ,n 7,fig4 #c72 and as the lines ,,nm 7,fig4 #c7 and ,n,m 7,fig4 #c7 have been made just the right distance apart to fit the thickness of the three-cornered piece abc 7,fig4 #b71 everything will go together just right4 ,the point c 7,fig4 #b7 will not quite reach to ,m 7,fig4 #c71 but will be on the line ,,nm 7,fig4 #c7 at a distance of three inches from ,m4 ,the two pieces of wood will be fastened together with three screws going through the bottom-board ,,abcd 7,figs4 #a and #c7 and into the edge ac 7,fig4 #b7 of the three-cornered piece4 ,the whole instrument will then look something like ,fig4 #a4 ,after you have got your sun-dial put together1 you need only set it in the sun in a level place1 on a piazza or window-sill1 and turn it round until it tells the right time by the shadow4 ,you can get your local time from a watch near enough for setting up the dial4 ,once the dial is set right you can screw it down or mark its position1 and it will continue to give correct solar time every day in the year4 ,if you wish to adjust the dial very closely1 you must go out some fine day and note the error of the dial by a watch at about ten in the morning1 and at noon1 and again at about two in the afternoon4 ,if the error is the same each time1 the dial is rightly set4 ,if not1 you must try1 by turning the dial slightly1 to get it so placed that your three errors will be nearly the same4 ,when you have got them as nearly alike as you can1 the dial will be sufficiently near right4 ,the solar or dial time may1 however1 differ somewhat from ordinary watch time1 but the difference will never be great enough to matter1 when we remember that sun-dials are only rough timekeepers after all1 and useful principally for amusement4 ,,footnote3 ,a ,this chapter is especially intended for boys and girls and others who like to make things with carpenters' tools4 ,,photography ,,in ,,astronomy ,new highways of science have been monumented now and again by the masterful efforts of genius1 working single-handed2 but more often it is slow-moving time that ripens discovery1 and1 at the proper moment1 opens some new path to men whose intellectual power is but willingness to learn4 ,so the annals of astronomical photography do not recount the achievements of extraordinary genius4 ,it would have been strange1 indeed1 if the discovery of photography had not been followed by its application to astronomy4 ,the whole range of chemical science contains no experiment of greater inherent interest than the development of a photographic plate4 ,let but the smallest ray of light fall upon its strangely sensitive surface1 and some subtle invisible change takes place4 ,it is then merely necessary to plunge the plate into a properly prepared chemical bath1 and the gradual process of developing the picture begins4 ,slowly1 very slowly1 the colorless surface darkens wherever light has touched it4 ,let us imagine that the exposure has been made with an ordinary lens and camera1 and that it is a landscape seeming to grow beneath the experimenter's eyes4 ,at first only the most conspicuous objects make their appearance4 ,but gradually the process extends1 until finally every tiny detail is reproduced with marvellous fidelity to the original4 ,the photographic plate1 when developed in this way1 is called a negative4 ,for in ,nature luminous points1 or sources of light1 are bright1 while the developing negative turns dark wherever light has acted4 ,thus the negative1 while true to ,nature1 reproduces everything in a reversed way2 bright things are dark1 and shadows appear light4 ,for ordinary purposes1 therefore1 the negative has to be replaced by a new photograph made by copying it again photographically4 ,in this way it is again reversed1 giving us a picture corresponding correctly to the facts as seen4 ,such a copy from a negative is what is ordinarily called a photograph2 technically1 it is known as a positive4 ,one of the remarkable things about the sensitive plate is its complete indifference to the distance from which the light comes4 ,it is ready to yield obediently to the ray of some distant star that may have journeyed1 as it were1 from the very vanishing point of space1 or to the bright glow of an electric light upon the photographer's table4 ,this quality makes its use especially advantageous in astronomy1 since we can gain knowledge of remote stars only by a study of the light they send us4 ,in such study the photographic plate possesses a supreme advantage over the human eye4 ,if the conditions of weather and atmosphere are favorable1 an observer looking through an ordinary telescope will see nearly as much at the first glance as he will ever see4 ,attentive and continued study will enable him to fix details upon his memory1 and to record them by means of drawings and diagrams4 ,occasional moments of especially undisturbed atmospheric conditions will allow him to glimpse faint objects seldom visible4 ,but on the whole1 telescopic astronomers add little to their harvest by continued husbandry in the same field of stars4 ,photography is different4 ,the effect of light upon the sensitive surface of the plate is strictly cumulative4 ,if a given star can bring about a certain result when it has been allowed to act upon the plate for one minute1 then in two or three minutes it will accomplish much more4 ,perhaps a single minute's exposure would have produced a mark scarcely perceptible upon the developed negative4 ,in that case1 three or four minutes would give us a perfectly well defined black image of the star4 ,illustration3 ,star-,field in ,constellation ,monoceros4 ,photographed by ,barnard1 ,february #a1 #ahid4 ,exposure1 three hours4 ,thus1 by lengthening the exposure we can make the fainter stars impress themselves upon the plate4 ,if their light is not able to produce the desired effect in minutes1 we can let its action accumulate for hours4 ,in this manner it becomes possible and easy to photograph objects so faint that they have never been seen1 even with our most powerful telescopes4 ,this achievement ranks high among those which make astronomy appeal so strongly to the imagination4 ,scientific men are not given to fancies2 nor should they be4 ,but the first long-exposure photograph must have been an exciting thing4 ,after coming from the observatory1 the chemical development was1 of course1 made in a dark room1 so that no additional light might harm the plate until the process was complete4 ,carrying it out then into the light1 that early experimenter cannot but have felt a thrill of triumph2 for his hand held a true picture of dim stars to the eye unlighted1 lifted into view as if by magic4 ,plates have been thus exposed as long as twenty-five hours1 and the manner of doing it is very interesting4 ,of course1 it is impossible to carry on the work continuously for so long a period1 since the beginning of daylight would surely ruin the photograph4 ,in fact1 the astronomer must stop before even the faintest streak of dawn begins to redden the eastern sky4 ,moreover1 making astronomical negatives requires excessively close attention1 and this it is impossible to give continuously during more than a few hours4 ,but the exposure of a single plate can be extended over several nights without difficulty4 ,it is merely necessary to close the plate-holder with a light-tight cover when the first night's work is finished4 ,to begin further exposure of the same plate on another night1 we simply aim the photographic telescope at precisely the same point of the sky as before4 ,the light-tight plate-holder being again opened1 the exposure can go on as if there had been no interruption4 ,astronomers have invented a most ingenious device for making sure that the telescope's aim can be brought back again to the same point with great exactness4 ,this is a very important matter2 for the slightest disturbance of the plate before the second or subsequent portions of the exposure would ruin everything4 ,instead of a very complete single picture1 we should have two partial ones mixed up together in inextricable confusion4 ,to prevent this1 photographic telescopes are made double1 not altogether unlike an opera-glass4 ,one of the tubes is arranged for photography proper1 while the other is fitted with lenses suitable for an ordinary visual telescope4 ,the two tubes are made parallel4 ,thus the astronomer1 by looking through the visual glass1 can watch objects in the heavens even while they are being photographed4 ,the visual half of the instrument is provided with a pair of very fine cross-wires movable at will in the field of view4 ,these can be made to bisect some little star exactly1 before beginning the first night's work4 ,afterward1 everything about the instrument having been left unchanged1 the astronomer can always assure himself of coming back to precisely the same point of the sky1 by so adjusting the instrument that the same little star is again bisected4 ,it must not be supposed1 however1 that the entire instrument remains unmoved1 even during the whole of a single night's exposure4 ,for in that case1 the apparent motion of the stars as they rise or set in the sky would speedily carry them out of the telescope's field of view4 ,consequently1 this motion has to be counteracted by shifting the telescope so as to follow the stars4 ,this can be accomplished accurately and automatically by means of clock-work mechanism4 ,such contrivances have already been applied in the past to visual telescopes1 because even then they facilitated the observer's work4 ,they save him the trouble of turning his instrument every few minutes1 and allow him to give his undivided attention to the actual business of observation4 ,for photographic purposes the telescope needs to follow the stars far more accurately than in the older kind of observing with the eye4 ,nor is it possible to make a clock that will drive the instrument satisfactorily and quite automatically4 ,but by means of the second or visual telescope1 astronomers can always ascertain whether the clock is working correctly at any given moment4 ,it requires only a glance at the little star bisected by the cross-wires1 and1 if there has been the slightest imperfection in the following by clock-work1 the star will no longer be cut exactly by the wires4 ,the astronomer can at once correct any error by putting in operation a very ingenious mechanical device sometimes called a mouse-control4 ,he need only touch an electric button1 and a signal is sent into the clock-work4 ,instantly there is a shifting of the mechanism4 ,for one of the regular driving wheels is substituted1 temporarily1 another having an extra tooth4 ,this makes the clock run a little faster so long as the electric current passes4 ,in a similar way1 by means of another button1 the clock can be made to run slower temporarily4 ,thus by watching the cross-wires continuously1 and manipulating his two electric buttons1 the photographic astronomer can compel his telescope to follow exactly the object under observation1 and he can make certain of obtaining a perfect negative4 ,these long-exposure plates are intended especially for what may be called descriptive astronomy4 ,with them1 as we have seen1 advantage is taken of cumulative light-effects on the sensitive plate1 and the telescope's light-gathering and space-penetrating powers are vastly increased4 ,we are enabled to carry our researches far beyond the confines of the old visible universe4 ,extremely faint objects can be recorded1 even down to their minutest details1 with a fidelity unknown to older visual methods4 ,but at present we intend to consider principally applications of photography in the astronomy of measurement1 rather than the descriptive branch of our subject4 ,instead of describing pictures made simply to see what certain objects look like in the sky1 we shall consider negatives intended for precise measurement1 with all that the word precision implies in celestial science4 ,taking up first the photography of stars1 we must begin by mentioning the work of ,rutherfurd at ,new ,york4 ,more than thirty years ago he had so far perfected methods of stellar photography that he was able to secure excellent pictures of stars as faint as the ninth magnitude4 ,in those days the modern process of dry-plate photography had not been invented4 ,to-day1 plates exposed in the photographic telescope are made of glass covered with a perfectly dry film of sensitized gelatine4 ,but in the old wet-plate process the sensitive film was first wetted with a chemical solution2 and this solution could not be allowed to dry during the exposure4 ,consequently1 ,rutherfurd was limited to exposures a few minutes in length1 while nowadays1 as we have said1 their duration can be prolonged at will4 ,when we add to this the fact that the old plates were far less sensitive to light than those now available1 it is easy to see what were the difficulties in the way of photographing faint stars in ,rutherfurd's time4 ,nor did he possess the modern ingenious device of a combined visual and photographic instrument4 ,he had no electric controlling apparatus4 ,in fact1 the younger generation of astronomers can form no adequate idea of the patience and personal skill ,rutherfurd must have had at his command4 ,for he certainly did produce negatives that are but little inferior to the best that can be made to-day4 ,his only limitation was that he could not obtain images of stars much below the ninth magnitude4 ,to understand just what is meant here by the ninth magnitude1 it is necessary to go back in imagination to the time of ,hipparchus1 the father of sidereal astronomy4 7,see page #ci47 ,he adopted the convenient plan of dividing all the stars visible to the naked eye 7of course1 he had no telescope7 into six classes1 according to their brilliancy4 ,the faintest visible stars were put in the sixth class1 and all the others were assigned somewhat arbitrarily to one or the other of the brighter classes4 ,modern astronomers have devised a more scientific system1 which has been made to conform very nearly to that of ,hipparchus1 just as it has come down to us through the ages4 ,we have adopted a certain arbitrary degree of luminosity as the standard first-magnitude2 compared with sunlight1 this may be represented roughly by a fraction of which the numerator is #a1 and the denominator about eighty thousand millions4 ,the standard second-magnitude star is one whose light1 compared with a first-magnitude1 may be represented approximately by the fraction 4 ,the third magnitude1 in turn1 may be compared with the second by the same fraction 2 and so the classification is extended to magnitudes below those visible to the unaided eye4 ,each magnitude compares with the one above it1 as the light of two candles would compare with the light of five4 ,rutherfurd did not stop with mere photographs4 ,he realized very clearly the obvious truth that by making a picture of the sky we simply change the scene of our operations4 ,upon the photograph we can measure that which we might have studied directly in the heavens2 but so long as they remain unmeasured1 celestial pictures have a potential value only4 ,locked within them may lie hidden some secret of our universe4 ,but it will not come forth unsought4 ,patient effort must precede discovery1 in photography1 as elsewhere in science4 ,there is no royal road4 ,rutherfurd devised an elaborate measuring-machine in which his photographs could be examined under the microscope with the most minute exactness4 ,with this machine he measured a large number of his pictures2 and it has been shown quite recently that the results obtained from them are comparable in accuracy with those coming from the most highly accredited methods of direct eye-observation4 ,and photographs are far superior in ease of manipulation4 ,convenient day-observing under the microscope in a comfortable astronomical laboratory is substituted for all the discomforts of a midnight vigil under the stars4 ,the work of measurement can proceed in all weathers1 whereas formerly it was limited strictly to perfectly clear nights4 ,lastly1 the negatives form a permanent record1 to which we can always return to correct errors or re-examine doubtful points4 ,rutherfurd's stellar work extended down to about #ahgg1 and included especially parallax determinations and the photography of star-clusters4 ,each of these subjects is receiving close attention from later investigators1 and1 therefore1 merits brief mention here4 ,stellar parallax is in one sense but another name for stellar distance4 ,its measurement has been one of the important problems of astronomy for centuries1 ever since men recognized that the ,copernican theory of our universe requires the determination of stellar distances for its complete demonstration4 ,if the earth is swinging around the sun once a year in a mighty path or orbit1 there must be changes of its position in space comparable in size with the orbit itself4 ,and the stars ought to shift their apparent places on the sky to correspond with these changes in the terrestrial observer's position4 ,the phenomenon is analogous to what occurs when we look out of a room1 first through one window1 and then through another4 ,any object on the opposite side of the street will be seen in a changed direction1 on account of the observer's having shifted his position from one window to the other4 ,if the object seemed to be due north when seen from the first window1 it will1 perhaps1 appear a little east of north from the other4 ,but this change of direction will be comparatively small1 if the object under observation is very far away1 in comparison with the distance between the two windows4 ,this is what occurs with the stars4 ,the earth's orbit1 vast as it is1 shrinks into almost absolute insignificance when compared with the profound distances by which we are sundered from even the nearest fixed stars4 ,consequently1 the shifting of their positions is also very small--so small as to be near the extreme limit separating that which is measurable from that which is beyond human ken4 ,photography lends itself most readily to a study of this matter4 ,suppose a certain star is suspected of having a parallax4 ,in other words1 we have reason to believe it near enough to admit of a successful measurement of distance4 ,perhaps it is a very bright star2 and1 other things being equal1 it is probably fair to assume that brightness signifies nearness4 ,and astronomers have certain other indications of proximity that guide them in the selection of proper objects for investigation1 though such evidence1 of course1 never takes the place of actual measurement4 ,the star under examination is sure to have near it on the sky a number of stars so very small that we may safely take them to be immeasurably far away4 ,the parallax star is among them1 but not of them4 ,we see it projected upon the background of the heavens1 though it may in reality be quite near us1 astronomically speaking4 ,if this is really so1 and the star1 therefore1 subject to the slight parallactic shifting already mentioned1 we can detect it by noting the suspected star's position among the surrounding small stars4 ,for these1 being immeasurably remote1 will remain unchanged1 within the limits of our powers of observation1 and thus serve as points of reference for marking the apparent shifting of the brighter star we are actually considering4 ,we have merely to photograph the region at various seasons of the year4 ,careful examination of the photographs under the microscope will then enable us to measure the slightest displacement of the parallax star4 ,from these measures1 by a process of calculation1 astronomers can then obtain the star's distance4 ,it will not become known in miles2 we shall only ascertain how many times the distance between the earth and sun would have to be laid down like a measuring-rod1 in order to cover the space separating us from the star3 and the subsequent evaluation of this distance earth to sun in miles is another important problem in whose solution photography promises to be most useful4 ,the above method of measuring stellar distance is1 of course1 subject to whatever slight uncertainty arises from the assumption that the small stars used for comparison are themselves beyond the possibility of parallactic shifting4 ,but astronomy possesses no better method4 ,moreover1 the number of small stars used in this way is1 of course1 much larger in photography than it ever can be in visual work4 ,in the former process1 all surrounding stars can be photographed at once2 in the latter each star must be measured separately1 and daylight soon intervenes to impose a limit on numbers4 ,usually only two can be used2 so that here photography has a most important advantage4 ,it minimizes the chance of our parallax being rendered erroneous1 by the stars of comparison not being really infinitely remote4 ,this might happen1 perhaps1 in the case of one or two2 but with an average result from a large number we know it to be practically impossible4 ,cluster work is not altogether unlike parallax hunting in its preliminary stage of securing the photographic observations4 ,the object is to obtain an absolutely faithful picture of a star group1 just as it exists in the sky4 ,we have every reason to suppose that a very large number of stars condensed into one small spot upon the heavens means something more than chance aggregation4 ,the ,pleiades group 7page #aj7 contains thousands of massive stars1 doubtless held together by the force of their mutual gravitational attraction4 ,if this be true1 there must be complex orbital motion in the cluster2 and1 as time goes on1 we should actually see the separate components change their relative positions1 as it were1 before our eyes4 ,the details of such motion upon the great scale of cosmic space offer one of the many problems that make astronomy the grandest of human sciences4 ,we have said that time must pass before we can see these things2 there may be centuries of waiting4 ,but one way exists to hurry on the perfection of our knowledge2 we must increase the precision of observations4 ,motions that would need the growth of centuries to become visible to the older astronomical appliances1 might yield in a few decades to more delicate observational processes4 ,here photography is most promising4 ,having once obtained a surpassingly accurate picture of a star-cluster1 we can subject it easily to precise microscopic measurement4 ,the same operations repeated at a later date will enable us to compare the two series of measures1 and thus ascertain the motions that may have occurred in the interval4 ,the ,rutherfurd photographs furnish a veritable mine of information in researches of this kind2 for they antedate all other celestial photographs of precision by at least a quarter-century1 and bring just so much nearer the time when definite knowledge shall replace information based on reasoning from probabilities4 ,rutherfurd's methods showed the advantages of photography as applied to individual star-clusters4 ,it required only the attention of some astronomer disposing of large observational facilities1 and accustomed to operations upon a great scale1 to apply similar methods throughout the whole heavens4 ,in the year #ahhb a bright comet was very conspicuous in the southern heavens4 ,it was extensively observed from the southern hemisphere1 and especially at the ,british ,royal ,observatory at the ,cape of ,good ,hope4 ,gill1 director of that institution1 conceived the idea that this comet might be bright enough to photograph4 ,at that time1 comet photography had been attempted but little1 if at all1 and it was by no means sure that the experiment would be successful4 ,nor was ,gill well acquainted with the work of ,rutherfurd2 for the best results of that astronomer had lain dormant many years4 ,he was one of those men with whom personal modesty amounts to a fault4 ,loath to put himself forward in any way1 and disliking to rush into print1 ,rutherfurd had given but little publicity to his work4 ,this peculiarity has1 doubtless1 delayed his just reputation2 but he will lose nothing in the end from a brief postponement4 ,gill must1 however1 be credited with more penetration than would be his due if ,rutherfurd had made it possible for others to know that he had anticipated many of the newer ideas4 ,however this may be1 the comet was photographed with the help of a local portrait photographer named ,allis4 ,when ,gill and ,allis fastened a simple portrait camera belonging to the latter upon the tube of one of the ,cape telescopes1 and pointed it at the great comet1 they little thought the experiment would lead to one of the greatest astronomical works ever attempted by men4 ,yet this was destined to occur4 ,the negative they obtained showed an excellent picture of the comet2 but what was more important for the future of sidereal astronomy1 it was also quite thickly dotted with little black points corresponding to stars4 ,the extraordinary ease with which the whole heavens could be thus charted photographically was brought home to ,gill as never before4 ,it was this comet picture that interested him in the application of photography to star-charting2 and without his interest the now famous astro-photographic catalogue of the heavens would probably never have been made4 ,after considerable preliminary correspondence1 a congress of astronomers was finally called to meet at ,paris in #ahhg4 ,representatives of the principal observatories and civilized governments were present4 ,they decided that the end of the nineteenth century should see the making of a great catalogue of all the stars in the sky1 upon a scale of completeness and precision surpassing anything previously attempted4 ,it is impossible to exaggerate the importance of such a work2 for upon our star-catalogues depends ultimately the entire structure of astronomical science4 ,the work was far too vast for the powers of any observatory alone4 ,therefore1 the whole sky1 from pole to pole1 was divided into eighteen belts or zones of approximately equal area2 and each of these was assigned to a single observatory to be photographed4 ,a series of telescopes was specially constructed1 so that every part of the work should be done with the same type of instrument4 ,as far as possible1 an attempt was made to secure uniformity of methods1 and particularly a uniform scale of precision4 ,to cover the entire sky upon the plan proposed no less than #dd1ajh negatives are required2 and most of these have now been finished4 ,the further measurement of the pictures and the drawing up of a vast printed star-catalogue are also well under way4 ,one of the participating observatories1 that at ,potsdam1 ,germany1 has published the first volume of its part of the catalogue4 ,it is estimated that this observatory alone will require twenty quarto volumes to contain merely the final results of its work on the catalogue4 ,altogether not less than two million stars will find a place in this1 our latest directory of the heavens4 ,such wholesale methods of attacking problems of observational astronomy are particularly characteristic of photography4 ,the great catalogue is1 perhaps1 the best illustration of this tendency2 but of scarcely smaller interest1 though less important in reality1 is the photographic method of dealing with minor planets4 ,we have already said 7page #fc7 that in the space between the orbits of ,mars and ,jupiter several hundred small bodies are moving around the sun in ordinary planetary orbits4 ,these bodies are called asteroids1 or minor planets4 ,the visual method of discovering unknown members of this group was painfully tedious2 but photography has changed matters completely1 and has added immensely to our knowledge of the asteroids4 ,wolf1 of ,heidelberg1 first made use of the new process for minor-planet discovery4 ,his method is sufficiently ingenious to deserve brief mention again4 ,a photograph of a suitable region of the sky was made with an exposure lasting two or three hours4 ,throughout all this time the instrument was manipulated so as to follow the motion of the heavens in the way we have already explained1 so that each star would appear on the negative as a small1 round1 black dot4 ,but if a minor planet happened to be in the region covered by the plate1 its photographic image would be very different4 ,for the orbital motion of the planet about the sun would make it move a little among the stars even in the two or three hours during which the plate was exposed4 ,this motion would be faithfully reproduced in the picture1 so that the planet would appear as a short curved line rather than a well-defined dot like a star4 ,thus the presence of such a line-image infallibly denotes an asteroid4 ,subsequent calculations are necessary to ascertain whether the object is a planet already known or a genuine new discovery4 ,wolf1 and others using his method in recent years1 have made immense additions to our catalogue of asteroids4 ,indeed1 the matter was beginning to lose interest on account of the frequency and sameness of these discoveries1 when the astronomical world was startled by the finding of the ,planet of #ahih4 7,page #eh47 ,on ,august #bg1 #ahih1 ,witt1 of ,berlin1 discovered the small body that bears the number #dcc in the list of minor planets1 and has received the name ,eros4 ,its important peculiarity consists in the exceptional position of the orbit4 ,while all the other asteroids are farther from the sun than ,mars1 and less distant than ,jupiter1 ,eros can pass within the orbit of the former4 ,at times1 therefore1 it will approach our earth more closely than any other permanent member of the solar system1 excepting our own moon4 ,so it is1 in a sense1 our nearest neighbor2 and this fact alone makes it the most interesting of all the minor planets4 ,the nineteenth century was opened by ,piazzi's well-known discovery of the first of these bodies 7page #ei72 it is1 therefore1 fitting that we should find the most important one at its close4 ,we are almost certain that it will be possible to make use of ,eros to solve with unprecedented accuracy the most important problem in all astronomy4 ,this is the determination of our earth's distance from the sun4 ,when considering stellar parallax1 we have seen how our observations enable us to measure some of the stars' distances in terms of the distance earth to sun as a unit4 ,it is1 indeed1 the fundamental unit for all astronomical measures1 and its exact evaluation has always been considered the basal problem of astronomy4 ,astronomers know it as the problem of ,solar ,parallax4 ,we shall not here enter into the somewhat intricate details of this subject1 however interesting they may be4 ,the problem offers difficulties somewhat analogous to those confronting a surveyor who has to determine the distance of some inaccessible terrestrial point4 ,to do this1 it is necessary first to measure a base-line1 as we call it4 ,then the measurement of angles with a theodolite will make it possible to deduce the required distance of the inaccessible point by a process of calculation4 ,to insure accuracy1 however1 as every surveyor knows1 the base-line must be made long enough2 and this is precisely what is impossible in the case of the solar parallax4 ,for we are necessarily limited to marking out our base-line on the earth2 and the entire planet is too small to furnish one of really sufficient size4 ,the best we can do is to use the distance between two observatories situated1 as near as may be1 on opposite sides of the earth4 ,but even this base is wofully small4 ,however1 the smallness loses some of its harmful effect if we operate upon a planet that is comparatively near us4 ,we can measure such a planet's distance more accurately than any other2 and this being known1 the solar distance can be computed by the aid of mathematical considerations based upon ,newton's law of gravitation and observational determinations of the planetary orbital elements4 ,photography is by no means limited to investigations in the older departments of astronomical observation4 ,its powerful arm has been stretched out to grasp as well the newer instruments of spectroscopic study4 ,here the sensitive plate has been substituted for the human eye with even greater relative advantage4 ,the accurate microscopic measurement of difficult lines in stellar spectra was indeed possible by older methods2 but photography has made it comparatively easy2 and1 above all1 has rendered practicable series of observations extensive enough in numbers to furnish statistical information of real value4 ,only in this way have we been able to determine whether the stars1 in their varied and unknown orbits1 are approaching us or moving farther away4 ,even the speed of this approach or recession has become measurable1 and has been evaluated in the case of many individual stars4 7,see page #ba47 ,illustration3 ,solar ,corona4 ,total ,eclipse4 ,photographed by ,campbell1 ,january #bb1 #ahih2 ,jeur1 ,india4 ,the subject of solar physics has become a veritable department of astronomy in the hands of photographic investigators4 ,ingenious spectro-photographic methods have been devised1 whereby we have secured pictures of the sun from which we have learned much that must have remained forever unknown to older methods4 ,especially useful has photography proved itself in the observation of total solar eclipses4 ,it is only when the sun's bright disk is completely obscured by the interposed moon that we can see the faintly luminous structure of the solar corona1 that great appendage of our sun1 whose exact nature is still unexplained4 ,only during the few minutes of total eclipse in each century can we look upon it2 and keen is the interest of astronomers when those few minutes occur4 ,but it is found that eye observations made in hurried excitement have comparatively little value4 ,half a dozen persons might make drawings of the corona during the same eclipse1 yet they would differ so much from one another as to leave the true outline very much in doubt4 ,but with photography we can obtain a really correct picture whose details can be studied and discussed subsequently at leisure4 ,if we were asked to sum up in one word what photography has accomplished1 we should say that observational astronomy has been revolutionized4 ,there is to-day scarcely an instrument of precision in which the sensitive plate has not been substituted for the human eye2 scarcely an inquiry possible to the older method which cannot now be undertaken upon a grander scale4 ,novel investigations formerly not even possible are now entirely practicable by photography2 and the end is not yet4 ,valuable as are the achievements already consummated1 photography is richest in its promise for the future4 ,astronomy has been called the perfect science2 it is safe to predict that the next generation will wonder that the knowledge we have to-day should ever have received so proud a title4 ,,time ,,standards ,,of ,,the ,,world ,the question is often asked1 ,what is the practical use of astronomy8 ,we know1 of course1 that men would profit greatly from a study of that science1 even if it could not be turned to any immediate bread-and-butter use2 for astronomy is essentially the science of big things1 and it makes men bigger to fix their minds on problems that deal with vast distances and seemingly endless periods of time4 ,no one can look upon the quietly shining stars without being impressed by the thought of how they burned--then as now--before he himself was born1 and so shall continue after he has passed away--aye1 even after his latest descendants shall have vanished from the earth4 ,of all the sciences1 astronomy is at once the most beautiful poetically1 and yet the one offering the grandest and most difficult problems to the intellect4 ,a study of these problems has ever been a labor of love to the greatest minds2 their solution has been counted justly among man's loftiest achievements4 ,and yet of all the difficult and abstruse sciences1 astronomy is1 perhaps1 the one that comes into the ordinary practical daily life of the people more definitely and frequently than any other4 ,there exist at least three things we owe to astronomy that must be regarded as quite indispensable1 from a purely practical point of view4 ,in the first place1 let us consider the maps in a work on geography4 ,how many people ever think to ask how these maps are made8 ,it is true that the ordinary processes of the surveyor would enable us to draw a map showing the outlines of a part of the earth's surface4 ,even the locations of towns and rivers might be marked in this way4 ,but one of the most important things of all could not be added without the aid of astronomical observations4 ,the latitude and longitude lines1 which are essential to show the relation of the map to the rest of the earth1 we owe to astronomy4 ,the longitude lines1 particularly1 as we shall see farther on1 play a most important part in the subject of time4 ,the second indispensable application of astronomy to ordinary business affairs relates to the subject of navigation4 ,how do ships find their way across the ocean8 ,there are no permanent marks on the sea1 as there are on the land1 by which the navigator can guide his course4 ,nevertheless1 seamen know their path over the trackless ocean with a certainty as unerring as would be possible on shore2 and it is all done by the help of astronomy4 ,the navigator's observations of the sun are astronomical observations2 the tables he uses in calculating his observations--the tables that tell him just where he is and in what direction he must go--are astronomical tables4 ,indeed1 it is not too much to say that without astronomy there could be no safe ocean navigation4 ,but the third application of astronomy is of still greater importance in our daily life--the furnishing of correct time standards for all sorts of purposes4 ,it is to this practical use of astronomical science that we would direct particular attention4 ,few persons ever think of the complicated machinery that must be put in motion in order to set a clock4 ,a man forgets some evening to wind his watch at the accustomed hour4 ,the next morning he finds it run down4 ,it must be re-set4 ,most people simply go to the nearest clock1 or ask some friend for the time1 so as to start the watch correctly4 ,more careful persons1 perhaps1 visit the jeweller's and take the time from his regulator4 ,but the regulator itself needs to be regulated4 ,after all1 it is nothing more than any other clock1 except that greater care has been taken in the mechanical construction and arrangement of its various parts4 ,yet it is but a machine built by human hands1 and1 like all human works1 it is necessarily imperfect4 ,no matter how well it has been constructed1 it will not run with perfectly rigid accuracy4 ,every day there will be a variation from the true time by a small amount1 and in the course of days or weeks the accumulation of these successive small amounts will lead to a total of quite appreciable size4 ,just as the ordinary citizen looks to the jeweller's regulator to correct his watch1 so the jeweller applies to the astronomer for the correction of his regulator4 ,ever since the dawn of astronomy1 in the earliest ages of which we have any record1 the principal duty of the astronomer has been the furnishing of accurate time to the people4 ,we shall not here enter into a detailed account1 however interesting it would be1 of the gradual development by which the very perfect system at present in use has been reached2 but shall content ourselves with a description of the methods now employed in nearly all the civilized countries of the world4 ,in the first place1 every observatory is1 of course1 provided with what is known as an astronomical clock4 ,this instrument1 from the astronomer's point of view1 is something very different from the ordinary popular idea4 ,to the average person an astronomical clock is a complicated and elaborate affair1 giving the date1 day of the week1 phases of the moon1 and other miscellaneous information4 ,but in reality the astronomer wants none of these things4 ,his one and only requirement is that the clock shall keep as near uniform time as may be possible to a machine constructed by human hands4 ,no expense is spared in making the standard clock for an observatory4 ,real artists in mechanical construction--men who have attained a world-wide celebrity for delicate skill in fashioning the parts of a clock--such are the astronomer's clock-makers4 ,to increase precision of motion in the train of wheels1 it is necessary that the mechanism be as simple as possible4 ,for this reason all complications of date1 etc41 are left out4 ,we have even abandoned the usual convenient plan of having the hour and minute hands mounted at the same centre2 for this kind of mounting makes necessary a slightly more intricate form of wheelwork4 ,the astronomer's clock usually has the centres of the second hand1 minute hand1 and hour hand in a straight line1 and equally distant from each other4 ,each hand has its own dial2 all drawn1 of course1 upon the same clock-face4 ,even after such a clock has been made as accurately as possible1 it will1 nevertheless1 not give the very best performance unless it is taken care of properly4 ,it is necessary to mount it very firmly indeed4 ,it should not be fastened to an ordinary wall1 but a strong pier of masonry or brick must be built for it on a very solid foundation4 ,moreover1 this pier is best placed underground in a cellar1 so that the temperature of the clock can be kept nearly uniform all the year round2 for we find that clocks do not run quite the same in hot weather as they do in cold4 ,makers have1 indeed1 tried to guard against this effect of temperature1 by ingenious mechanical contrivances4 ,but these are never quite perfect in their action1 and it is best not to test them too severely by exposing the clock to sharp changes of heat and cold4 ,another thing affecting the going of fine clocks1 strange as it may seem1 is the variation of barometric pressure4 ,there is a slight but noticeable difference in their running when the barometer is high and when it is low4 ,to prevent this1 some of our best clocks have been enclosed in air-tight cases1 so that outside barometric changes may not be felt in the least by the clock itself4 ,but even after all this has been accomplished1 and the astronomer is in possession of a clock that may be called a masterpiece of mechanical construction1 he is not any better off than was the jeweller with his regulator4 ,after all1 even the astronomical clock needs to be set1 and its error must be determined from time to time4 ,a final appeal must then be had to astronomical observations4 ,the clock must be set by the stars and sun4 ,for this purpose the astronomer uses an instrument called a transit4 ,this is simply a telescope of moderate size1 possibly five or six feet long1 and firmly attached to an axis at right angles to the tube of the telescope4 ,this axis is supported horizontally in such a way that it points as nearly as may be exactly east and west4 ,the telescope itself being square with the axis1 always points in a north-and-south direction4 ,it is possible to rotate the telescope about its axis so as to reach all parts of the sky that are directly north or south of the observatory4 ,in the field of view of the telescope certain very fine threads are mounted so as to form a little cross4 ,as the telescope is rotated this cross traces out1 as it were1 a great circle on the sky2 and this great circle is called the astronomical meridian4 ,now we are in possession of certain star-tables1 computed from the combined observations of astronomers in the last #aej years4 ,these tables tell us the exact moment of time when any star is on the meridian4 ,to discover1 therefore1 whether our clock is right on any given night1 it is merely necessary to watch a star with the telescope1 and note the exact instant by the clock when it reaches the little cross in the field of view4 ,knowing from the astronomical tables the time when the star ought to have been on the meridian1 and having observed the clock time when it is actually there1 the difference is1 of course1 the error of the clock4 ,the result can be checked by observations of other stars1 and the slight personal errors of observation can be rendered harmless by taking the mean from several stars4 ,by an hour's work on a fine night it is possible to fix the clock error quite easily within the one-twentieth part of a second4 ,we have not space to enter into the interesting details of the methods by which the astronomical transit is accurately set in the right position1 and how any slight residual error in its setting can be eliminated from our results by certain processes of computation4 ,it must suffice to say that practically all time determinations in the observatory depend substantially upon the procedure outlined above4 ,the observatory clock having been once set right by observations of the sky1 its error can be re-determined every few days quite easily4 ,thus even the small irregularities of its nearly perfect mechanism can be prevented from accumulating until they might reach a harmful magnitude4 ,but we obtain in this way only a correct standard of time within the observatory itself4 ,how can this be made available for the general public8 ,the problem is quite simple with the aid of the electric telegraph4 ,we shall give a brief account of the methods now in use in ,new ,york ,city1 and these may be taken as essentially representative of those employed elsewhere4 ,every day1 at noon precisely1 an electric signal is sent out by the ,united ,states ,naval ,observatory in ,washington4 ,the signal is regulated by the standard clock of the observatory1 of course taking account of star observations made on the next preceding fine night4 ,this signal is received in the central ,new ,york office of the telegraph company1 where it is used to keep correct a very fine clock1 which may be called the time standard of the telegraph company4 ,this clock1 in turn1 has automatic electric connections1 by means of which it is made to send out signals over what are called time wires that go all over the city4 ,jewellers1 and others who desire correct time1 can arrange to have a small electric sounder in their offices connected with the time wires4 ,thus the ticks of the telegraph company's standard clock are repeated automatically in the jeweller's shop1 and used for controlling the exactness of his regulator4 ,this1 in brief1 is the method by which the astronomer's careful determination of correct time is transferred and distributed to the people at large4 ,having thus outlined the manner of obtaining and distributing correct time1 we shall now consider the question of time differences between different places on the earth4 ,this is a matter which many persons find most perplexing1 and yet it is essentially quite simple in principle4 ,travellers1 of course1 are well acquainted with the fact that their watches often need to be reset when they arrive at their destination4 ,yet few ever stop to ask the cause4 ,let us consider for a moment our method of measuring time4 ,we go by the sun4 ,if we leave out of account some small irregularities of the sun's motion that are of no consequence for our present purpose1 we may lay down this fundamental principle3 ,when the sun reaches its highest position in the sky it is twelve o'clock or noon4 ,the sun1 as everyone knows1 rises each morning in the east1 slowly goes up higher and higher in the sky1 and at last begins to descend again toward the west4 ,but it is clear that as the sun travels from east to west1 it must pass over the eastern one of any two cities sooner than the western one4 ,when it reaches its greatest height over a western city it has1 therefore1 already passed its greatest height over an eastern one4 ,in other words1 when it is noon1 or twelve o'clock1 in the western city1 it is already after noon in the eastern city4 ,this is the simple and evident cause of time differences in different parts of the country4 ,of any two places the eastern one always has later time than the western4 ,when we consider the matter in this way there is not the slightest difficulty in understanding how time differences arise4 ,they will1 of course1 be greatest for places that are very far apart in an east-and-west direction4 ,and this brings us again to the subject of longitude1 which1 as we have already said1 plays an important part in all questions relating to time2 for longitude is used to measure the distance in an east-and-west direction between different parts of the earth4 ,if we consider the earth as a large ball we can imagine a series of great circles drawn on its surface and passing directly from the ,north ,pole to the ,south ,pole4 ,such a circle could be drawn through any point on the earth4 ,if we imagine a pair of them drawn through two cities1 such as ,new ,york and ,london1 the longitude difference of these two cities is defined as the angle at the ,north ,pole between the two great circles in question4 ,the size of this angle can be expressed in degrees4 ,if we then wish to know the difference in time between ,new ,york and ,london in hours1 we need only divide their longitude difference in degrees by the number #ae4 ,in this simple way we can get the time difference of any two places4 ,we merely measure the longitude difference on a map1 and then divide by #ae to get the time difference4 ,these time differences can sometimes become quite large4 ,indeed1 for two places differing #ahj degrees in longitude1 the time difference will evidently be no less than twelve hours4 ,most civilized nations have agreed informally to adopt some one city as the fundamental point from which all longitudes are to be counted4 ,up to the present we have considered only longitude differences2 but when we speak of the longitude of a city we mean its longitude difference from the place chosen by common consent as the origin for measuring longitudes4 ,the town almost universally used for this purpose is ,greenwich1 near ,london1 ,england4 ,here is situated the ,british ,royal ,observatory1 one of the oldest and most important institutions of its kind in the world4 ,the great longitude circle passing through the centre of the astronomical transit at the ,greenwich observatory is the fundamental longitude circle of the earth4 ,the longitude of any other town is then simply the angle at the pole between the longitude circle through that town and the fundamental ,greenwich one here described4 ,longitudes are counted both eastward and westward from ,greenwich4 ,thus ,new ,york is in #gd degrees west longitude1 while ,berlin is in #ad degrees east longitude4 ,this has led to a rather curious state of affairs in those parts of the earth the longitudes of which are nearly #ahj degrees east or west4 ,there are a number of islands in that part of the world1 and if we imagine for a moment one whose longitude is just #ahj degrees1 we shall have the following remarkable result as to its time difference from ,greenwich4 ,we have seen that of any two places the eastern always has the later time4 ,now1 since our imaginary island is exactly #ahj degrees from ,greenwich1 we can consider it as being either #ahj degrees east or #ahj degrees west4 ,but if we call it #ahj degrees east1 its time will be twelve hours later than ,greenwich1 and if we call it #ahj degrees west1 its time will be twelve hours earlier than ,greenwich4 ,evidently there will be a difference of just twenty-four hours1 or one whole day1 between these two possible ways of reckoning its time4 ,this circumstance has actually led to considerable confusion in some of the islands of the ,pacific ,ocean4 ,the navigators who discovered the various islands naturally gave them the date which they brought from ,europe4 ,and as some of these navigators sailed eastward1 around the ,cape of ,good ,hope1 and others westward1 around ,cape ,horn1 the dates they gave to the several islands differed by just one day4 ,the state of affairs at the present time has been adjusted by a sort of informal agreement4 ,an arbitrary line has been drawn on the map near the #ahjth longitude circle1 and it has been decided that the islands on the east side of this line shall count their longitudes west from ,greenwich1 and those west of the line shall count longitude east from ,greenwich4 ,thus ,samoa is nearly #ahj degrees west of ,greenwich1 while the ,fiji ,islands are nearly #ahj degrees east4 ,yet the islands are very near each other1 though the arbitrary line passes between them4 ,as a result1 when it is ,sunday in ,samoa it is ,monday in the ,fiji ,islands4 ,the arbitrary line described here is sometimes called the ,international ,date-,line4 ,it does not pass very near the ,philippine ,islands1 which are situated in about #abj degrees east longitude1 and1 therefore1 use a time about eight hours later than ,greenwich4 ,new ,york1 being about #gd degrees west of ,greenwich1 is about five hours earlier in time4 ,consequently1 as we may remark in passing1 ,philippine time is about thirteen hours later than ,new ,york time4 ,thus1 five o'clock1 ,sunday morning1 ,may #ast1 in ,manila1 would correspond to four o'clock1 ,saturday afternoon1 ,april #cjth1 in ,new ,york4 ,there is another kind of time which we shall explain briefly--the so-called standard1 or railroad time1 which came into general use in the ,united ,states some few years ago1 and has since been generally adopted throughout the world4 ,it requires but a few moments' consideration to see that the accidental situation of the different large cities in any country will cause their local times to differ by odd numbers of hours1 minutes1 and seconds4 ,thus a great deal of inconvenience has been caused in the past4 ,for instance1 a train might leave ,new ,york at a certain hour by ,new ,york time4 ,it would then arrive in ,buffalo some hours later by ,new ,york time4 ,but it would leave ,buffalo by ,buffalo time1 which is quite different4 ,thus there would be a sort of jump in the time-table at ,buffalo1 and it would be a jump of an odd number of minutes4 ,it would be different in different cities1 and very hard to remember4 ,indeed1 as each railway usually ran its trains by the time used in the principal city along its line1 it might happen that three or four different railroad times would be used in a single city where several roads met4 ,this has all been avoided by introducing the standard time system4 ,according to this the whole country is divided into a series of time zones1 fifteen degrees wide1 and so arranged that the middle line of each zone falls at a point whose longitude from ,greenwich is #fj1 #ge1 #ij1 #aje1 or #abj degrees4 ,the times at these middle lines are1 therefore1 earlier than ,greenwich time by an even number of hours4 ,thus1 for instance1 the #ge-;degree line is just five even hours earlier than ,greenwich time4 ,all cities simply use the time of the nearest one of these special lines4 ,this does not result in doing away with time differences altogether--that would1 of course1 be impossible in the nature of things--but for the complicated odd differences in hours and minutes1 we have substituted the infinitely simpler series of differences in even hours4 ,the traveller from ,chicago to ,new ,york can reset his watch by putting it just one hour later on his arrival--the minute hand is kept unchanged1 and no ,new ,york timepiece need be consulted to set the watch right on arriving4 ,there can be no doubt that this standard-time system must be considered one of the most important contributions of astronomical science to the convenience of man4 ,its value has received the widest recognition1 and its use has now extended to almost all civilized countries--,france is the only nation of importance still remaining outside the time-zone system4 ,in the following table we give the standard time of the various parts of the earth as compared with ,greenwich1 together with the date of adopting the new time system4 ,it will be noticed that in certain cases even half-hours have been employed to separate the time-zones1 instead of even hours as used in the ,united ,states4 ,,table ,,of ,,the ,,world',s ,,time ,,standards ---------------------------------------- --------------- ,when it is ,noon ,date of ,adopting at ,greenwich ,in ,standard ,time it is ,system4 ---------------------------------------- --------------- ,noon ,great ,britain4 ,belgium4 ,may1 #ahib4 ,holland4 ,may1 #ahib4 ,spain4 ,january1 #aija4 #a ,p4,m4 ,germany4 ,april1 #ahic4 ,italy4 ,november1 #ahic4 ,denmark4 ,january1 #ahid4 ,switzerland4 ,june1 #ahid4 ,norway4 ,january1 #ahie4 ,austria 7railways74 #a4cj ,p4,m4 ,cape ,colony4 #ahib4 ,orange ,river ,colony4 #ahib4 ,transvaal4 #ahib4 #b ,p4,m4 ,natal4 ,september1 #ahie4 ,turkey 7railways74 ,egypt4 ,october1 #aijj4 #h ,p4,m4 ,west ,australia4 ,february1 #ahie4 #i ,p4,m4 ,japan4 #ahif4 #i4cj ,p4,m4 ,south ,australia4 ,may1 #ahii4 #aj ,p4,m4 ,victoria4 ,february1 #ahie4 ,new ,south ,wales4 ,february1 #ahie4 ,queensland4 ,february1 #ahie4 #aa ,p4,m4 ,new ,zealand4 ---------------------------------------- --------------- ,in the ,united ,states and ,canada it is #d ,a4,m4 by ,pacific ,time when it is ,noon at ,greenwich4 #e ,a4,m4 ,mountain #f ,a4,m4 ,central #g ,a4,m4 ,eastern #h ,a4,m4 ,colonial ,,motions ,,of ,,the ,,earth',s ,,pole ,students of geology have been puzzled for many years by traces remaining from the period when a large part of the earth was covered with a heavy cap of ice4 ,these shreds of evidence all seem to point to the conclusion that the centre of the ice-covered region was quite far away from the present position of the north pole of the earth4 ,if we are to regard the pole as very near the point of greatest cold1 it becomes a matter of much interest to examine whether the pole has always occupied its present position1 or whether it has been subject to slow changes of place upon the earth's surface4 ,therefore1 the geologists have appealed to astronomers to discover whether they are in possession of any observational evidence tending to show that the pole is in motion4 ,now we may say at once that astronomical research has not as yet revealed the evidence thus expected4 ,astronomy has been unable to come to the rescue of geological theory4 ,from about the year #agej1 which saw the beginning of precise observation in the modern sense1 down to very recent times1 astronomers were compelled to deny the possibility of any appreciable motion of the pole4 ,observational processes1 it is true1 furnished slightly divergent pole positions from time to time4 ,yet these discrepancies were always so minute as to be indistinguishable from those slight personal errors that are ever inseparable from results obtained by the fallible human eye4 ,but in the last few years improved methods of observation1 coupled with extreme diligence in their application by astronomers generally1 have brought to light a certain small motion of the pole which had never before been demonstrated in a reliable way4 ,this motion1 it is true1 is not of the character demanded by geological theory1 for the geologists had been led to expect a motion which would be continuous in the same direction1 no matter how slow might be its annual amount2 for the vast extent of geologic time would give even the slowest of motions an opportunity to produce large effects1 provided its results could be continuously cumulative4 ,given time enough1 and the pole might move anywhere on the earth1 no matter how slow might be its tortoise speed4 ,but the small motion we have discovered is neither cumulative nor continuous in one direction4 ,it is what we call a periodic motion1 the pole swinging now to one side1 and now to the other1 of its mean or average position4 ,thus this new discovery cannot be said to unravel the mysterious puzzle of the geologists4 ,yet it is not without the keenest interest1 even from their point of view2 for the proof of any form of motion in a pole previously supposed to be absolutely at rest may mean everything4 ,no man can say what results will be revealed by the further observations now being continued with great diligence4 ,in the first place1 it is important to explain that any such motions as we have under consideration will show themselves to ordinary observational processes principally in the form of changes of terrestrial latitudes4 ,let us imagine a pair of straight lines passing through the centre of the earth and terminating1 one at the observer's station on the earth's surface1 and the other at that point of the equator which is nearest the observer4 ,then1 according to the ordinary definition of latitude1 the angle between these two imaginary lines is called the latitude of the point of observation4 ,now we know1 of course1 that the equator is everywhere just #ij degrees from the pole4 ,consequently1 if the pole is subject to any motion at all1 the equator must also partake of the motion4 ,thus the angle between our two imaginary lines will be affected directly by polar movement1 and the latitude obtained by astronomical observation will be subject to quite similar changes4 ,to clear up the whole question1 so far as this can be done by the gathering of observational evidence1 it is only necessary to keep up a continual series of latitude determinations at several observatories4 ,these determinations should show small variations similar in magnitude to the wabblings of the pole4 ,let us now consider for a moment what is meant by the axis of the earth4 ,it has long been known that the planet has in general the shape of a ball or sphere4 ,that this is so can be seen at once from the way ships at sea disappear at the horizon4 ,as they go farther and farther from us1 we first lose sight of the hull1 and then slowly and gradually the spars and sails seem to sink down into the ocean4 ,this proves that the earth's surface is curved4 ,that it is more or less like a sphere is evident from the fact that it always casts a round shadow in eclipses4 ,sometimes the earth passes between the sun and eclipsed moon4 ,then we see the earth's black shadow projected on the moon1 which would otherwise be quite bright4 ,this shadow has been observed in a very large number of such eclipses1 and it has always been found to have a circular edge4 ,while1 therefore1 the earth is nearly a round ball1 it must not be supposed that it is exactly spherical in form4 ,we may disregard the small irregularities of its surface1 for even the greatest mountains are insignificant in height when compared with the entire diameter of the earth itself4 ,but even leaving these out of account1 the earth is not perfectly spherical4 ,we can describe it best as a flattened sphere4 ,it is as though one were to press a round rubber ball between two smooth boards4 ,it would be flattened at the top and bottom and bulged out in the middle4 ,this is the shape of the earth4 ,it is flattened at the poles and bulges out near the equator4 ,the shortest straight line that can be drawn through the earth's centre and terminated by the flattened parts of its surface may be called the earth's axis of figure2 and the two points where this axis meets the surface are called the poles of figure4 ,but the earth has another axis1 called the axis of rotation4 ,this is the one about which the planet turns once in a day1 giving rise to the well-known phenomena called the rising and setting of sun1 moon1 and stars4 ,for these motions of the heavenly bodies are really only apparent ones1 caused by an actual motion of the observer on the earth4 ,the observer turns with the earth on its axis1 and is thus carried past the sun and stars4 ,this daily turning of the earth1 then1 takes place about the axis of rotation4 ,now1 it so happens that all kinds of astronomical observations for the determination of latitude lead to values based on the rotation axis of the earth1 and not on its axis of figure4 ,we have seen how the earth's equator1 from which we count our latitudes1 is everywhere #ij degrees distant from the pole4 ,but this pole is the pole of rotation1 or the point at which the rotation axis pierces the earth's surface4 ,it is not the pole of figure4 ,it is clear that the latitude of any observatory will remain constant only if the pole of figure and the rotation pole maintain absolutely the same positions relatively one to the other4 ,these two poles are actually very near together2 indeed1 it was supposed for a very long time that they were absolutely coincident1 so that there could not be any variations of latitude4 ,but it now appears that they are separated slightly4 ,strange to say1 one of them is revolving about the other in a little curve4 ,the pole of figure is travelling around the pole of rotation4 ,the distance between them varies a little1 never becoming greater than about fifty feet1 and it takes about fourteen months to complete a revolution4 ,there are some slight irregularities in the motion1 but1 in the main1 it takes place in the manner here stated4 ,in consequence of this rotation of the one pole about the other1 the pole of figure is now on one side of the rotation pole and now on the opposite side1 but it never travels continuously in one direction4 ,thus1 as we have already seen1 the sort of continuous motion required to explain the observed geological phenomena has not yet been found by astronomers4 ,observations for the study of latitude variations have been made very extensively within recent years both in ,europe and the ,united ,states4 ,it has been found practically most advantageous to carry out simultaneous series of observations at two observatories situated in widely different parts of the earth1 but having very nearly the same latitude4 ,it is then possible to employ the same stars for observation in both places1 whereas it would be necessary to use different sets of stars if there were much difference in the latitudes4 ,there is a special advantage in using the same stars in both places4 ,we can then determine the small difference in latitude between the two participating observatories in a manner which will make it quite free from any uncertainty in our knowledge of the positions on the sky of the stars observed2 for1 strange as it may seem1 our star-catalogues do not contain absolutely accurate numbers4 ,like all other data depending on fallible human observation1 they are affected with small errors4 ,but if we can determine simply the difference in latitude of the two observatories1 we can discover from its variation the path in which the pole is moving4 ,if1 for instance1 the observatories are separated by one-quarter the circumference of the globe1 the pole will be moving directly toward one of them1 when it is not changing its distance from the other one at all4 ,this method was used for seven years with good effect at the observatories of ,columbia ,university in ,new ,york1 and the ,royal ,observatory at ,naples1 ,italy4 ,for obtaining its most complete advantages it is1 of course1 better to establish several observing stations on about the same parallel of latitude4 ,this was done in #ahii by the ,international ,geodetic ,association4 ,two stations are in the ,united ,states1 one in ,japan1 and one in ,sicily4 ,we can1 therefore1 hope confidently that our knowledge as to the puzzling problem of polar motion will soon receive very material advancement4 ,,saturn',s ,,rings ,the death of ,james ,e4 ,keeler1 ,director of the ,lick ,observatory1 in ,california 7p4 #cb71 recalls to mind one of the most interesting and significant of later advances in astronomical science4 ,only seven years have elapsed since ,keeler made the remarkable spectroscopic observations which gave for the first time an ocular demonstration of the true character of those mysterious luminous rings surrounding the brilliant planet ,saturn4 ,his results have not yet been made sufficiently accessible to the public at large1 nor have they been generally valued at their true worth4 ,we consider this work of ,keeler's interesting1 because the problem of the rings has been a classic one for many generations2 and we have been particular1 also1 to call it significant1 because it is pregnant with the possibilities of newer methods of spectroscopic research1 applied in the older departments of observational astronomy4 ,the troubles of astronomers with the rings began with the invention of the telescope itself4 ,they date back to #afaj1 when ,galileo first turned his new instrument to the heavens 7p4 #di74 ,it may be imagined easily that the bright planet ,saturn was among the very first objects scrutinized by him4 ,his powerful instrument magnified only about thirty times1 and was1 doubtless1 much inferior to our pocket telescopes of to-day4 ,but it showed1 at all events1 that something was wrong with ,saturn4 ,galileo put it1 ,ultimam planet am tergeminam observavi 7,i have observed the furthest planet to be triple74 ,it is easy to understand now how ,galileo's eyes deceived him4 ,for a round luminous ball like ,saturn1 surrounded by a thin flat ring seen nearly edgewise1 really looks as if it had two little attached appendages4 ,strange1 indeed1 it is to-day to read a scientific book so old that the planet ,saturn could be called the furthest planet4 ,but it was the outermost known in ,galileo's day1 and for nearly two centuries afterward4 ,not until #agha did ,william ,herschel discover ,uranus 7p4 #ei72 and ,neptune was not disclosed by the marvellous mathematical perception of ,le ,verrier until #ahdf 7p4 #fa74 ,galileo's further observations of ,saturn bothered him more and more4 ,the planet's behavior became much worse as time went on4 ,has ,saturn devoured his children1 according to the old legend8 he inquired soon afterward2 for the changed positions of earth and planet in the course of their motions around the sun in their respective orbits had become such that the ring was seen quite edgewise1 and was1 therefore1 perfectly invisible to ,galileo's optic tube4 ,the puzzle remained unsolved by ,galileo2 it was left for another great man to find the true answer4 ,huygens1 in #afef1 first announced that the ring is a ring4 ,the manner in which this announcement was made is characteristic of the time2 to-day it seems almost ludicrous4 ,huygens published a little pamphlet in #afef called ,de ,saturni ,luna ,observatio ,nova or1 ,a ,new ,observation of ,saturn's ,moon4 ,he gave the explanation of what had been observed by himself and preceding astronomers in the form of a puzzle1 or logogriph4 ,here is what he had to say of the phenomenon in question3 aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu4 ,it was not until #afei1 three years later1 in a book entitled ,systema ,saturnium1 that ,huygens rearranged the above letters in their proper order1 giving the ,latin sentence3 ,annulo cingitur1 tenui plano1 nusquam cohaerente1 ad eclipticam inclinato4 ,translated into ,english1 this sentence informs us that the planet is girdled with a thin1 flat ring1 nowhere touching ,saturn1 and inclined to the ecliptic6 ,this was a perfectly correct and wonderfully sagacious explanation of those complex and exasperatingly puzzling phenomena that had been too difficult for no less a person than ,galileo himself4 ,it was an explanation that explained4 ,the reason for its preliminary announcement in the above manner must have been the following3 ,huygens was probably not quite sure of his ground in #afef1 while three years afterward he had become quite certain4 ,by the publication of the logogriph of #afef he secured for himself the credit of what he had done4 ,if any other astronomer had published the true explanation after #afef1 ,huygens could have proved his claim to priority by rearranging the letters of his puzzle4 ,on the other hand1 if further researches showed him that he was wrong1 he would never have made known the true meaning of his logogriph1 and would thus have escaped the ignominy of making an erroneous explanation4 ,thus1 the method of announcement was comparable in ingenuity with the ,huygenian explanation itself4 ,we are compelled to pass over briefly the entertaining history of subsequent observations of the ring1 in order to explain the new work of ,keeler and others4 ,cassini1 about #afge1 been able to show that the ring was double2 that there are really two independent rings1 with a distinct dark space between them4 ,it was a case of wheels within wheels4 ,to our own eminent countryman1 ,w4 ,c4 ,bond1 of ,cambridge1 ,mass41 we owe the further discovery 7,harvard ,college ,observatory1 ,november1 #ahej7 of the third ring4 ,this is also concentric with the other two1 and interior to them1 but difficult to observe1 because of its much smaller luminosity4 ,it is almost transparent1 and the brilliant light of the planet's central ball is capable of shining directly through it4 ,for this reason the inner ring is called the gauze or crape ring4 ,if we add to the above details the fact that our modern large telescopes show slight irregularities in the surface of the rings1 especially when seen edgewise1 we have a brief statement of all that the telescope has been able to reveal to us since ,galileo's time4 ,but of far greater interest than the mere fact of their existence is the important cosmic question as to the constitution1 structure1 and1 above all1 durability of the ring system4 ,astronomers often use the term stability with regard to celestial systems like the ring system of ,saturn4 ,by this they mean permanent durability4 ,a system is stable if its various parts can continue in their present relationship to one another1 without violating any of the known laws of astronomy4 ,whenever we study any collection of celestial objects1 and endeavor to explain their motions and peculiarities1 we always seek some explanation not inconsistent with the continued existence of the phenomena in question4 ,for this there is1 perhaps1 no sufficient philosophical basis4 ,probably much of the great celestial procession is but a passing show1 to be but for a moment in the endless vista of cosmic time4 ,however this may be1 we are bound to assume as a working theory that ,saturn has always had these rings1 and will always have them2 and it is for us to find out how this is possible4 ,the problem has been attacked mathematically by various astronomers1 including ,laplace2 but no conclusive mathematical treatment was obtained until #aheg1 when ,james ,clerk ,maxwell proved in a masterly manner that the rings could be neither solid nor liquid4 ,he showed1 indeed1 that they would not last if they were continuous bodies like the planets4 ,a big solid wheel would inevitably be torn asunder by any slight disturbance1 and then precipitated upon the planet's surface4 ,therefore1 the rings must be composed of an immense number of small detached particles1 revolving around ,saturn in separate orbits1 like so many tiny satellites4 ,this mathematical theory of the ring system being once established1 astronomers were more eager than ever to obtain a visual confirmation of it4 ,we had1 indeed1 a sort of analogy in the assemblage of so-called minor planets 7p4 #fd71 which are known to be revolving around our sun in orbits situated between ,mars and ,jupiter4 ,some hundreds of these are known to exist1 and probably there are countless others too small for us to see4 ,such a swarm of tiny particles of luminous matter would certainly give the impression of a continuous solid body1 if seen from a distance comparable to that separating us from ,saturn4 ,but arguments founded on analogy are of comparatively little value4 ,astronomers need direct and conclusive telescopic evidence1 and this was lacking until ,keeler made his remarkable spectroscopic observation in #ahie4 ,the spectroscope is a peculiar instrument1 different in principle from any other used in astronomy2 we study distant objects with it by analyzing the light they send us1 rather than by examining and measuring the details of their visible surfaces4 ,the reader will recall that according to the modern undulatory theory1 light consists simply of a series of waves4 ,now1 the nature of waves is very far from being understood in the popular mind4 ,most people1 for instance1 think that the waves of ocean consist of great masses of water rolling along the surface4 ,this notion doubtless arises from the behavior of waves when they break upon the shore1 forming what we call surf4 ,when a wave meets with an immovable body like a sand beach1 the wave is broken1 and the water really does roll upon the beach4 ,but this is an exceptional case4 ,farther away from the shore1 where the waves are unimpeded1 they consist simply of particles of water moving straight up and down4 ,none of the water is carried by mere wave-action away from the point over which it was situated at first4 ,tides or other causes may move the water1 but not simple wave-motion alone4 ,that this is so can be proved easily4 ,if a chip of wood be thrown overboard from a ship at sea it will be seen to rise and fall a long time on the waves1 but it will not move4 ,similarly1 wind-waves are often quite conspicuous on a field of grain2 but they are caused by the individual grain particles moving up and down4 ,the grain certainly cannot travel over the ground1 since each particle is fast to its own stalk4 ,but while the particles do not travel1 the wave-disturbance does4 ,at times it is transmitted to a considerable distance from the point where it was first set in motion4 ,thus1 when a stone is dropped into still water1 the disturbance 7though not the water7 travels in ever-widening circles1 until at last it becomes too feeble for us to perceive4 ,light is just such a travelling wave-disturbance4 ,beginning1 perhaps1 in some distant star1 it travels through space1 and finally the wave impinges on our eyes like the ocean-wave breaking on a sand beach4 ,such a light-wave affects the eye in some mysterious way4 ,we call it seeing4 ,the spectroscope 7p4 #ba7 enables us to measure and count the waves reaching us each second from any source of light4 ,no matter how far away the origin of stellar light may be1 the spectroscope examines the character of that light1 and tells us the number of waves set up every second4 ,it is this characteristic of the instrument that has enabled us to make some of the most remarkable observations of modern times4 ,if the distant star is approaching us in space1 more light-waves per second will reach us than we should receive from the same star at rest4 ,thus if we find from the spectroscope that there are too many waves1 we know that the star is coming nearer2 and if there are too few1 we can conclude with equal certainty that the star is receding4 ,keeler was able to apply the spectroscope in this way to the planet ,saturn and to the ring system4 ,the observations required dexterity and observational manipulative skill in a superlative degree4 ,these ,keeler had2 and this work of his will always rank as a classic observation4 ,he found by examining the light-waves from opposite sides of the planet that the luminous ball rotated2 for one side was approaching us and the other receding4 ,this observation was1 of course1 in accord with the known fact of ,saturn's rotation on his axis4 ,with regard to the rings1 ,keeler showed in the same way the existence of an axial rotation1 which appears not to have been satisfactorily proved before1 strange as it may seem4 ,but the crucial point established by his spectroscope was that the interior part of the rings rotates faster than the exterior4 ,the velocity of rotation diminishes gradually from the inside to the outside4 ,this fact is absolutely inconsistent with the motion of a solid ring2 but it fits in admirably with the theory of a ring comprised of a vast assemblage of small separate particles4 ,thus1 for the first time1 astronomy comes into possession of an observational determination of the nature of ,saturn's rings1 and ,galileo's puzzle is forever solved4 ,,the ,,heliometer ,astronomical discoveries are always received by the public with keen interest4 ,every new fact read in the great open book of nature is written eagerly into the books of men4 ,for there exists a strong curiosity to ascertain just how the greater world is built and governed2 and it must be admitted that astronomers have been able to satisfy that curiosity with no small measure of success4 ,but it is seldom that we hear of the means by which the latest and most refined astronomical observations are effected4 ,popular imagination pictures the astronomer1 as he doubtless once was1 an aged gentleman1 usually having a long white beard1 and spending entire nights staring at the sky through a telescope4 ,but the facts to-day are very different4 ,the working astronomer is an active man in the prime of life1 often a young man4 ,he wastes no time in star-gazing4 ,his observations consist of exact measurements made in a precise1 systematic1 and almost business-like manner4 ,a night's watch at the telescope is seldom allowed to exceed about three hours1 since it is found that more continued exertions fatigue the eye and lead to less accurate results4 ,to this1 of course1 there have been many notable exceptions1 for endurance of sight1 like any form of physical strength1 differs greatly in different individuals4 ,astronomical research does not include picking out the constellations1 and learning the ,arabic names of individual stars4 ,these things are not without interest2 but they belong to astronomy's ancient history1 and are of little value except to afford amusement and instruction to successive generations of amateurs4 ,among the instruments for carefully planned measurements of precision the heliometer probably takes first rank4 ,it is at once the most exquisitely accurate in its results1 and the most fatiguing to the observer1 of all the varied apparatus employed by the astronomer4 ,the principle upon which its construction depends is very peculiar1 and applies to all telescopes1 even ordinary ones for terrestrial purposes4 ,if part of a telescope lens be covered up with the hand1 it will still be possible to see through the instrument4 ,the glass lens at the end of the tube farthest from the observer's eye helps to magnify distant objects and make them seem nearer by gathering to a single point1 or focus1 a greater amount of their light than could be brought together by the far smaller lens in the unaided eye4 ,the telescope might very properly be likened to an enlarged eye1 which can see more than we can1 simply because it is bigger4 ,if a telescope lens has a surface one hundred times as large as that of the lens in our eye1 it will gather and bring to a focus one hundred times as much light from a distant object4 ,now1 if any part of this telescope be covered1 the remaining part will1 nevertheless1 gather and focus light just as though the whole lens were in action2 only1 there will be less light collected at the focus within the tube4 ,the small lens at the telescope's eye-end is simply a magnifier to help our eye examine the image of any distant object formed at the focus by the large lens at the farther end of the instrument4 ,for of this simple character is the operation of any telescope3 the large glass lens at one end collects a distant planet's light1 and brings it to a focus near the other end of the tube1 where it forms a tiny picture of the planet1 which1 in turn1 is examined with the little magnifier at the eye-end4 ,having arrived at the fundamental principle that part of a lens will act in a manner similar to a whole one1 it is easy to explain the construction of a heliometer4 ,an ordinary telescope lens is sawed in half by means of a thin round metal disk revolved rapidly by machinery1 and fed continually with emery and water at its edge4 ,the cutting effect of emery is sufficient to make such a disk enter glass much as an ordinary saw penetrates wood4 ,the two semi-lenses1 as they are called1 are then mounted separately in metal holders4 ,these are attached to one end of the heliometer1 called the head1 in such a way that the two semi-lenses can slide side by side upon metal guides4 ,this head is then fastened to one end of a telescope tube mounted in the usual way4 ,the head end of the instrument is turned toward the sky in observing1 and at the eye-end is placed the usual little magnifier we have already described4 ,the observer at the eye-end has control of certain rods by means of which he can push the semi-lenses on their slides in the head at the other end of the tube4 ,now1 if he moves the semi-lenses so as to bring them side by side exactly1 the whole arrangement will act like an ordinary telescope4 ,for the semi-lenses will then fit together just as if the original glass had never been cut4 ,but if the half-lenses are separated a little on their slides1 each will act by itself4 ,being slightly separated1 each will cover a different part of the sky4 ,the whole affair acts as if the observer at the eye-end were looking through two telescopes at once4 ,for each semi-lens acts independently1 just as if it were a complete glass of only half the size4 ,now1 suppose there were a couple of stars in the sky1 one in the part covered by the first semi-lens1 and one in the part covered by the second4 ,the observer would1 of course1 see both stars at once upon looking into the little magnifier at the eye-end of the heliometer4 ,we must remember that these stars will appear in the field of view simply as two tiny points of light4 ,the astronomer1 as we have said1 is provided with a simple system of long rods1 by means of which he can manipulate the semi-lenses while the observation is being made4 ,if he slides them very slowly one way or the other1 the two star-points in the field of view will be seen to approach each other4 ,in this way they can at last be brought so near together that they will form but a single dot of light4 ,observation with the heliometer consists in thus bringing two star-images together1 until at last they are superimposed one upon the other1 and we see one image only4 ,means are provided by which it is then possible to measure the amount of separation of the two half-lenses4 ,evidently the farther asunder on the sky are the two stars under observation1 the greater will be the separation of the semi-lenses necessary to make a single image of their light4 ,thus1 measurement of the lenses' separation becomes a means of determining the separation of the stars themselves upon the sky4 ,the two slides in the heliometer head are supplied with a pair of very delicate measures or scales usually made of silver4 ,these can be examined from the eye-end of the instrument by looking through a long microscope provided for this special purpose4 ,thus an extremely precise value is obtained both of the separation of the sliders and of the distance on the sky between the stars under examination4 ,measures made in this way with the heliometer are counted the most precise of astronomical observations4 ,having thus described briefly the kind of observations obtained with the heliometer1 we shall now touch upon their further utilization4 ,we shall take as an example but one of their many uses--that one which astronomers consider the most important--the measurement of stellar distances4 7,see also p4 #id47 ,even the nearest fixed star is almost inconceivably remote from us4 ,and astronomers are imprisoned on this little earth2 we cannot bridge the profound distance separating us from the stars1 so as to use direct measurement with tape-line or surveyor's chain4 ,we are forced to have recourse to some indirect method4 ,suppose a certain star is suspected1 on account of its brightness1 or for some other reason1 of being near us in space1 and so giving a favorable opportunity for a determination of distance4 ,a couple of very faint stars are selected close by4 ,these1 on account of their faintness1 the astronomer may regard as quite immeasurably far away4 ,he then determines with his heliometer the exact position on the sky of the bright star with respect to the pair of faint ones4 ,half a year is then allowed to pass4 ,during that time the earth has been swinging along in its annual path or orbit around the sun4 ,half a year will have sufficed to carry the observer on the earth to the other side of that path1 and he is then #ahe1jjj1jjj miles away from his position at the first observation4 ,another determination is made of the bright star's position as referred to the two faint ones4 ,now1 if all these stars were equally distant1 their relative positions at the second observation would be just the same as at the former one4 ,but if1 as is very probable1 the bright star is very much nearer us than are the two faint ones1 we shall obtain a different position from our second observation4 ,for the change of #ahe1jjj1jjj miles in the observer's location will1 of course1 affect the direction in which we see the near star1 while it will leave the distant ones practically unchanged4 ,without entering into technical details1 we may say that from a large number of observations of this kind1 we can obtain the distance of the bright star by a process of calculation4 ,the only essential is to have an instrument that can make the actual observations of position accurately enough2 and in this respect the heliometer is still unexcelled4 ,,occultations ,scarcely anyone can have watched the sky without noticing how different is the behavior of our moon from that of any other object we can see4 ,of course1 it has this in common with the sun and stars and planets1 that it rises in the eastern horizon1 slowly climbs the dome of the sky1 and again goes down and sets in the west4 ,this motion of the heavenly bodies is known to be an apparent one merely1 and caused by the turning of our own earth upon its axis4 ,a man standing upon the earth's surface can look up and see above him one-half the great celestial vault1 gemmed with twinkling stars1 and bearing1 perhaps1 within its ample curve one or two serenely shining planets and the lustrous moon4 ,but at any given moment the observer can see nothing of the other half of the heavenly sphere4 ,it is beneath his feet1 and concealed by the solid bulk of the earth4 ,the earth1 however1 is turning on an axis1 carrying the observer with it4 ,and so it is continually presenting him to a new part of the sky4 ,at any moment he sees but a single half-sphere2 yet the very next instant it is no longer the same2 a small portion has passed out of sight on one side by going down behind the turning earth1 while a corresponding new section has come into view on the opposite side4 ,it is this coming into view that we call the rising of a star2 and the corresponding disappearance on the other side is called setting4 ,thus rising and setting are1 of course1 due entirely to a turning of the earth1 and not at all to actual motions of the stars2 and for this reason1 all objects in the sky1 without exception1 must rise and set again4 ,but the moon really has a motion of its own in addition to this apparent one caused by the earth's rotation4 ,somewhere in the dawn of time early watchers of the stars thought out those fancied constellations that survive even down to our own day4 ,they placed the mighty lion1 king of beasts1 upon the face of night1 and the great hunter1 too1 armed with club and dagger1 to pursue him4 ,among these constellations the moon threads her destined way1 night after night1 so rapidly that the unaided eye can see that she is moving4 ,it needs but little power of fancy's magic to recall from the dim past a picture of some aged astronomer graving upon his tablets the ,records of the ,moon4 ,to-night she is near the bright star in the eye of the ,bull4 ,and again3 ,to-night she rides full1 and near to the heart of the ,virgin4 ,and why does the moon ride thus through the stars of night8 ,modern science has succeeded in disentangling the intricacies of her motion1 until to-day only one or two of the very minutest details of that motion remain unexplained4 ,but it has been a hard problem4 ,someone has well said that lunar theory should be likened to a lofty cliff upon whose side the intellectual giants among men can mark off their mental stature1 but whose height no one of them may ever hope to scale4 ,but for our present purpose it is unnecessary to pursue the subject of lunar motion into its abstruser details4 ,to understand why the moon moves rapidly among the stars1 it is sufficient to remember that she is whirling quickly round the earth1 so as to complete her circuit in a little less than a month4 ,we see her at all times projected upon the distant background of the sky on which are set the stellar points of light1 as though intended for beacons to mark the course pursued by moon and planets4 ,the stars themselves have no such motions as the moon2 situated at a distance almost inconceivably great1 they may1 indeed1 be travellers through empty space1 yet their journeys shrink into insignificance as seen from distant earth4 ,it requires the most delicate instruments of the astronomer to so magnify the tiny displacements of the stars on the celestial vault that they may be measured by human eyes4 ,let us again recur to our supposed observer watching the moon night after night1 so as to record the stars closely approached by her4 ,why should he not some time or other be surprised by an approach so close as to amount apparently to actual contact8 ,the moon covers quite a large surface on the sky1 and when we remember the nearly countless numbers of the stars1 it would1 indeed1 be strange if there were not some behind the moon as well as all around her4 ,a moment's consideration shows that this must often be the case2 and in fact1 if the moon's advancing edge be scrutinized carefully through a telescope1 small stars can be seen frequently to disappear behind it4 ,this is a most interesting observation4 ,at first we see the moon and star near each other in the telescope's field of view4 ,but the distance between them lessens perceptibly1 even quickly1 until at last1 with a startling suddenness1 the star goes out of sight behind the moon4 ,after a time1 ranging from a few moments to1 perhaps1 more than an hour1 the moon will pass1 and we can see the star suddenly reappear from behind the other edge4 ,these interesting observations1 while not at all uncommon1 can be made only very rarely by naked-eye astronomers4 ,the reason is simple4 ,the moon's light is so brilliant that it fairly overcomes the stars whenever they are at all near1 except in the case of very bright ones4 ,small stars that can be followed quite easily up to the moon's edge in a good telescope1 disappear from a naked-eye view while the moon is still a long distance away4 ,but the number of very bright stars is comparatively small1 so that it is quite unusual to find anyone not a professional astronomer who has actually seen one of these so-called occultations4 ,moreover1 most people are not informed in advance of the occurrence of an opportunity to make such observations1 although they can be predicted quite easily by the aid of astronomical calculations4 ,sometimes we have occultations of planets1 and these are the most interesting of all4 ,when the moon passes between us and one of the larger planets1 it is worth while to observe the phenomenon even without a telescope4 ,up to this point we have considered occultations chiefly as being of interest to the naked-eye astronomer4 ,yet occultations have a real scientific value4 ,it is by their means that we can1 perhaps1 best measure the moon's size4 ,by noting with a telescope the time of disappearance and reappearance of known stars1 astronomers can bring the direct measurement of the moon's diameter within the range of their numerical calculations4 ,sometimes the moon passes over a condensed cluster of stars like the ,pleiades4 ,the results obtainable on these occasions are valuable in a very high degree1 and contribute largely to making precise our knowledge of the lunar diameter4 ,there is another thing of scientific interest about occultations1 though it has lost some of its importance in recent years4 ,when such an event has been observed1 the agreement of the predicted time with that actually recorded by the astronomer offers a most searching test of the correctness of our theory of lunar motion4 ,we have already called attention to the great inherent difficulty of this theory4 ,it is easy to see that by noting the exact instant of disappearance of a star at a place on the earth the latitude and longitude of which are known1 we can both check our calculations and gather material for improving our theory4 ,the same principle can be used also in the converse direction4 ,within the limits of precision imposed by the state of our knowledge of lunar theory1 we can utilize occultations to help determine the position on the earth of places whose longitude is unknown4 ,it is a very interesting bit of history that the first determination of the longitude of ,washington was made by means of occultations1 and that this early determination led to the founding of the ,united ,states ,naval ,observatory4 ,on ,march #bh1 #ahaj1 ,mr4 ,pitkin1 of ,connecticut1 reported to the ,house of ,representatives on laying a foundation for the establishment of a first meridian for the ,united ,states1 by which a further dependence on ,great ,britain or any other foreign nation for such meridian may be entirely removed4 ,this report was the result of a memorial presented by one ,william ,lambert1 who had deduced the longitude of the ,capitol from an occultation observed ,october #bj1 #ahjd4 ,various proceedings were had in ,congress and in committee1 until at last1 in #ahba1 ,lambert was appointed to make astronomical observations by lunar occultations of fixed stars1 solar eclipses1 or any approved method adapted to ascertain the longitude of the ,capitol from ,greenwich4 ,lambert's reports were made in #ahbb and #ahbc1 but ten years passed before the establishment of a formal ,naval ,observatory under ,goldsborough1 ,wilkes1 and ,gilliss4 ,but to ,lambert belongs the honor of having marked out the first fundamental official meridian of longitude in the ,united ,states4 ,,mounting ,,great ,,telescopes ,there are many interesting practical things about an astronomical observatory with which the public seldom has an opportunity to become acquainted4 ,among these1 perhaps1 the details connected with setting up a great telescope take first rank4 ,the writer happened to be present at the ,cape of ,good ,hope ,observatory when the photographic equatorial telescope was being mounted1 and the operation of putting it in position may be taken as typical of similar processes elsewhere4 7,see also p4 #hf47 ,illustration3 ,forty-,inch ,telescope1 ,yerkes ,observatory1 ,university of ,chicago4 ,in the first place1 it is necessary to explain what is meant by an equatorial telescope4 ,one of the chief difficulties in making ordinary observations arises from the rising and setting of the stars4 ,they are all apparently moving across the face of the sky1 usually climbing up from the eastern horizon1 only to go down again and set in the west4 ,if1 therefore1 we wish to scrutinize any given object for a considerable time1 we must move the telescope continuously so as to keep pace with the motion of the heavens4 ,for this purpose1 the tube must be attached to axles1 so that it can be turned easily in any direction4 ,the equatorial mounting is a device that permits the telescope to be thus aimed at any part of the sky1 and at the same time facilitates greatly the operation of keeping it pointed correctly after a star has once been brought into the field of view4 ,to understand the equatorial mounting it is necessary to remember that the rising and setting motions of the heavenly bodies are apparent ones only1 and due in reality to the turning of the earth on its own axis4 ,as the earth goes around1 it carries observer1 telescope1 and observatory past the stars fixed upon the distant sky4 ,consequently1 to keep a telescope pointed continuously at a given star1 it is merely necessary to rotate it steadily backward upon a suitable axis just fast enough to neutralize exactly the turning of our earth4 ,by a suitable axis for this purpose we mean one so mounted as to be exactly parallel to the earth's own axis of rotation4 ,a little reflection shows how simply such an arrangement will work4 ,all the heavenly bodies may be regarded1 for practical purposes1 as excessively remote in comparison with the dimensions of our earth4 ,the entire planet shrinks into absolute insignificance when compared with the distances of the nearest objects brought under observation by astronomers4 ,it follows that if we have our telescope attached to such a rotation-axis as we have described1 it will be just the same for purposes of observation as though the telescope's axis were not only parallel to the earth's axis1 but actually coincident with it4 ,the two axes may be separated by a distance equal to that between the earth's surface and its centre2 but1 as we have said1 this distance is insignificant so far as our present object is concerned4 ,there is another way to arrive at the same result4 ,we know that the stars in rising and setting all seem to revolve about the pole star1 which itself seems to remain immovable4 ,consequently1 if we mount our telescope so that it can turn about an axis pointing at the pole1 we shall be able to neutralize the rotation of the stars by simply turning the telescope about the axis at the proper speed and in the right direction4 ,astronomical considerations teach us that an axis thus pointing at the pole will be parallel to the earth's own axis4 ,thus we arrive at the same fundamental principle for mounting an astronomical telescope from whichever point of view we consider the subject4 ,every large telescope is provided with such an axis of rotation2 and for the reason stated it is called the polar axis4 ,the telescope itself is then called an equatorial4 ,the advantage of this method of mounting is very evident4 ,since we can follow the stars' motions by turning the telescope about one axis only1 it becomes a very simple matter to accomplish this turning automatically by means of clock-work4 ,the following of a star being thus provided for by the device of a polar axis1 it is1 of course1 also necessary to supply some other motion so as to enable us to aim the tube at any point in the heavens4 ,for it is obvious that if it were rigidly attached to the polar axis1 we could1 indeed1 follow any star that happened to be in the field of view1 but we could not change this field of view at will so as to observe other stars or planets4 ,to accomplish this1 the telescope is attached to the polar axis by means of a pivot4 ,by turning the telescope around its polar axis1 and also on this pivot1 we can find any object in the heavens2 and once found1 we can leave to the polar axis and its automatic clock-work the task of keeping that object before the observer's eye4 ,in setting up the ,cape of ,good ,hope instrument the astronomers were obliged to do a large part of the work of adjustment personally4 ,far away from ,european instrument-makers1 the parts of the mounting and telescope had to be assembled1 or put together1 by the astronomers of the ,cape ,observatory4 ,a heavy pier of brick and masonry had been prepared in advance4 ,upon this was placed a massive iron base1 intended to support the superstructure of polar axis and telescope4 ,this base rested on three points1 one of which could be screwed in and out1 so as to tilt the whole affair a little forward or backward4 ,by means of this screw we effected the final adjustment of the polar axis to exact parallelism with that of the earth4 ,other screws were provided with which the base could be twisted a little horizontally either to the right or left4 ,once set up in a position almost correct1 it was easy to perfect the adjustment by the aid of these screws4 ,afterward the tube and lenses were put in place1 and the clock properly attached inside the big cast-iron base4 ,this clock-work looked more like a piece of heavy machinery than a delicate clock mechanism4 ,but it had heavy work to do1 carrying the massive telescope with its weighty lenses1 and needed to be correspondingly strong4 ,it had a driving-weight of about #b1jjj pounds1 and was so powerful that turning the telescope affected it no more than the hour-hand of an ordinary clock affects the mechanism within its case4 ,the final test of the whole adjustment consisted in noting whether stars once brought into the telescopic field of view could be maintained there automatically by means of the clock4 ,this object having been attained successfully1 the instrument stood ready to be used in the routine business of the observatory4 ,before leaving the subject of telescope-mountings1 we must mention the giant instrument set up at the ,paris ,exposition of #aijj4 ,the project of having a ,grande ,lunette had been hailed by newspapers throughout the world and by the general public in their customary excitable way4 ,it was tremendously over-advertised2 exaggerated notions of the instrument's powers were spread abroad and eagerly credited2 the moon was to be dragged down1 as it were1 from its customary place in the sky1 so near that we should be able almost to touch its surface4 ,as to the planets--free license was given to the journalistic imagination1 and there was no effective limitation to the magnificence of astronomical discovery practically within our grasp1 beyond the necessity for printed space demanded by sundry wars1 pestilences1 and other mundane trifles4 ,illustration3 ,yerkes ,observatory1 ,university of ,chicago4 ,now1 the present writer is very far from advocating a lessening of the attention devoted to astronomy4 ,rather would he magnify his office than diminish it4 ,but let journalistic astronomy be as good an imitation of sober scientific truth as can be procured at space rates2 let editors encourage the public to study those things in the science that are ennobling and cultivating to the mind2 let there be an end to the frenzied effort to fabricate a highly colored account of alleged discoveries of yesterday1 capable of masquerading to-day under heavy head-lines as ,news4 ,the manner in which the big telescope came to be built is not without interest1 and shows that enterprise is far from dead1 even in the old countries4 ,a stock company was organized--we should call it a corporation--under the name ,socit de l',optique4 ,it would appear that shares were regularly put on the market1 and that a prospectus1 more or less alluring1 was widely distributed4 ,we may say at once that the investing public did not respond with obtrusive alacrity2 but at all events1 the promoters' efforts received sufficient encouragement to enable them to begin active work4 ,from the very first a vigorous attempt was made to utilize both the resources of genuine science and the devices of quasi-charlatanry4 ,it was announced that the public were to be admitted to look through the big glass 7apparently at so much an eye71 and many1 doubtless1 expected that the man in the street would be able to make personal acquaintance with the man in the moon4 ,a telescopic image of the sun was to be projected on a big screen1 and exhibited to a concourse of spectators assembled in rising tiers of seats within a great amphitheatre4 ,and when clouds or other circumstances should prevent observing the planets or scrutinizing the sun1 a powerful stereopticon was to be used4 ,artificial pictures of the wonders of heaven were to be projected on the screen1 and the public would never be disappointed4 ,it was arranged that skilled talkers should be present to explain all marvels3 and1 in short1 financial profit was to be combined with machinery for advancing scientific discovery4 ,astronomers the world over were circularized1 asked to become shareholders1 and1 in default of that1 to send lantern-slides or photographs of remarkable celestial objects for exhibition in the magic-lantern part of the show4 ,the project thus brought to the attention of scientific men three years ago did not have an attractive air4 ,it savored too much of charlatanism4 ,but it soon appeared that effective government sanction had been given to the enterprise2 and1 above all1 that men of reputation were allowing the use of their names in connection with the affair4 ,more important still1 we learned that the actual construction had been undertaken by ,gautier1 of ,paris1 that finances were favorable1 and that real work on parts of the instrument was to commence without delay4 ,gautier is a first-class instrument-builder2 he has established his reputation by constructing successfully several telescopes of very large size1 including the equatorial coud of the ,paris ,observatory1 a unique instrument of especial complexity4 ,the present writer believes that1 if sufficient time and money were available1 the ,grande ,lunette would stand a reasonable chance of success in the hands of such a man4 ,and by a reasonable chance1 we mean that there is a large enough probability of genuine scientific discovery to justify the necessary financial outlay4 ,but the project should be divorced from its popular features1 and every kind of advertising and charlatanism excluded with rigor4 ,as planned originally1 and actually constructed1 the ,grande ,lunette presents interesting peculiarities1 distinguishing it from other telescopes4 ,previous instruments have been built on the principle of universal mobility4 ,it is possible to move them in all directions1 and thus bring any desired star under observation1 irrespective of its position in the sky4 ,but this general mobility offers great difficulties in the case of large and ponderous telescopes4 ,delicacy of adjustment is almost destroyed when the object to be adjusted weighs several tons4 ,and the excessive weight of telescopes is not due to unavoidably heavy lenses alone4 ,it is essential that the tube be long2 and great length involves appreciable thickness of material1 if stiffness and solidity are to remain unsacrificed4 ,length in the tube is necessitated by certain peculiar optical defects of all lenses1 into the nature of which we shall not enter at present4 ,the consequences of these defects can be rendered harmless only if the instrument is so arranged that the observer's eye is far from the other end of the tube4 ,the length of a good telescope should be at least twelve times the diameter of its large lens4 ,if the relative length can be still further increased1 so much the better2 for then the optical defects can be further reduced4 ,in the case of the ,paris instrument a radical departure consists in making the tube of unprecedented length1 #aig feet1 with a lens diameter of #di inches4 ,this great length1 while favorable optically1 precludes the possibility of making the instrument movable in the usual sense4 ,in fact1 the entire tube is attached to a fixed horizontal base1 and no attempt is made to change its position4 ,outside the big lens1 and disconnected altogether from the telescope proper1 is mounted a smooth mirror1 so arranged that it can be turned in any direction1 and thus various parts of the sky examined by reflection in the telescope4 ,while this method unquestionably has the advantage of leaving the optician quite free as to how long he will make his tube1 it suffers from the compensating objection that a new optical surface is introduced into the combination1 viz41 the mirror4 ,any slight unavoidable imperfection in the polishing of its surface will infallibly be reproduced on a magnified scale in the image of a distant star brought before the observer's eye4 ,but it is not yet possible to pronounce definitely upon the merit of this form of instrument1 since1 as we have said1 the maker has not been given time enough to try the idea to the complete satisfaction of scientific men4 ,in the early part of ,august1 #aijj1 when the informant of the present writer left ,paris1 after serving as a member of the international jury for judging instruments of precision at the ,exposition1 the condition of the ,grande ,lunette was as follows3 ,two sets of lenses had been contemplated1 one intended for celestial photography1 and the other to be used for ordinary visual observation4 ,only the photographic lenses had been completed1 however1 and for this reason the public could not be permitted to look through the instrument4 ,the photographic lenses were in place in the tube1 but at that time their condition was such that1 though some photographs had been obtained1 it was not thought advisable to submit them to the jury4 ,consequently1 the ,lunette did not receive a prize4 ,since that time various newspapers have reported wonderful results from the telescope2 but1 disregarding the fusillade from the sensational press1 we may sum up the present state of affairs very briefly4 ,gautier is still experimenting2 and1 given sufficient time and money1 he may succeed in producing what astronomers hope for--an instrument capable of advancing our knowledge1 even if that advance be only a small one4 ,,the ,,astronomer',s ,,pole ,the pole of the frozen ,north is not the only pole sought with determined effort by more than one generation of scientific men4 ,up in the sky astronomers have another pole which they are following up just as vigorously as ever ,arctic explorer struggled toward the difficult goal of his terrestrial journeying4 ,the celestial pole is1 indeed1 a fundamentally important thing in astronomical science1 and the determination of its exact position upon the sky has always engaged the closest attention of astronomers4 ,quite recently new methods of research have been brought to bear1 promising a degree of success not hitherto attained in the astronomers' pursuit of their pole4 ,in the first place1 we must explain what is meant by the celestial pole4 ,we have already mentioned the poles of the earth 7p4 #acf74 ,our planet turns once daily upon an axis passing through its centre1 and it is this rotation that causes all the so-called diurnal phenomena of the heavens4 ,rising and setting of sun1 moon1 and stars are simply results of this turning of the earth4 ,heavenly bodies do not really rise2 it is merely the man on the earth who is turned round on an axis until he is brought into a position from which he can see them4 ,the terrestrial poles are those two points on the earth's surface where it is pierced by the rotation axis of the planet4 ,now we can1 if we choose1 imagine this axis lengthened out indefinitely1 further and further1 until at last it reaches the great round vault of the sky4 ,here it will again pierce out two polar points2 and these are the celestial poles4 ,the whole thing is thus quite easy to understand4 ,on the sky the poles are marked by the prolongation of the earth's axis1 just as on the earth the poles are marked by the axis itself4 ,and this explains at once why the stars seem nightly to revolve about the pole4 ,if the observer is being turned round the earth's axis1 of course it will appear to him as if the stars were rotating around the same axis in the opposite direction1 just as houses and fields seem to fly past a person sitting in a railway train1 unless he stops to remember that it is really himself who is in motion1 and not the trees and houses4 ,the existence of such a centre of daily motions among the stars once recognized1 it becomes of interest to ascertain whether the centre itself always retains precisely the same position in the sky4 ,it was discovered as early as the time of ,hipparchus 7p4 #ci7 that such is not the case1 and that the celestial pole is subject to a slow motion among the stars on the sky4 ,if a given star were to-day situated exactly at the pole1 it would no longer be there after the lapse of a year's time2 for the pole would have moved away from it4 ,this motion of the pole is called precession4 ,it means that certain forces are continually at work1 compelling the earth's axis to change its position1 so that the prolongation of that axis must pierce the sky at a point which moves as time goes on4 ,these forces are produced by the gravitational attractions of the sun1 moon1 and planets upon the matter composing our earth4 ,if the earth were perfectly spherical in shape1 the attractions of the other heavenly bodies would not affect the direction of the earth's rotation-axis in the least4 ,but the earth is not quite globular in form2 it is flattened a little at the poles and bulges out somewhat at the equator4 7,see p4 #ace47 ,this protuberant matter near the equator gives the other bodies in the solar system an opportunity to disturb the earth's rotation4 ,the general effect of all these attractions is to make the celestial pole move upon the sky in a circle having a radius of about #bc degrees2 and it requires #be1hjj years to complete a circuit of this precessional cycle4 ,one of the most striking consequences of this motion will be the change of the polar star4 ,just at present the bright star ,polaris in the constellation of the ,little ,bear is very close to the pole4 ,but after the lapse of sufficient ages the first-magnitude star ,vega of the constellation ,lyra will in its turn become ,guardian of the ,pole4 ,it must not be supposed1 however1 that the motion of the pole proceeds quite uniformly1 and in an exact circle2 the varying positions of the heavenly bodies whose attractions cause the phenomena in question are such as to produce appreciable divergencies from exact circular motion4 ,sometimes the pole deviates a little to one side of the precessional circle1 and sometimes it deviates on the other side4 ,the final result is a sort of wavy line1 half on one side and half on the other of an average circular curve4 ,it takes only nineteen years to complete one of these little waves of polar motion1 so that in the whole precessional cycle of #be1hjj years there are about #a1djj indentations4 ,this disturbance of the polar motion is called by astronomers nutation4 ,the first step in a study of polar motion is to devise a method of finding just where the pole is on any given date4 ,if the astronomer can ascertain by observational processes just where the pole is among the stars at any moment1 and can repeat his observations year after year and generation after generation1 he will possess in time a complete chart of a small portion at least of the celestial pole's vast orbit4 ,from this he can obtain necessary data for a study of the mathematical theory of attractions1 and thus1 perhaps1 arrive at an explanation of the fundamental laws governing the universe in which we live4 ,the instrument which has been used most extensively for the study of these problems is the transit 7p4 #aah7 or the meridian circle4 ,this latter consists of a telescope firmly attached to a metallic axis about which it can turn4 ,the axis itself rests on massive stone supports1 and is so placed that it points as nearly as possible in an east-and-west direction4 ,consequently1 when the telescope is turned about its axis1 it will trace out on the sky a great circle 7the meridian7 which passes through the north and south points of the horizon and the point directly overhead4 ,the instrument has also a metallic circle very firmly fastened to the telescope and its axis4 ,let into the surface of this circle is a silver disk upon which are engraved a series of lines or graduations by means of which it is possible to measure angles4 ,observers with the meridian circle begin by noting the exact instant when any given star passes the centre of the field of view of the telescope4 ,this centre is marked with a cross made by fastening into the focus some pieces of ordinary spider's web1 which give a well-marked1 delicate set of lines1 even under the magnifying power of the telescope's eye-piece4 ,in addition to thus noting the time when the star crosses the field of the telescope1 the astronomer can measure by means of the circle1 how high up it was in the sky at the instant when it was thus observed4 ,if the telescope of the meridian circle be turned toward the north1 and we observe stars close to the pole1 it is possible to make two different observations of the same star4 ,for the close polar stars revolve in such small circles around the pole of the heavens that we can observe them when they are on the meridian either above the pole or below it4 ,double observations of this class enable us to obtain the elevation of the pole above the horizon1 and to fix its position with respect to the stars4 ,now1 there is one very serious objection to this method4 ,in order to secure the two necessary observations of the same star1 it is essential to be stationed at the instrument at two moments of time separated by exactly twelve hours2 and if one of the observations occurs in the night1 the other corresponding observation will occur in daylight4 ,it is a fact not generally known that the brighter stars can be seen with a telescope1 even when the sun is quite high above the horizon4 ,unfortunately1 however1 there is only one star close to the pole which is bright enough to be thus observed in daylight--the polar star already mentioned under the name ,polaris4 ,the fact that we are thus limited to observations of a single star has made it difficult even for generations of astronomers to accumulate with the meridian circle a very large quantity of observational material suitable for the solution of our problem4 ,the new method of observation to which we have referred above consists in an application of photography to the polar problem4 ,if we aim at the pole a powerful photographic telescope1 and expose a photographic plate throughout the entire night1 we shall find that all stars coming within the range of the plate will mark out little circles or trails upon the developed negative4 ,it is evident that as the stars revolve about the pole on the sky1 tracing out their daily circular orbits1 these same little circles must be reproduced faithfully upon the photographic plate4 ,the only condition is that the stars shall be bright enough to make their light affect the sensitive gelatine surface4 ,but even if observations of this kind are continued throughout all the hours of darkness1 we do not obtain complete circles1 but only those portions of circles traced out on the sky between sunset and sunrise4 ,if the night is twelve hours in length1 we get half-circles on the plate2 if it is eighteen hours long1 we get circles that lack only one-quarter of being complete4 ,in other words1 we get a series of circular arcs1 one corresponding to each close polar star4 ,there are no fewer than sixteen stars near enough to the pole to come within the range of a photographic plate1 and bright enough to cause measurable impressions upon the sensitive surface4 ,the fact that the circular arcs are not complete circles does not in the least prevent our using them for ascertaining the position of their common centre2 and that centre is the pole4 ,moreover1 as the arcs are distributed at all sorts of distances from the pole and in all directions1 corresponding to the accidental positions of the stars on the sky1 we have a state of affairs extremely favorable to the accurate determination of the pole's place among the stars by means of microscopic measurements of the plate4 ,it will be perceived that this method is extremely simple1 and1 therefore1 likely to be successful2 though its simplicity is slightly impaired by the phenomenon known to astronomers as atmospheric refraction4 ,the rays of light coming down to our telescopes from a distant star must pass through the earth's atmosphere before they reach us2 and in passing thus from the nothingness of outer space into the denser material of the air1 they are bent out of their straight course4 ,the phenomenon is analogous to what we see when we push a stick down through the surface of still water2 we notice that the stick appears to be bent at the point where it pierces the surface of the water2 and in just the same way the rays of light are bent when they pierce into the air4 ,fortunately1 the mathematical theory of this atmospheric bending of light is well understood1 so that it is possible to remove the effects of refraction from our results by a process of calculation4 ,in other words1 we can transform our photographic measures into what they would have been if no such thing as atmospheric refraction existed4 ,this having been done1 all the arcs on the plate should be exactly circular1 and their common centre should be the position of the pole among the stars on the night when the photograph was made4 ,it is possible to facilitate the removal of refraction effects very much by placing our photographic telescope at some point on the earth situated in a very high latitude4 ,the elevation of the pole above the horizon is greatest in high latitudes4 ,indeed1 if ,arctic voyagers could ever reach the pole of the earth they would see the pole of the heavens directly overhead4 ,now1 the higher up the pole is in the sky1 the less will be the effects of atmospheric refraction2 for the rays of light will then strike the atmosphere in a direction nearly perpendicular to its surface1 which is favorable to diminishing the amount of bending4 ,there is also another very important advantage in placing the telescope in a high latitude2 in the middle of winter the nights are very long there2 if we could get within the ,arctic4 ,circle itself1 there would be nights when the hours of darkness would number twenty-four1 and we could substitute complete circles for our broken arcs4 ,this would1 indeed1 be most favorable from the astronomical point of view2 but the essential condition of convenience for the observer renders an expedition to the frozen ,arctic regions unadvisable4 ,but it is at least possible to place the telescope as far north as is consistent with retaining it within the sphere of civilized influences4 ,we can put it in that one of existing observatories on the earth which has the highest latitude2 and this is the observatory of ,helsingfors1 in ,finland1 which belongs to a great university1 is manned by competent astronomers1 and has a latitude greater than #fj degrees4 ,dr4 ,anders ,donner1 ,director of the ,helsingfors ,observatory1 has at its disposal a fine photographic telescope1 and with this some preliminary experimental trail photographs were made in #ahie4 ,these photographs were sent to ,columbia ,university1 ,new ,york1 and were there measured under the writer's direction4 ,calculations based on these measures indicate that the method is promising in a very high degree2 and it was1 therefore1 decided to construct a special photographic telescope better adapted to the particular needs of the problem in hand4 ,the desirability of a new telescope arises from the fact that we wish the instrument to remain absolutely unmoved during all the successive hours of the photographic exposure4 ,it is clear that if the telescope moves while the stars are tracing out their little trails on the plate1 the circularity of the curves will be disturbed4 ,now1 ordinary astronomical telescopes are always mounted upon very stable foundations1 well adapted to making the telescope stand still2 but the polar telescope which we wish to use in a research fundamental to the entire science of astronomy ought to possess immobility and stability of an order higher than that required for ordinary astronomical purposes4 ,it is a remarkable peculiarity of the instrument needed for the new trail photographs that it is never moved at all4 ,once pointed at the pole1 it is ready for all the observations of successive generations of astronomers4 ,it should have no machinery1 no pivots1 axes1 circles1 clocks1 or other paraphernalia of the usual equatorial telescope4 ,all we want is a very heavy stone pier1 with a telescope tube firmly fastened to it throughout its entire length4 ,the top of the pier having been cut to the proper angle of the pole's elevation1 and the telescope cemented down1 everything is complete from the instrumental side2 and just such an instrument as this is now ready for use at ,helsingfors4 ,the late ,miss ,catharine ,wolfe ,bruce1 of ,new ,york1 was much interested in the writer's proposed polar investigations1 and in ,october1 #ahih1 she contributed funds for the construction of the new telescope1 and the ,russian authorities have generously undertaken the expense of a building to hold the instrument and the granite foundation upon which it rests4 ,photographs are now being secured with the new instrument1 and they will be sent to ,columbia ,university1 ,new ,york1 for measurement and discussion4 ,it is hoped that they will carry out the promise of the preliminary photographs made in #ahie with a less suitable telescope of the ordinary form4 ,,the ,,moon ,,hoax ,the public attitude toward matters scientific is one of the mysteries of our time4 ,it can be described best by the single word1 ,credulity2 simple1 absolute credulity4 ,perfect confidence is the most remarkable characteristic of this unbelieving age4 ,no charlatan1 necromancer1 or astrologer of three centuries ago commanded more respectful attention than does his successor of to-day4 ,any person can be a scientific authority2 he has but to call himself by that title1 and everyone will give him respectful attention4 ,numerous instances can be adduced from the experience of very recent years to show how true are these remarks4 ,we have had the ,keeley motor and the liquid-air power schemes for making something out of nothing4 ,extracting gold from sea-water has been duly heralded on scientific authority as an easy source of fabulous wealth for the million4 ,hard-headed business men not only believe in such things1 but actually invest in them their most valued possession1 capital4 ,venders of nostrums and proprietary medicines acquire wealth as if by magic1 though it needs but a moment's reflection to realize that these persons cannot possibly be in possession of any drugs1 or secret methods of compounding drugs1 that are unknown to scientific chemists4 ,if the world1 then1 will persistently intrust its health and wealth into the safe-keeping of charlatans1 what can we expect when things supposedly of far less value are at stake8 ,the famous ,moon ,hoax1 as we now call it1 is truly a classic piece of lying4 ,though it dates from as long ago as #ahce1 it has never had an equal as a piece of modern journalism4 ,nothing could be more useful than to recall it to public attention at least once every decade2 for it teaches an important lesson that needs to be iterated again and again4 ,on ,november #ac1 #ahcc1 ,sir ,john ,herschel embarked on the ,mountstuart ,elphinstone1 bound for the ,cape of ,good ,hope4 ,he took with him a collection of astronomical instruments1 with which he intended to study the heavens of the southern hemisphere1 and thus extend his father's great work to the south polar stars4 ,an earnest student of astronomy1 he asked no better than to be left in peace to seek the truth in his own fashion4 ,little did he think that his expedition would be made the basis for a fabrication of alleged astronomical discoveries destined to startle a hemisphere4 ,yet that is precisely what happened4 ,some time about the middle of the year #ahce the ,new ,york ,sun began the publication of certain articles1 purporting to give an account of ,great ,astronomical ,discoveries1 lately made by ,sir ,john ,herschel at the ,cape of ,good ,hope4 ,it was alleged that these articles were taken from a supplement to the ,edinburgh ,journal of ,science2 yet there is no doubt that they were manufactured entirely in the ,united ,states1 and probably in ,new ,york4 ,the hoax begins at once in a grandiloquent style1 calculated to attract popular attention1 and well fitted to the marvels about to be related4 ,here is an introductory remark1 as a specimen3 ,it has been poetically said that the stars of heaven are the hereditary regalia of man as the intellectual sovereign of the animal creation4 ,he may now fold the zodiac around him with a loftier consciousness of his mental supremacy4 ,then follows a circumstantial and highly plausible account of the manner in which early and exclusive information was obtained from the ,cape4 ,this was1 of course1 important in order to make people believe in the genuineness of the whole2 but we pass at once to the more interesting account of ,herschel's supposed instrument4 ,nothing could be more skilful than the way in which an air of truth is cast over the coming account of marvellous discoveries by explaining in detail the construction of the imaginary ,herschelian instrument4 ,sir ,john is supposed to have had an interesting conversation in ,england with ,sir ,david ,brewster1 upon the merits of some ingenious suggestion by the latter1 in his article on optics in the ,edinburgh ,encyclopdia 7p4 #fdd71 for improvements in the ,newtonian reflectors4 ,the exact reference to a particular page is here quite delightful4 ,after some further talk1 the conversation became directed to that all-invincible enemy1 the paucity of light in powerful magnifiers4 ,after a few moments' silent thought1 ,sir ,john diffidently inquired whether it would not be possible to effect a transfusion of artificial light through the focal object of vision6 ,sir ,david1 somewhat startled at the originality of the idea1 paused awhile1 and then hesitatingly referred to the refrangibility of rays1 and the angle of incidence4444 ,sir ,john continued1 ',why cannot the illuminated microscope1 say the hydro-oxygen1 be applied to render distinct1 and1 if necessary1 even to magnify the focal object8' ,sir ,david sprang from his chair in an ecstasy of conviction1 and leaping half-way to the ceiling1 exclaimed1 ',thou art the man4' ,this absurd imaginary conversation contains nothing but an assemblage of optical jargon1 put together without the slightest intention of conveying any intelligible meaning to scientific people4 ,yet it was well adapted to deceive the public2 and we should not be surprised if it would be credited by many newspaper readers to-day4 ,the authors go on to explain how money was raised to build the new instrument1 and then describe ,herschers embarkation and the difficulties connected with transporting his gigantic machines to the place selected for the observing station4 ,sir ,john accomplished the ascent to the plains by means of two relief teams of oxen1 of eighteen each1 in about four days1 and1 aided by several companies of ,dutch boors sic1 proceeded at once to the erecting of his gigantic fabric4 ,the place really selected by ,herschel cannot be described better than in his own words1 contained in a genuine letter dated ,january #ba1 #ahce3 ,a perfect paradise in rich and magnificent mountain scenery1 sheltered from all winds4444 ,i must reserve for my next all description of the gorgeous display of flowers which adorn this splendid country1 as well as the astonishing brilliancy of the constellations4 ,the author of the hoax could have had no knowledge of ,herschers real location1 as described in this letter4 ,the present writer can bear witness to the correctness of ,herschel's words4 ,feldhausen is truly an ideal secluded spot for astronomical study4 ,a small obelisk under the sheer cliff of far-famed ,table ,mountain now marks the site of the great reflecting telescope4 ,here ,herschel carried on his scrutiny of the ,southern skies4 ,he observed #a1bjb double stars and #a1gjh nebul and clusters1 of which only #dci were already known4 ,he studied the famous ,magellanic clouds1 and made the first careful drawings of the keyhole nebula in the constellation ,argo4 ,very recent researches of the present royal astronomer at the ,cape have shown that changes of import have certainly taken place in this nebula since ,herschel's time1 when a sudden blazing up of the wonderful star ,eta ,argus was seen within the nebula4 ,this object has1 perhaps1 undergone more remarkable changes of light than any other star in the heavens4 ,it is as though there were some vast conflagration at work1 now blazing into incandescence1 and again sinking almost into invisibility4 ,in #ahdc ,maclear estimated the brilliancy of ,eta to be about equal to that of ,sirius1 the brightest star in the whole sky4 ,later it diminished in light1 and cannot be seen to-day with the naked eye1 though the latest telescopic observations indicate that it is again beginning to brighten4 ,such was ,herschel's quiet study of his beloved science1 in glaring contrast to the supposed discoveries of the ,hoax4 ,here are a few things alleged to have been seen on the moon4 ,the first time the instrument was turned upon our satellite the field of view was covered throughout its entire area with a beautifully distinct and even vivid representation of basaltic rock4 ,there were forests1 too1 and water1 fairer shores never angels coasted on a tour of pleasure4 ,a beach of brilliant white sand1 girt with wild castellated rocks1 apparently of green marble4 ,there was animal life as well2 we beheld continuous herds of brown quadrupeds1 having all the external characteristics of the bison1 but more diminutive than any species of the bos genus in our natural history4 ,there was a kind of beaver1 that carries its young in its arms like a human being1 and lives in huts4 ,from the appearance of smoke in nearly all of them1 there is no doubt of its 7the beaver's7 being acquainted with the use of fire4 ,finally1 as was1 of course1 unavoidable1 human creatures were discovered4 ,whilst gazing in a perspective of about half a mile1 we were thrilled with astonishment to perceive four successive flocks of large-winged creatures1 wholly unlike any kind of birds1 descend with a slow1 even motion from the cliffs on the western side1 and alight upon the plain4444 ,certainly they were like human beings1 and their attitude in walking was both erect and dignified4 ,we have not space to give more extended extracts from the hoax1 but we think the above specimens will show how deceptive the whole thing was4 ,the rare reprint from which we have extracted our quotations contains also some interesting ,opinions of the ,american ,press ,respecting the ,foregoing ,discovery4 ,the ,daily ,advertiser said3 ,no article1 we believe1 has appeared for years1 that will command so general a perusal and publication4 ,sir ,john has added a stock of knowledge to the present age that will immortalize his name and place it high on the page of science4 ,the ,mercantile ,advertiser said3 ,discoveries in the ,moon4--,we commence to-day the publication of an interesting article which is stated to have been copied from the ,edinburgh ,journal of ,science1 and which made its first appearance here in a contemporary journal of this city4 ,it appears to carry intrinsic evidence of being an authentic document4 ,many other similar extracts are given4 ,the ,new ,york ,evening ,post did not fall into the trap4 ,the ,evening ,post's remarks were as follows3 ,it is quite proper that the ,sun should be the means of shedding so much light on the ,moon4 ,that there should be winged people in the moon does not strike us as more wonderful than the existence of such a race of beings on the earth2 and that there does or did exist such a race rests on the evidence of that most veracious of voyagers and circumstantial of chroniclers1 ,peter ,wilkins1 whose celebrated work not only gives an account of the general appearance and habits of a most interesting tribe of flying ,indians1 but also of all those more delicate and engaging traits which the author was enabled to discover by reason of the conjugal relations he entered into with one of the females of the winged tribe4 ,we shall limit our extracts from the contemporary press to the few quotations here given1 hoping that enough has been said to direct attention once more to that important subject1 the ,possibility of ,being ,deceived4 ,,the ,,sun',s ,,destination ,three generations of men have come and gone since the ,marquis de ,laplace stood before the ,academy of ,france and gave his demonstration of the permanent stability of our solar system4 ,there was one significant fault in ,newton's superbly simple conception of an eternal law governing the world in which we live4 ,the labors of mathematicians following him had shown that the planets must trace out paths in space whose form could be determined in advance with unerring certainty by the aid of ,newton's law of gravitation4 ,but they proved just as conclusively that these planetary orbits1 as they are called1 could not maintain indefinitely the same shapes or positions4 ,slow indeed might be the changes they were destined to undergo2 slow1 but sure1 with that sureness belonging to celestial science alone4 ,and so men asked3 ,has this magnificent solar system been built upon a scale so grand1 been put in operation subject to a law sublime in its very simplicity1 only to change and change until at length it shall lose every semblance of its former self1 and end1 perhaps1 in chaos or extinction8 ,laplace was able to answer confidently1 ,no4 ,nor was his answer couched in the enthusiastic language of unbalanced theorists who work by the aid of imagination alone4 ,based upon the irrefragable logic of correct mathematical reasoning1 and clad in the sober garb of mathematical formul1 his results carried conviction to men of science the world over4 ,so was it demonstrated that changes in our solar system are surely at work1 and shall continue for nearly countless ages2 yet just as surely will they be reversed at last1 and the system will tend to return again to its original form and condition4 ,the objection that the ,newtonian law meant ultimate dissolution of the world was thus destroyed by ,laplace4 ,from that day forward the law of gravitation has been accepted as holding sway over all phenomena visible within our planetary world4 ,the intricacies of our own solar system being thus illumined1 the restless activity of the human intellect was stimulated to search beyond for new problems and new mysteries4 ,even more fascinating than the movements of our sun and planets are all those questions that relate to the clustered stellar congeries hanging suspended within the deep vault of night4 ,does the same law of gravitation cast its magic spell over that hazy cloud of ,pleiades1 binding them1 like ourselves1 with bonds indissoluble8 ,who shall answer1 yes or no8 ,we can only say that astronomers have as yet but stepped upon the threshold of the universe1 and fixed the telescope's great eye upon that which is within4 ,let us then begin by reminding the reader what is meant by the ,newtonian law of gravitation4 ,it appears all things possess the remarkable property of attracting or pulling each other4 ,newton declared that all substances1 solid1 liquid1 or even gaseous--from the massive cliff of rock down to the invisible air--all matter can no more help pulling than it can help existing4 ,his law further formulates certain conditions governing the manner in which this gravitational attraction is exerted2 but these are mere matters of detail2 interest centres about the mysterious fact of attraction itself4 ,how can one thing pull another with no connecting link through which the pull can act8 ,just here we touch the point that has never yet been explained4 ,nature withholds from science her ultimate secrets4 ,they that have pondered longest1 that have descended farthest of all men into the clear well of knowledge1 have done so but to sound the depths beyond1 never touching bottom4 ,this inability of ours1 to give a good physical explanation of gravitation1 has led certain makers of paradoxes to doubt or even deny that there is any such thing4 ,but1 fortunately1 we have a simple laboratory experiment that helps us4 ,unexplained it may ever remain1 but that there can be attraction between physical objects connected by no visible link is proved by the behavior of an ordinary magnet4 ,place a small piece of steel or iron near a magnetized bar1 and it will at once be so strongly attracted that it will actually fly to the magnet4 ,anyone who has seen this simple experiment can never again deny the possibility1 at least1 of the law of attraction as stated by ,newton4 ,its possibility once admitted1 the fact that it can predict the motions of all the planets1 even down to their minutest details1 transforms the possibility of its truth into a certainty as strong as any human certainty can ever be4 ,but this demonstration of ,newton's law is limited strictly to the solar system itself4 ,we may1 indeed1 reason by analogy1 and take for granted that a law which holds within our immediate neighborhood is extremely likely to be true also of the entire visible universe4 ,but men of science are loath to reason thus2 and hence the fascination of researches in cosmic astronomy4 ,analogy points out the path4 ,the astronomer is not slow to follow2 but he seeks ever to establish upon incontrovertible evidence those truths which at first only his daring imagination had led him to half suspect4 ,if we are to extend the law of gravitation to the utmost1 we must be careful to consider the law itself in its most complete form4 ,a heavenly body like the sun is often said to govern the motions of its family of planets2 but such a statement is not strictly accurate4 ,the governing body is no despot2 'tis an abject slave of law and order1 as much as the tiniest of attendant planets4 ,the action of gravitation is mutual1 and no cosmic body can attract another without being itself in turn subject to that other's gravitational action4 ,if there were in our solar system but two bodies1 sun and planet1 we should find each one pursuing a path in space under the influence of the other's attraction4 ,these two paths or orbits would be oval1 and if the sun and planet were equally massive1 the orbits would be exactly alike1 both in shape and size4 ,but if the sun were far larger than the planet1 the orbits would still be similar in form1 but the one traversed by the larger body would be small4 ,for it is not reasonable to expect a little planet to keep the big sun moving with a velocity as great as that derived by itself from the attraction of the larger orb4 ,whenever the preponderance of the larger body is extremely great1 its orbit will be correspondingly insignificant in size4 ,this is in fact the case with our own sun4 ,so massive is it in comparison with the planets that the orbit is too small to reveal its actual existence without the aid of our most refined instruments4 ,the path traced out by the sun's centre would not fill a space as large as the sun's own bulk4 ,nevertheless1 true orbital motion is there4 ,so we may conclude that as a necessary consequence of the law of gravitation every object within the solar system is in motion4 ,to say that planets revolve about the sun is to neglect as unimportant the small orbit of the sun itself4 ,this may be sufficiently accurate for ordinary purposes2 but it is unquestionably necessary to neglect no factor1 however small1 if we propose to extend our reasoning to a consideration of the stellar universe4 ,for we shall then have to deal with systems in which the planets are of a size comparable with the sun2 and in such systems all the orbits will also be of comparatively equal importance4 ,mathematical analysis has derived another fact from discussion of the law of gravitation which1 perhaps1 transcends in simple grandeur everything we have as yet mentioned4 ,it matters not how great may be the number of massive orbs threading their countless interlacing curved paths in space1 there yet must be in every cosmic system one single point immovable4 ,this point is called the ,centre of ,gravity4 ,if it should so happen that in the beginning of things1 some particle of matter were situated at this centre1 then would that atom ever remain unmoved and imperturbable throughout all the successive vicissitudes of cosmic evolution4 ,it is doubtful whether the mind of man can form a conception of anything grander than such an immovable atom within the mysterious intricacies of cosmic motion4 ,but in general1 we cannot suppose that the centres of gravity in the various stellar systems are really occupied by actual physical bodies4 ,the centre may be a mere mathematical point in space1 situated among the several bodies composing the system1 but1 nevertheless1 endowed1 in a certain sense1 with the same remarkable property of relative immobility4 ,having thus defined the centre of gravity in its relation to the constituent parts of any cosmic system1 we can pass easily to its characteristic properties in connection with the inter-relation of stellar systems with one another4 ,it can be proved mathematically that our solar system will pull upon distant stars just as though the sun and all the planets were concentrated into one vast sphere having its centre in the centre of gravity of the whole4 ,it is this property of the centre of gravity which makes it pre-eminently important in cosmic researches4 ,for1 while we know that centre to be at rest relatively to all the planets in the system1 it may1 nevertheless1 in its quality as a sort of concentrated essence of them all1 be moving swiftly through space under the pull of distant stars4 ,in that case1 the attendant bodies will go with it--but they will pursue their evolutions within the system1 all unconscious that the centre of gravity is carrying them on a far wider circuit4 ,what is the nature of that circuit8 ,this question has been for many years the subject of earnest study by the clearest minds among astronomers4 ,the greatest difficulty in the way is the comparatively brief period during which men have been able to make astronomical observations of precision4 ,space and time are two conceptions that transcend the powers of definition possessed by any man4 ,but we can at least form a notion of how vast is the extent of time1 if we remember that the period covered by man's written records is registered but as a single moment upon the great revolving dial of heaven's dome4 ,one hundred and fifty years have elapsed since ,james ,bradley built the foundations of modern sidereal astronomy upon his masterly series of observations at the ,royal ,observatory of ,greenwich1 in ,england4 ,yet so slowly do the movements of the stars unroll themselves upon the firmament1 that even to this day no one of them has been seen by men to trace out more than an infinitesimal fraction of its destined path through the voids of space4 ,travellers upon a railroad cannot tell at any given moment whether they are moving in a straight line1 or whether the train is turning upon some curve of huge size4 ,the ,st4 ,gothard railway has several so-called corkscrew tunnels1 within which the rails make a complete turn in a spiral1 the train finally emerging from the tunnel at a point almost vertically over the entrance4 ,in this way the train is lifted to a higher level4 ,passengers are wont to amuse themselves while in these tunnels by watching the needle of an ordinary pocket-compass4 ,this needle1 of course1 always points to the north2 and as the train turns upon its curve1 the needle will make a complete revolution4 ,but the passenger could not know without the compass that the train was not moving in a perfectly straight line4 ,just so we passengers on the earth are unaware of the kind of path we are traversing1 until1 like the compass1 the astronomer's instruments shall reveal to us the truth4 ,but as we have seen1 astronomical observations of precision have not as yet extended through a period of time corresponding to the few minutes during which the ,st4 ,gothard traveller watches the compass4 ,we are still in the dark1 and do not know as yet whether mankind shall last long enough upon the earth to see the compass needle make its revolution4 ,we are compelled to believe that the motion in space of our sun is progressing upon a curved path2 but so far as precise observations allow us to speak1 we can but say that we have as yet moved through an infinitesimal element only of that mighty curve4 ,however1 we know the point upon the sky toward which this tiny element of our path is directed1 and we have an approximate knowledge of the speed at which we move4 ,more than a century ago ,sir ,william ,herschel was able to fix roughly what we call the apex of the sun's way in space1 or the point among the stars toward which that way is for the moment directed4 ,we say for the moment1 but we mean that moment of which ,bradley saw the beginning in #agej1 and upon whose end no man of those now living shall ever look4 ,herschel found that a comparison of old stellar observations seemed to indicate that the stars in a certain part of the sky were opening out1 as it were1 and that the constellations in the opposite part of the heavens seemed to be drawing in1 or becoming smaller4 ,there can be but one reasonable explanation of this4 ,we must be moving toward that part of the sky where the stars are separating4 ,just so a man watching a regiment of soldiers approaching1 will see at first only a confused body of men2 but as they come nearer1 the individual soldiers will seem to separate1 until at length each one is seen distinct from all the others4 ,herschel fixed the position of the apex at a point in the constellation ,hercules4 ,the most recent investigations of ,newcomb and others have1 on the whole1 verified ,herschel's conclusions4 ,with the intuitive power of rare genius1 ,herschel had been able to sift truth out of error4 ,the observational data at his disposal would now be called rude1 but they disclosed to the scrutiny of his acute understanding the germ of truth that was in them4 ,later investigators have increased the precision of our knowledge1 until we can now say that the present direction of the solar motion is known within very narrow limits4 ,a tiny circle might be drawn on the sky1 to which an astronomer might point his hand and say3 ,yonder little circle contains the goal toward which the sun and planets are hastening to-day4 ,even the speed of this motion has been subjected to measurement1 and found to be about ten miles per second4 ,the objective point and the rate of motion thus stated1 exact science holds her peace4 ,here genuine knowledge stops2 and we can proceed further only by the aid of that imagination which men of science need to curb at every moment4 ,but let no one think that the sun will ever reach the so-called apex4 ,to do so would mean cosmic motion upon a straight line1 while every consideration of celestial mechanics points to motion upon a curve4 ,when shall we turn sufficiently upon that curve to detect its bending8 ',tis a problem we must leave as a rich heritage to later generations that are to follow us4 ,the visionary theorist's notion of a great central sun1 controlling our own sun's way in space1 must be dismissed as far too daring4 ,but for such a central sun we may substitute a central centre of gravity belonging to a great system of which our sun is but an insignificant member4 ,then we reach a conception that has lost nothing in the grandeur of its simplicity1 and is yet in accord with the probabilities of sober mechanical science4 ,we cease to be a lonely world1 and stretch out the bonds of a common relationship to yonder stars within the firmament4 ,,index ,,page ,airy1 ,astronomer ,royal1 #a ,allis1 photographs comet1 #aja ,andromeda nebula1 #bh temporary star1 #bh1 #bi1 #de ,apex1 of solar motion1 explained1 #bba ,aquila1 constellation1 temporary star in1 #dj ,arctic regions1 position of pole in1 #aid ,argo1 constellation1 variable star in1 #bje ,association1 international geodetic1 #aci ,asteroids1 first discovery by ,piazzi1 #ei1 #ajf discovery by photography1 #fd group of1 #fc photography of1 invented by ,wolf1 #ajd ,astronomer1 royal1 #a working1 description of1 #aeb ,,astronomer',s ,,pole1 ,,the1 #ahd ,astronomy1 journalistic1 #agf practical uses of1 #aab ,atmospheric refraction1 explained1 #aic ,axis1 of figure of the earth1 #acf of rotation of the earth1 #acf polar1 of telescope1 #agc ,barnard1 discovers satellite of ,jupiter1 #ea ,bessel1 measures ,pleiades1 #ae ,bond1 discovers crape ring of ,saturn1 #add ,bradley1 observes at ,greenwich1 #bai ,brahe1 ,tycho1 his temporary star1 #dj ,bruce1 endows polar photography1 #aig ,campbell1 observes ,pole-star1 #ah ,cape of ,good ,hope1 observatory1 photography at1 #aja telescope1 #agj1 #agd ,capriccio1 ,galileo's1 #ee ,cassini1 shows ,saturn's rings to be double1 #add ,cassiopeia1 temporary star in1 #dj ,celestial pole1 #ahd ,central sun theory1 #bbc ,centre of gravity1 #bag ,chart-room1 on ship-board1 #e ,chronometer1 invention of1 #h ,circle1 meridian1 explained1 #ahi ,clerk ,maxwell1 discusses ,saturn's rings1 #adf ,clock1 affected by temperature1 #aag affected by barometric pressure1 #aag astronomical1 #aae astronomical1 how mounted1 #aaf astronomical1 its dial1 #aaf error of1 determined with transit1 #aah jeweller's regulator1 #aad of telescope1 #age ,clusters of stars1 photography of1 #ih ,columbia ,university ,observatory1 latitude observations1 #aci polar photography1 #aif ,common1 his reflecting telescope1 #cb ,confusion of dates1 in ,pacific ,ocean1 #abe ,congress of ,astronomers1 ,paris1 #ahhg1 #ajb ,constellations1 #afb ,control1 mouse1 for photography1 #hh ,copernican theory of universe1 #ec1 #ef demonstration1 #id ,corkscrew tunnels1 #bbj ,crape ring of ,saturn1 #add ,cumulative effect1 in photography1 #hd ,date1 confusion of1 in ,pacific ,ocean1 #abe ,date-line1 international1 explained1 #abf ,development of photograph1 #ha ,dial1 of astronomical clock1 #aaf ,dialogue of ,galileo1 #ec ,differences of time1 explained1 #aba ,directions1 telescopic measurement of1 #ba ,directory of the heavens1 #ajc ,distance1 of light-source in photography1 #hc of stars1 #id1 #ajf1 #aeh of ,sun1 #fg1 #ig1 #ajf ,donner1 polar photography1 #aie ,double telescopes1 for photography1 #hf ,earth1 motions of its pole1 #aca rotation of1 #acf1 #afb1 #aga1 #ahd shape of1 #ace ,eclipses1 photography of1 #aji ,elkin1 measures ,pleiades1 #ae ,equatorial telescope1 explained1 #agj ,eros1 discovered by ,witt1 #ff1 #aje its importance1 #fg ,error of clock1 determined by transit1 #aah ,exposure1 length of1 in photography1 #hd ,feldhausen1 ,herschel's observatory near ,capetown1 #bjd ,fiji ,islands1 their date1 #abf ,fixed polar telescope1 #aig ,following the stars1 #hh1 #agc ,four-day cycle of pole-star1 #bd ,france1 outside time-zone system1 #abi ,fundamental longitude meridian1 #abd ,,galileo1 #dg and the ,church1 #dh discoveries of1 #di observes ,saturn1 #ada ,galle1 discovers ,neptune1 #fa ,gauss1 computes first asteroid orbit1 #fj ,gautier1 ,paris1 constructs big telescope1 #agi ,geodetic ,association1 international1 #aci ,geography1 maps1 astronomical side of1 #aab ,geology1 polar motion in1 #aca ,gill1 photographs comet1 #ajj ,gilliss1 at ,naval ,observatory1 ,washington1 #afi ,goldsborough1 at ,naval ,observatory1 ,washington1 #afi ,grande ,lunette1 ,paris1 #aijj1 #agf1 #ahj ,gravitation1 #ac in ,pleiades1 #ad1 #bab law of1 ,newton's1 #bab ,gravity1 centre of1 #bag ,greenwich1 origin of longitudes1 #g1 #abd time1 #g ,groombridge1 ,english astronomer1 #a ,harrison1 inventor of chronometer1 #h ,head1 of heliometer1 #aef ,heidelberg1 photography at1 #ajd ,,heliometer1 #aeb head of1 #aef how used1 #aeg principle of1 #aed scales of1 #aeh semi-lenses of1 #aee ,helsingfors observatory1 polar photography at1 #aie ,henry1 measures ,pleiades1 #aa1 #ag ,hercules1 constellation1 solar motion toward1 #bbb ,herschel1 discovers apex of solar motion1 #bba discovers ,uranus1 #ei1 #ada ,john1 the moon hoax1 #bjj ,hipparchus1 discovers precession1 #ahf early star-catalogue1 #ba1 #ci invents star magnitudes1 #ia ,huygens1 announces rings of ,saturn1 #adb his logogriph1 #adc ,ice-cap1 of ,earth1 #aca ,index ,librorum ,prohibitorum1 #ec ,international1 date-line1 explained1 #abf geodetic association1 #aci ,inter-stellar motion1 in clusters1 #ih in ,pleiades1 #ad ,islands of ,pacific1 their longitude and time1 #abe ,japan1 latitude station in1 #aci ,jewellers' correct time1 #aba ,journalistic astronomy1 #agf ,jupiter's satellites1 discovered by ,galileo1 #ej discovered by ,barnard1 #ea ,keeler1 observes ,saturn's rings1 #adj1 #adg1 #aej photographs nebul1 #cb ,keyhole nebula1 #bje ,lambert1 determines longitude of ,washington1 #afh ,laplace1 discusses ,saturn's rings1 #adf nebular hypothesis1 #cc stability of solar system1 #baj ,latitude1 changes of1 #acc1 #ach definition of1 #acd determining the1 #f ,leverrier1 predicts discovery of ,neptune1 #fa1 #adb ,lick ,observatory1 ,keeler's observations1 #adj ,light1 undulatory theory of1 #ai1 #adh ,light-waves1 measuring length of1 #bj1 #adi ,logogriph1 by ,huygens1 #adc ,long-exposure photography1 #he ,longitude1 counted ,east and ,west1 #abe determining1 #f determining by occultations1 #afg effect on time differences1 #abc explained1 #abc of ,washington1 first determined1 #afh ,maclear1 observes ,eta ,argus1 #bje ,magnitudes1 stellar1 #ia ,manila1 its time1 #abg ,maps1 astronomical side of1 #aab ,meridian circle1 explained1 #ahi ,milky-way1 poor in nebul1 #cc ,minor ,planets1 see ,asteroids4 ,,moon1 ,,hoax1 #aii motion among stars1 #afc mountains discovered by ,galileo1 #di size of1 measured1 #aff ,motion of moon1 #afc ,,motions of the ,,earth',s ,pole1 #aca ,,mounting ,,great ,,telescopes1 #agj ,naked-eye nebul1 #bh ,naples1 ,royal ,observatory1 latitude observations1 #aci ,naval ,observatory1 ,washington1 noon signal1 #abj ,,navigation1 #a before chronometers1 #c use of astronomy in1 #aac ,,nebul1 #bg ,nebula1 in ,andromeda1 #bh in ,orion1 #cj keyhole1 #bje ,nebular1 hypothesis1 #cc structure in ,pleiades1 #ag ,nebulous stars1 #ca ,negative1 and positive1 in photography1 #hb ,neptune1 discovery predicted by ,leverrier1 #fa1 #adb discovery by ,galle1 #fa ,newcomb1 fixes apex of solar motion1 #bbb ,newton1 law of gravitation1 #bab longitude commission1 #h ,new ,york1 its telegraphic time system1 #abj ,noon ,signal1 ,washington1 #abj ,number1 of nebul1 #ca1 #cc of temporary stars1 #ch ,nutation1 explained1 #ahh ,occultations1 #afa explained1 #afe ,occultations1 use of1 #aff1 #afg ,orion nebula1 #cj ,pacific islands1 their longitude and time1 #abe ,parallax1 solar1 #fg1 #ajf stellar1 #id1 #ajf measured with heliometer1 #aeh ,paris1 congress of astronomers1 #ahhg1 #ajb exposition of #aijj1 #agf ,periodic motion of earth's pole1 #acc ,perseus1 constellation1 temporary star in1 #df ,philippine ,islands1 their time1 #abg ,photography1 asteroid1 invented by ,wolf1 #ajd congress of astronomical1 #ajb cumulative effect of light1 #hd distance of light-source1 #hc double telescopes for1 #hf general star-catalogue1 #ajb ,,in ,,astronomy1 #ha in discovery of asteroids1 #fd1 #ajd in solar physics1 #aji in spectroscopy1 #ajh length of exposure1 #hd measuring-machine1 ,rutherfurd1 #ic motion of telescope for1 #hg mouse control of telescope1 #hh of eclipses1 #aji of inter-stellar motion1 #ii ,paris congress1 #ahgg1 #ajb polar1 #aia ,rutherfurd pioneer in1 #ij star-clusters1 #ih star-distances measured by1 #id summarized1 #aaj wholesale methods in1 #ajc ,piazzi1 discovers first asteroid1 #ei1 #ajf ,pitkin1 report to ,house of ,representatives1 #afh ,planetary nebul1 #ca ,,planet ,,of #ahih1 #eh ,planetoids1 see ,asteroids4 ,planets known to ancients1 #eh ,,pleiades1 #aj gravitation among1 #bab motion among1 #ad1 #af1 #ih nebular structure1 #ag number visible1 #aa ,polar axis1 of telescope1 #agc ,polar photography1 #aia at ,helsingfors1 #aie ,pole1 celestial1 #ahd of the earth1 motions of1 #aca ,,the ,,astronomer',s1 #ahd ,,pole-,,star1 #ah as a binary1 #be as a triple1 #ah1 #bf change of1 #ahg its four-day cycle1 #bd motion toward us1 #bd ,positive1 and negative1 in photography1 #hb ,potsdam1 observatory1 photographic star-catalogue1 #ajc ,practical uses of astronomy1 #aab ,precession1 explained1 #ahf ,prize1 for invention of chronometer1 #h ,ptolemaic theory of universe1 #ef ,ptolemy1 writes concerning ,hipparchus1 #ci ,railroad time1 explained1 #abg ,refraction1 atmospheric1 explained1 #aic ,regulator1 the jeweller's clock1 #aad ,ring-nebul1 #ca ,rings1 of ,saturn1 see ,saturn's rings4 ,roberts1 ,andromeda nebula1 #bh ,rotation1 of ,earth1 #acf1 #afb1 #aga1 #ahd of ,saturn1 #aej ,royal ,astronomer1 his duties1 #b ,royal ,observatory1 ,greenwich1 #abd ,greenwich1 ,bradley's observations1 #bai ,naples1 latitude observations1 #aci ,rutherfurd1 cluster photography1 #ii invents photographic apparatus1 #ic pioneer in photography1 #ij stellar parallax1 #id ,sagredus1 character in ,galileo's ,dialogue1 #ee ,salusbury1 ,galileo's translator1 #ej1 #ed ,salviati1 character in ,galileo's ,dialogue1 #ee ,samoa1 its date1 #abf ,,saturn',s ,,rings1 #adj analogy to planetoids1 #adg announced by ,huygens1 #adb observed with spectroscope1 #adg shown to be double by ,cassini1 #add structure and stability1 #ade ,scales1 of heliometer1 #aeh ,scorpio1 constellation1 temporary star in1 #ci ,semi-lenses of heliometer1 #aee ,sextant1 how used1 #d ,sicily1 latitude station in1 #aci ,sidereus ,nuncius1 published by ,galileo1 #eb ,simplicio1 character in ,galileo's ,dialogue1 #ee ,sirius1 brightest star1 #bje ,size of ,moon1 measured1 #aff ,socit de l',optique1 #agg ,solar parallax1 see ,sun's distance4 physics1 by photography1 #aji system1 stability of1 #baj ,spectroscope1 its use explained1 #adg used on pole-star1 #ai to observe ,saturn's rings1 #adg ,spiral nebul1 #ca ,stability1 of ,saturn's rings1 #ade of ,solar ,system1 #baj ,standards1 time1 of the world1 #aaa table of1 #acj ,standard time1 explained1 #abg ,star-catalogue1 general photographic1 #ajb ,star-clusters1 photography of1 #ih ,star-distances #id1 #ajf measured with heliometer1 #aeh ,rutherfurd1 #id ,star magnitudes1 #ia ,star-motion1 toward us1 #ba ,star-tables1 astronomical1 #aah ,stars1 variable1 #db ,st ,gothard railway1 tunnels1 #bbj ,sun1 newspaper1 the moon hoax1 #bja ,,sun-,,dial1 ,,how ,,to ,,make ,a1 #fi ,,sun',s1 ,,destination1 #baj distance1 compared with star distance1 #ig measured with ,eros1 #fg1 #ajf motion1 apex of1 #bba ,sun-spots1 discovered by ,galileo1 #di ,systema ,saturnium1 ,huygens1 #adc ,telescope1 clock1 #age at ,paris ,exposition1 #agf1 #ahj double1 for photography1 #hf equatorial1 explained1 #agj first used by ,galileo1 #di motion of1 #hg mounting great1 #agj unmoving1 for polar photography1 #aig ,,temporary ,,stars1 #cg in ,andromeda nebula1 #bh1 #bi1 #de in ,aquila1 #dj in ,cassiopeia1 #dj in ,perseus1 #df in ,scorpio1 #ci their number1 #ch theory of1 #db ,time1 correct1 determined astronomically1 #aac differences between different places1 #aba ,,time ,,standards ,,of ,,the ,,world1 #aaa standards of the ,world1 table of1 #acj system1 in ,new ,york1 #abj zones1 explained1 #abh ,trails1 photographic1 #aia ,transit1 for determining clock error1 #aah ,tycho ,brahe1 his temporary star1 #dj ,ulugh ,beg1 early star-catalogue1 #ba ,undulatory theory1 of light1 #ai1 #adh ,universe1 theories of1 #cd1 #ec1 #ef ,uranus1 discovered by ,herschel1 #ei1 #adb ,use of occultations1 #aff1 #afg ,uses of astronomy1 practical1 #aab ,variable stars1 #db in ,argo1 #bje ,vega1 future pole-star1 #ahg ,visibility of stars1 in day-time1 #aia ,vision1 phenomenon of1 #bj1 #adi ,washington1 its longitude first determined1 #afh ,waves1 explained1 #adh of light1 #bj1 #adh ,wilkes1 at ,naval ,observatory1 ,washington1 #afi ,wilkins1 imaginary voyage of1 #bjh ,witt1 discovers ,eros1 #ff1 #aje ,wolf1 ,m1 invents asteroid photography1 #ajd measures ,pleiades1 #aa ,world's time standards1 table of1 #acj ,yale ,college1 ,pleiades measured at1 #ae ,zones1 time1 explained1 #abh ,,transcriber',s ,,note ,italic text is denoted by underscores4 ,fractions in the two tables on pg #gd and pg #gh are displayed in the form a-b_/c as #d-a_/b or #b-g_/af for example4 ,the original text in the tables used the form a b-c4 ,a few other basic fractions in the text such as and are displayed in this same form in the etext4 ,there is only one ,footnote in this book1 with its anchor on pg #fi4 ,it has been placed at the end of the chapter containing the anchor4 ,obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources4 ,except for those changes noted below1 all misspellings in the text1 and inconsistent or archaic usage1 have been retained4 ,for example1 time zone1 time-zone2 ,le ,verrier1 ,leverrier2 light wave1 light-wave2 intrust2 wabbling2 unexcelled2 crape2 monumented4 ,pg #adf1 ',james ,clark-,maxwell' replaced by ',james ,clerk ,maxwell'4 ,pg #ahi1 'impossible to measure' replaced by 'possible to measure'4