PRACTICA ASTRONOMER HAROLD LIBRARY OF THE UNIVERSITY OF CALIFORNIA. doss 157 r PRACTICAL TALKS BY AN ASTRONOMER The Moon. First Quarter. Photographed by Loewy and Fuiseux, February 13, 1894. PRACTICAL TALKS BY AN ASTRONOMER BY HAROLD JACOBY ADJUNCT PROFESSOR OF ASTRONOMY IN COLUMBIA UNIVERSITY ILLUSTRATED OF THE UNIVERSITY OF NEW YORK CHARLES SCRIBNER'S SONS 1902 COPYRIGHT, 1902, BY CHARLES SCRIBNER'S SONS Published, April, 1902 TROW DIRECTORY , AND BOOKBINDING COMPANY NEW YORK PREFACE THE present volume has not been designed as a systematic treatise on astronomy. There are many excellent books of that kind, suitable for serious students as well as the general reader ; but they are necessarily somewhat dry and un- attractive, because they must aim at complete- ness. Completeness means detail, and detail means dryness. But the science of astronomy contains subjects that admit of detached treatment ; and as many of these are precisely the ones of greatest general interest, it has seemed well to select several, and describe them in language free from technicalities. 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 whole, but who may take pleasure in devoting a half-hour V 101613 PREFACE now and then to a detached essay on some spe- cial topic. Preparation of the book in this form has made it suitable for prior publication in periodicals ; and the several essays have in fact all been printed before. But the intention of collecting them into a book was kept in mind from the first; and while no attempt has been made at consecutiveness, it is hoped that nothing of merely ephemeral value has been included. VI CONTENTS PAGE NAVIGATION AT SEA i THE PLEIADES 10 THE POLE-STAR 18 NEBULJE 271*** TEMPORARY STARS 37 GALILEO 47 THE PLANET OF 1898 58 How TO MAKE A SUN-DIAL 69 PHOTOGRAPHY IN ASTRONOMY 81 TIME STANDARDS OF THE WORLD 1 1 1 MOTIONS OF THE EARTH'S POLE 131 SATURN'S RINGS ... * 140 THE HELIOMETER 152 OCCULTATIONS . . . l6l MOUNTING GREAT TELESCOPES 170 THE ASTRONOMER'S POLE 184 THE MOON HOAX 199 THE SUN'S DESTINATION 210 VI 1 ILLUSTRATIONS THE MOON. FIRST QUARTER Frontispiece Photographed by Loeivy and Puiseux, February /?, 1894. FACING PAGE SPIRAL NEBULA IN CONSTELLATION LEO 26 Photographed by Keeler, February 24, iqoo, NEBULA IN ANDROMEDA 28 Photographed by Barnard, November ii, t8f)2. THE " DUMB-BELL" NEBULA 34 Photographed by Keeler, July 31, iSQQ* STAR-FIELD IN CONSTELLATION MONOCEROS .... 84 Photographed by Barnard, February /, / ON ON ON ON 7-16 7-16 5 16 Indianapolis Louisville 3 ,-16 2 5-1 6 3 1-16 3 X-IO 2 7-8 3 1-16 3 3-*6 2 7-8 3 1-16 2 1-2 2 7-8 2 1-2 2 7-8 3 36 3 3-8 2 7-8 7-16 7-16 7-16 7-16 1-2 5-J6 5-x6 1-2 1-2 7-16 New Orleans. . . . New York Omaha Philadelphia Pittsburg 7 11-16 7 1-8 7 7-16 8 8 7 1-8 6 5-8 8 Portland, Me Richmond Rochester San Diego San Francisco Savannah St. Paul 15-16 13-16 1-8 4 1-16 3 15-16 4 11-16 Seattle Washington, D. C II to X, and I to XL The number XII is marked at MM' as shown. If you desire to add lines (not shown in Fig. 3 to avoid confusion) for hours earlier than six in the morning, it is merely necessary to mark off a distance on the line KO, below the point O, and equal to the distance from O to VII. This will give the point where the 78 HOW TO MAKE A SUN-DIAL 5 A.M. shadow line drawn from N cuts the line KO. A corresponding line for 7 P.M. can be drawn from N' on the other side of the figure. After you have marked out the dial very care- fully, you must fasten the three-cornered shadow- piece to it in such a way that the whole instru- ment will look like Fig. i. The edge ac (Fig. 2) goes on NM (Fig. 3). The point a (Fig. 2) must come exactly on N (Fig. 3) ; and as the lines NM (Fig. 3) and N'M' (Fig. 3) have been made just the right distance apart to fit the thickness of the three-cornered piece abc (Fig. 2), everything will go together just right. The point c (Fig. 2) will not quite reach to M (Fig. 3), but will be on the line NM (Fig. 3) at a distance of three inches from M. The two pieces of wood will be fastened together with three screws going through the bottom-board ABCD (Figs. i and 3) and into the edge ac (Fig. 2) of the three-cornered piece. The whole instrument will then look something like Fig. i. After you have got your sun-dial put together, you need only set it in the sun in a level place, on a piazza or window-sill, and turn it round 79 HOW TO MAKE A SUN-DIAL until it tells the right time by the shadow. You can get your local time from a watch near enough for setting up the dial. Once the dial is set right you can screw it down or mark its position, and it will continue to give correct solar time every day in the year. If you wish to adjust the dial very closely, you must go out some fine day and note the er- ror of the dial by a watch at about ten in the morning, and at noon, and again at about two in the afternoon. If the error is the same each time, the dial is rightly set. If not, you must try, by turning the dial slightly, to get it so placed that your three errors will be nearly the same. When you have got them as nearly alike as you can, the dial will be sufficiently near right. The solar or dial time may, however, differ somewhat from ordinary watch time, but the difference will never be great enough to matter, when we remember that sun-dials are only rough timekeepers after all, and useful principally for amusement. 80 PHOTOGRAPHY IN ASTRONOMY NEW highways of science have been monu- mented now and again by the masterful efforts of genius, working single-handed ; but more often it is slow-moving time that ripens discovery, and, at the proper moment, opens some new path to men whose intellectual power is but willingness to learn. So the annals of astronomical photog- raphy do not recount the achievements of extra- ordinary genius. It would have been strange, indeed, if the discovery of photography had not been followed by its application to astronomy. The whole range of chemical science contains no experiment of greater inherent interest than the development of a photographic plate. Let but the smallest ray of light fall upon its strangely sensitive surface, and some subtle invis- ible change takes place. It is then merely nec- essary to plunge the plate into a properly pre- pared chemical bath, and the gradual process of 81 PHOTOGRAPHY IN ASTRONOMY developing the picture begins. Slowly, very slowly, the colorless surface darkens wherever light has touched it. Let us imagine that the exposure has been made with an ordinary lens and camera, and that it is a landscape seeming to grow beneath the experimenter's eyes. At first only the most conspicuous objects make their appearance. But gradually the process extends, until finally every tiny detail is reproduced with marvellous fidelity to the original. The photo- graphic plate, when developed in this way, is called a " negative." For in Nature luminous points, or sources of light, are bright, while the developing negative turns dark wherever light has acted. Thus the negative, while true to Nat- ure, reproduces everything in a reversed way; bright things are dark, and shadows appear light. For ordinary purposes, therefore, the negative has to be replaced by a new photograph made by copy- ing it again photographically. In this way it is again reversed, giving us a picture corresponding correctly to the facts as seen. Such a copy from a negative is what is ordinarily called a photo- graph ; technically, it is known as a " positive." 82 PHOTOGRAPHY IN ASTRONOMY One of the remarkable things about the sensi- tive plate is its complete indifference to the dis- tance from which the light comes. It is ready to yield obediently to the ray of some distant star that may have journeyed, as it were, from the very vanishing point of space, or to the bright glow of an electric light upon the photog- rapher's table. This quality makes its use es- pecially advantageous in astronomy, since we can gain knowledge of remote stars only by a study of the light they send us. In such study the photographic plate possesses a supreme advan- tage over the human eye. If the conditions of weather and atmosphere are favorable, an ob- server looking through an ordinary telescope will see nearly as much at the first glance as he will ever see. Attentive and continued study will enable him to fix details upon his memory, and to record them by means of drawings and dia- grams. Occasional moments of especially un- disturbed atmospheric conditions will allow him to glimpse faint objects seldom visible. But on the whole, telescopic astronomers add little to their harvest by continued husbandry in the 83 PHOTOGRAPHY IN ASTRONOMY same field of stars. Photography is different. The effect of light upon the sensitive surface of the plate is strictly cumulative. If a given star can bring about a certain result when it has been allowed to act upon the plate for one minute, then in two or three minutes it will accomplish much more. Perhaps a single minute's exposure would have produced a mark scarcely perceptible upon the developed negative. In that case, three or four minutes would give us a perfectly well defined black image of the star. Thus, by lengthening the exposure we can make the fainter stars impress themselves upon the plate. If their light is not able to produce the desired effect in minutes, we can let its action accumulate for hours. In this manner it be- comes possible and easy to photograph objects so faint that they have never been seen, even with our most powerful telescopes. This achievement ranks high among those which make astronomy appeal so strongly to the imag- ination. Scientific men are not given to fancies ; nor should they be. But the first long-exposure photograph must have been an exciting thing. 8 4 Star-Field in Constellation Monoceros. Photographed by Barnard, February I, 1894. Exposure, three hours. PHOTOGRAPHY IN ASTRONOMY After coming from the observatory, the chemical development was, of course, made in a dark room, so that no additional light might harm the plate until the process was complete. Carrying it out then into the light, that early experi- menter cannot but have felt a thrill of triumph ; for his hand held a true picture of dim stars to the eye unlighted, lifted into view as if by magic. Plates have been thus exposed as long as twenty-five hours, and the manner of doing it is very interesting. Of course, it is impossible to carry on the work continuously for so long a period, since the beginning of daylight would surely ruin the photograph. In fact, the astron- omer must stop before even the faintest streak of dawn begins to redden the eastern sky. More- over, making astronomical negatives requires ex- cessively close attention, and this it is impossible to give continuously during more than a few hours. But the exposure of a single plate can be extended over several nights without difficulty. It is merely necessary to close the plate-holder with a " light-tight " cover when the first night's work is finished. To begin further exposure of 85 PHOTOGRAPHY IN ASTRONOMY the same plate on another night, we simply aim the photographic telescope at precisely the same point of the sky as before. The light-tight plate-holder being again opened, the exposure can go on as if there had been no interruption. 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 exactness. This is a very important mat- ter ; for the slightest disturbance of the plate be- fore the second or subsequent portions of the ex- posure would ruin everything. Instead of a very complete single picture, we should have two partial ones mixed up together in inextricable confusion. To prevent this, photographic telescopes are made double, not altogether unlike an opera-glass. One of the tubes is arranged for photography proper, while the other is fitted with lenses suitable for an ordinary visual telescope. The two tubes are made parallel. Thus the astronomer, by look- ing through the visual glass, can watch objects in the heavens even while they are being photo- graphed. The visual half of the instrument is provided with a pair of very fine cross-wires mov- 86 PHOTOGRAPHY IN ASTRONOMY able at will in the field of view. These can be made to bisect some little star exactly, before be- ginning the first night's work. A fterward, every- thing about the instrument having been left un- changed, the astronomer can always assure himself of coming back to precisely the same point of the sky, by so adjusting the instrument that the same little star is again bisected. It must not be supposed, however, that the entire instrument remains unmoved, even during the whole of a single night's exposure. For in that case, 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 view. Consequent- ly, this motion has to be counteracted by shift- ing the telescope so as to follow the stars. This can be accomplished accurately and automatically by means of clock-work mechanism. Such con- trivances have already been applied in the past to visual telescopes, because even then they facili- tated the observer's work. They save him the trouble of turning his instrument every few min- utes, and allow him to give his undivided atten- tion to the actual business of observation. 87 PHOTOGRAPHY IN ASTRONOMY For photographic purposes the telescope needs to " follow " the stars far more accurately than in the older kind of observing with the eye. Nor is it possible to make a clock that will drive the in- strument satisfactorily and quite automatically. But by means of the second or visual telescope, astronomers can always ascertain whether the clock is working correctly at any given moment. It requires only a glance at the little star bisected by the cross-wires, and, if there has been the slightest imperfection in the following by clock- work, the star will no longer be cut exactly by the wires. The astronomer can at once correct any error by putting in operation a very ingenious me- chanical device sometimes called a "mouse- control." He need only touch an electric but- ton, and a signal is sent into the clock-work. Instantly there is a shifting of the mechanism. For one of the regular driving wheels is substi- tuted, temporarily, another having an extra tooth. This makes the clock run a little faster so long as the electric current passes. In a similar way, by means of another button, the clock can be 88 PHOTOGRAPHY IN ASTRONOMY made to run slower temporarily. Thus by watching the cross-wires continuously, and ma- nipulating his two electric buttons, the photo- graphic astronomer can compel his telescope to follow exactly the object under observation, and he can make certain of obtaining a perfect neg- ative. These long-exposure plates are intended espe- cially for what may be called descriptive astron- omy. With them, as we have seen, advantage is taken of cumulative light-effects on the sensitive plate, and the telescope's light - gathering and space - penetrating powers are vastly increased. We are enabled to carry our researches far be- yond the confines of the old visible universe. Extremely faint objects can be recorded, even down to their minutest details, with a fidelity un- known to older visual methods. But at present we intend to consider principally applications of photography in the astronomy of measure- ment, rather than the descriptive branch of our subject. Instead of describing pictures made simply to see what certain objects look like in the sky, we shall consider negatives intended for 8 9 PHOTOGRAPHY IN ASTRONOMY precise measurement, with all that the word pre- cision implies in celestial science. Taking up first the photography of stars, we must begin by mentioning the work of Ruther- furd at New York. More than thirty years ago he had so far perfected methods of stellar pho- tography that he was able to secure excellent pictures of stars as faint as the ninth magnitude. In those days the modern process of dry-plate photography had not been invented. To-day, plates exposed in the photographic telescope are made of glass covered with a perfectly dry film of sensitized gelatine. But in the old wet-plate process the sensitive film was first wetted with a chemical solution ; and this solution could not be allowed to dry during the exposure. Conse- quently, Rutherfurd was limited to exposures a few minutes in length, while nowadays, as we have said, their duration can be prolonged at will. When we add to this the fact that the old plates were far less sensitive to light than those now available, it is easy to see what were the diffi- culties in the way of photographing faint stars in Rutherfurd's time. Nor did he possess the 90 PHOTOGRAPHY IN ASTRONOMY modern ingenious device of a combined visual and photographic instrument. He had no elec- tric controlling apparatus. In fact, the younger generation of astronomers can form no adequate idea of the patience and personal skill Ruther- furd must have had at his command. For he certainly did produce negatives that are but little inferior to the best that can be made to-day. His only limitation was that he could not obtain images of stars much below the ninth magnitude. To understand just what is meant here by the ninth magnitude, it is necessary to go back in im- agination to the time of Hipparchus, the father of sidereal astronomy. (See page 39.) He adopted the convenient plan of dividing all the stars visible to the naked eye (of course, he had no telescope) into six classes, according to their brilliancy. The faintest visible stars were put in the sixth class, and all the others were assigned somewhat arbitrarily to one or the other of the brighter classes. Modern astronomers have devised a more sci- entific system, which has been made to conform 91 PHOTOGRAPHY IN ASTRONOMY very nearly to that of Hipparchus, just as it has come down to us through the ages. We have adopted a certain arbitrary degree of luminosity as the standard " first-magnitude"; compared with sunlight, this may be represented roughly by a fraction of which the numerator is i, and the de- nominator about eighty thousand millions. The standard second- magnitude star is one whose light, compared with a first-magnitude, may be represented approximately by the fraction |-. The third magnitude, in turn, may be compared with the second by the same fraction f ; and so the classification is extended to magnitudes below those visible to the unaided eye. Each magni- tude compares with the one above it, as the light of two candles would compare with the light of five. Rutherfurd did not stop with mere photo- graphs. He realized very clearly the obvious truth that by making a picture of the sky we simply change the scene of our operations. Upon the photograph we can measure that which we might have studied directly in the heavens ; but so long as they remain unmeasured, celestial 92 PHOTOGRAPHY IN ASTRONOMY pictures have a potential value only. Locked within them may lie hidden some secret of our universe. But it will not come forth unsought. Patient effort must precede discovery, in pho- tography, as elsewhere in science. There is no royal road. Rutherfurd devised an elaborate measuring-machine in which his photographs could be examined under the microscope with the most minute exactness. With this machine he measured a large number of his pictures ; and it has been shown quite recently that the results obtained from them are comparable in accuracy with those coming from the most highly ac- credited methods of direct eye-observation. And photographs are far superior in ease of ma- nipulation. Convenient day-observing under the microscope in a comfortable astronomical labora- tory is substituted for all the discomforts of a midnight vigil under the stars. The work of measurement can proceed in all weathers, whereas formerly it was limited strictly to perfectly clear nights. Lastly, the negatives form a permanent record, to which we can always return to correct errors or re-examine doubtful points. 93 PHOTOGRAPHY IN ASTRONOMY Rutherford's stellar work extended down to about 1877, and included especially parallax de- terminations and the photography of star-clusters. Each of these subjects is receiving close attention from later investigators, and, therefore, merits brief mention here. Stellar parallax is in one sense but another name for stellar distance. Its measurement has been one of the important problems of astronomy for centuries, ever since men recognized that the Copernican theory of our universe requires the determination of stellar distances for its complete demonstration. If the earth is swinging around the sun once a year in a mighty path or orbit, there must be changes of its position in space comparable in size with the orbit itself. And the stars ought to shift their apparent places on the sky to correspond with these changes in the terrestrial observer's position. The phenomenon is analogous to what occurs when we look out of a room, first through one window, and then through another. Any object on the opposite side of the street will be seen in a changed direction, on account of the observer's having shifted his position from one 94 PHOTOGRAPHY IN ASTRONOMY window to the other. If the object seemed to be due north when seen from the first window, it will, perhaps, appear a little east of north from the other. But this change of direction will be comparatively small, if the object under observa- tion is very far away, in comparison with the dis- tance between the two windows. This is what occurs with the stars. The earth's orbit, vast as it is, shrinks into almost absolute insignificance when compared with the profound distances by which we are sundered from even the nearest fixed stars. Consequently, the shift- ing 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 be- yond human ken. Photography lends itself most readily to a study of this matter. Suppose a certain star is suspected of "having a parallax." In other words, we have reason to believe it near enough to admit of a successful measurement of distance. Perhaps it is a very bright star ; and, other things being equal, it is probably fair to assume that brightness signifies nearness. And astrono- 95 PHOTOGRAPHY IN ASTRONOMY mers have certain other indications of proximity that guide them in the selection of proper objects for investigation, though such evidence, of course, never takes the place of actual measurement. 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 away. The parallax star is among them, but not of them. We see it projected upon the background of the heavens, though it may in reality be quite near us, astronomically speaking. If this is really so, and the star, therefore, subject to the slight parallactic shifting already men- tioned, we can detect it by noting the suspected star's position among the surrounding small stars. For these, being immeasurably remote, will remain unchanged, within the limits of our powers of observation, and thus serve as points of reference for marking the apparent shifting of the brighter star we are actually considering. We have merely to photograph the region at various seasons of the year. Careful examina- tion of the photographs under the microscope will then enable us to measure the slightest dis- 9 6 PHOTOGRAPHY IN ASTRONOMY placement of the parallax star. From these measures, by a process of calculation, astrono- mers can then obtain the star's distance. It will not become known in miles ; we shall only ascer- tain how many times the distance between the earth and sun would have to be laid down like a measuring-rod, in order to cover the space sepa- rating us from the star : and the subsequent evaluation of this distance " earth to sun " in miles is another important problem in whose so- lution photography promises to be most useful. The above method of measuring stellar distance is, of course, subject to whatever slight uncertainty arises from the assumption that the small stars used for comparison are themselves beyond the possibility of parallactic shifting. But astron- omy possesses no better method. Moreover, the number of small stars used in this way is, of course, much larger in photography than it ever can be in visual work. In the former process, all surrounding stars can be photographed at once ; in the latter each star must be measured separately, and daylight soon intervenes to im- pose a limit on numbers. Usually only two can 97 PHOTOGRAPHY IN ASTRONOMY be used ; so that here photography has a most important advantage. It minimizes the chance of our parallax being rendered erroneous, by the stars of comparison not being really infinitely remote. This might happen, perhaps, in the case of one or two ; but with an average result from a large number we know it to be practically impossible. Cluster work is not altogether unlike " paral- lax hunting " in its preliminary stage of securing the photographic observations. The object is to obtain an absolutely faithful picture of a star group, just as it exists in the sky. We have every reason to suppose that a very large num- ber of stars condensed into one small spot upon the heavens means something more than chance aggregation. The Pleiades group (page 10) con- tains thousands of massive stars, doubtless held together by the force of their mutual gravita- tional attraction. If this be true, there must be complex orbital motion in the cluster ; and, as time goes on, we should actually see the sepa- rate components change their relative positions, as it were, before our eyes. The details of such 9 8 PHOTOGRAPHY IN ASTRONOMY motion upon the great scale of cosmic space offer one of the many problems that make astronomy the grandest of human sciences. We have said that time must pass before we can see these things ; there may be centuries of waiting. But one way exists to hurry on the perfection of our knowledge ; we must increase the precision of observations. Motions that would need the growth of centuries to become visible to the older astronomical appliances, might yield in a few decades to more delicate observational processes. Here photography is most promising. Having once obtained a sur- passingly accurate picture of a star-cluster, we can subject it easily to precise microscopic meas- urement. The same operations repeated at a later date will enable us to compare the two series of measures, and thus ascertain the mo- tions that may have occurred in the interval. The Rutherfurd photographs furnish a veritable mine of information in researches of this kind ; for they antedate all other celestial photographs of precision by at least a quarter-century, and bring just so much nearer the time when definite 99 PHOTOGRAPHY IN ASTRONOMY knowledge shall replace information based on reasoning from probabilities. Rutherfurd's methods showed the advantages of photography as applied to individual star- clusters. It required only the attention of some astronomer disposing of large observational facili- ties, and accustomed to operations upon a great scale, to apply similar methods throughout the whole heavens. In the year 1882 a bright comet was very conspicuous in the southern heavens. It was extensively observed from the southern hemisphere, and especially at the British Royal Observatory at the Cape of Good Hope. Gill, director of that institution, conceived the idea that this comet might be bright enough to photograph. At that time, comet photogra- phy had been attempted but little, if at all, and it was by no means sure that the experiment would be successful. Nor was Gill well acquainted with the work of Rutherfurd ; for the best results of that astronomer had lain dormant many years. He was one of those men with whom personal modesty amounts to a fault. Loath to put him- self forward in any way, and disliking to rush 100 PHOTOGRAPHY IN ASTRONOMY into print, Rutherfurd had given but little pub- licity to his work. This peculiarity has, doubt- less, delayed his just reputation ; but he will lose nothing in the end from a brief postponement. Gill must, however, be credited with more pene- tration than would be his due if Rutherfurd had made it possible for others to know that he had anticipated many of the newer ideas. However this may be, the comet was photo- graphed with the help of a local portrait photog- rapher named Allis. When Gill and Allis fast- ened a simple portrait camera belonging to the latter upon the tube of one of the Cape tele- scopes, and pointed it at the great comet, they little thought the experiment would lead to one of the greatest astronomical works ever at- tempted by men. Yet this was destined to oc- cur. The negative they obtained showed an excellent picture of the comet ; but what was more important for the future of sidereal astron- omy, it was also quite thickly dotted with little black points corresponding to stars. The extra- ordinary ease with which the whole heavens could be thus charted photographically was IOI PHOTOGRAPHY IN ASTRONOMY brought home to Gill as never before. It was this comet picture that interested him in the ap- plication of photography to star-charting ; and without his interest the now famous astro-photo- graphic catalogue of the heavens would probably never have been made. After considerable preliminary correspondence, a congress of astronomers was finally called to meet at Paris in 1887. Representatives of the principal observatories and civilized governments were present. They decided that the end of the nineteenth century should see the making of a great catalogue of all the stars in the sky, upon a scale of completeness and precision surpassing anything previously attempted. It is impossible to exaggerate the importance of such a work ; for upon our star-catalogues depends ultimately the entire structure of astronomical science. The work was far too vast for the powers of any observatory alone. Therefore, the whole sky, from pole to pole, was divided into eighteen belts or zones of approximately equal area ; and each of these was assigned to a single observa- tory to be photographed. A series of telescopes 102 PHOTOGRAPHY IN ASTRONOMY was specially constructed, so that every part of the work should be done with the same type of instrument. As far as possible, an attempt was made to secure uniformity of methods, and par- ticularly a uniform scale of precision. To cover the entire sky upon the plan proposed no less than 44,108 negatives are required; and most of these have now been finished. The further measurement of the pictures and the drawing up of a vast printed star-catalogue are also well un- der way. One of the participating observatories, that at Potsdam, Germany, has published the first volume of its part of the catalogue. It is esti- mated that this observatory alone will require twenty quarto volumes to contain merely the final results of its work on the catalogue. Alto- gether not less than two million stars will find a place in this, our latest directory of the heavens. Such wholesale methods of attacking problems of observational astronomy are particularly char- acteristic of photography. The great catalogue is, perhaps, the best illustration of this tendency ; but of scarcely smaller interest, though less im- portant in reality, is the photographic method of 103 PHOTOGRAPHY IN ASTRONOMY dealing with minor planets. We have already said (page 63) that in the space between the orbits of Mars and Jupiter several hundred small bodies are moving around the sun in ordinary planetary orbits. These bodies are called asteroids, or minor planets. The visual method of discover- ing unknown members of this group was pain- fully tedious ; but photography has changed matters completely, and has added immensely to our knowledge of the asteroids. Wolf, of Heidelberg, first made use of the new process for minor-planet discovery. His method is sufficiently ingenious to deserve brief mention again. A photograph of a suitable re- gion of the sky was made with an exposure last- ing two or three hours. Throughout all this time the instrument was manipulated so as to follow the motion of the heavens in the way we have already explained, so that each star would appear on the negative as a small, round, black dot. But if a minor planet happened to be in the region covered by the plate, its photographic image would be very different. For the orbital 104 PHOTOGRAPHY IN ASTRONOMY 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 ex- posed. This motion would be faithfully repro- duced in the picture, so that the planet would appear as a short curved line rather than a well- defined dot like a star. Thus the presence of such a line-image infallibly denotes an asteroid. Subsequent calculations are necessary to ascer- tain whether the object is a planet already known or a genuine new discovery. Wolf, and others using his method in recent years, have made im- mense additions to our catalogue of asteroids. Indeed, the matter was beginning to lose inter- est on account of the frequency and sameness of these discoveries, when the astronomical world was startled by the finding of the Planet of 1898. (Page 58.) On August 27, 1898, Witt, of Berlin, discov- ered the small body that bears the number "433 " in the list of minor planets, and has re- ceived the name Eros. Its important peculiar- ity consists in the exceptional position of the orbit. While all the other asteroids are farther 105 PHOTOGRAPHY IN ASTRONOMY from the sun than Mars, and less distant than Jupiter, Eros can pass within the orbit of the former. At times, therefore, it will approach our earth more closely than any other permanent member of the solar system, excepting our own moon. So it is, in a sense, our nearest neigh- bor ; and this fact alone makes it the most inter- esting of all the minor planets. The nineteenth century was opened by Piazzi's well-known dis- covery of the first of these bodies (page 59) ; it is, therefore, fitting that we should find the most important one at its close. We are almost cer- tain that it will be possible to make use of Eros to solve with unprecedented accuracy the most important problem in all astronomy. This is the determination of our earth's distance from the sun. When considering stellar parallax, 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 unit. It is, indeed, the fun- damental unit for all astronomical measures, and its exact evaluation has always been considered the basal problem of astronomy. Astronomers know it as the problem of Solar Parallax. 106 PHOTOGRAPHY IN ASTRONOMY We shall not here enter into the somewhat intricate details of this subject, however interest- ing they may be. The problem offers diffi- culties somewhat analogous to those confronting a surveyor who has to determine the distance of some inaccessible terrestrial point. To do this, it is necessary first to measure a " base-line," as we call it. 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 calculation. To insure accuracy, however, as every surveyor knows, the base-line must be made long enough ; and this is precise- ly what is impossible in the case of the solar parallax. For we are necessarily limited to marking out our base-line on the earth ; and the entire planet is too small to furnish one of really suffi- cient size. The best we can do is to use the dis- tance between two observatories situated, as near as may be, on opposite sides of the earth. But even this base is wofully small. However, the smallness loses some of its harmful effect if we operate upon a planet that is comparatively near 107 PHOTOGRAPHY IN ASTRONOMY us. We can measure such a planet's distance more accurately than any other ; and this being known, the solar distance can be computed by the aid of mathematical considerations based upon Newton's law of gravitation and observa- tional determinations of the planetary orbital elements. Photography is by no means limited to inves- tigations in the older departments of astronom- ical observation. Its powerful arm has been stretched out to grasp as well the newer instru- ments of spectroscopic study. Here the sensi- tive plate has been substituted for the human eye with even greater relative advantage. The accurate microscopic measurement of difficult lines in stellar spectra was indeed possible by older methods ; but photography has made it comparatively easy ; and, above all, has ren- dered practicable series of observations extensive enough in numbers to furnish statistical informa- tion of real value. Only in this way have we been able to determine whether the stars, in their varied and unknown orbits, are approaching us or moving farther away. Even the speed of this 108 Solar Corona. Total Eclipse. Photographed by Campbell, January 22, 1898 ; Jeur, India. PHOTOGRAPHY IN ASTRONOMY approach or recession has become measurable, and has been evaluated in the case of many in- dividual stars. (See page 21.) The subject of solar physics has become a ver- itable department of astronomy in the hands of photographic investigators. Ingenious spectro- photographic methods have been devised, where- by we have secured pictures of the sun from which we have learned much that must have remained forever unknown to older methods. Especially useful has photography proved itself in the observation of total solar eclipses. 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 co- rona, that great appendage of our sun, whose exact nature is still unexplained. Only during the few minutes of total eclipse in each century can we look upon it ; and keen is the interest of astronomers when those few minutes occur. But it is found that eye observations made in hur- ried excitement have comparatively little value. Half a dozen persons might make drawings of the corona during the same eclipse, yet they 109 PHOTOGRAPHY IN ASTRONOMY would differ so much from one another as to leave the true .outline very much in doubt. But with photography we can obtain a really correct picture whose details can be studied and dis- cussed subsequently at leisure. If we were asked to sum up in one word what photography has accomplished, we should say that observational astronomy has been revolu- tionized. There is to-day scarcely an instru- ment of precision in which the sensitive plate has not been substituted for the human eye ; scarcely an inquiry possible to the older method which cannot now be undertaken upon a grander scale. Novel investigations formerly not even possible are now entirely practicable by photog- raphy ; and the end is not yet. Valuable as are the achievements already consummated, photog- raphy is richest in its promise for the future. Astronomy has been called the " perfect sci- ence " ; it is safe to predict that the next gen- eration will wonder that the knowledge we have to-day should ever have received so proud a title. no TIME STANDARDS OF THE WORLD THE question is often asked, " What is the practical use of astronomy ? " We know, of course, that men would profit greatly from a study of that science, even if it could not be turned to any immediate bread-and-butter use ; for astron- omy is essentially the science of big things, and it makes men bigger to fix their minds on problems that deal with vast distances and seemingly end- less periods of time. 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 born, and so shall continue after he has passed away aye, even after his lat- est descendants shall have vanished from the earth. Of all the sciences, astronomy is at once the most beautiful poetically, and yet the one offering the grandest and most difficult problems to the intellect. A study of these problems has in TIME STANDARDS OF THE WORLD ing to wind his watch at the accustomed hour. The next morning he finds it run down. It must be re-set. Most people simply go to the nearest clock, or ask some friend for the time, so as to start the watch correctly. More careful per- sons, perhaps, visit the jeweller's and take the time from his " regulator." But the regulator itself needs to be regulated. After all, it is noth- ing more than any other clock, except that greater care has been taken in the mechanical construction and arrangement of its various parts. Yet it is but a machine built by human hands, and, like all human works, it is necessarily imperfect. No matter how well it has been constructed, it will not run with perfectly rigid accuracy. Every day there will be a variation from the true time by a small amount, and in the course of days or weeks the accumulation of these successive small amounts will lead to a total of quite appreciable size. Just as the ordinary citizen looks to the jewel- ler's regulator to correct his watch, so the jeweller applies to the astronomer for the correction of his regulator. Ever since the dawn of astronomy, in the earliest ages of which we have any record, the 114 TIME STANDARDS OF THE WORLD principal duty of the astronomer has been the fur- nishing of accurate time to the people. We shall not here enter into a detailed account, however in- teresting it would be, of the gradual development by which the very perfect system at present in use has been reached ; but shall content ourselves with a description of the methods now employed in nearly all the civilized countries of the world. In the first place, every observatory is, of course, provided with what is known as an astronomical clock. This instrument, from the astronomer's point of view, is something very different from the ordinary popular idea. To the average per- son an astronomical clock is a complicated and elaborate affair, giving the date, day of the week, phases of the moon, and other miscellaneous in- formation. But in reality the astronomer wants none of these things. His one and only require- ment is that the clock shall keep as near uniform time as may be possible to a machine constructed by human hands. No expense is spared in mak- ing the standard clock for an observatory. Real artists in mechanical construction men who have attained a world-wide celebrity for delicate skill "5 TIME STANDARDS OF THE WORLD in fashioning the parts of a clock such are the astronomer's clock-makers. To increase precision of motion in the train of wheels, it is necessary that the mechanism be as simple as possible. For this reason all complica- tions of date, etc., are left out. We have even abandoned the usual convenient plan of having the hour and minute hands mounted at the same centre ; for this kind of mounting makes neces- sary a slightly more intricate form of wheelwork. The astronomer's clock usually has the centres of the second hand, minute hand, and hour hand in a straight line, and equally distant from each other. Each hand has its own dial ; all drawn, of course, upon the same clock-face. Even after such a clock has been made as ac- curately as possible, it will, nevertheless, not give the very best performance unless it is taken care of properly. It is necessary to mount it very firmly indeed. It should not be fastened to an ordinary wall, but a strong pier of masonry or brick must be built for it on a very solid founda- tion. Moreover, this pier is best placed under- ground in a cellar, so that the temperature of the 116 TIME STANDARDS OF THE WORLD clock can be kept nearly uniform all the year round ; for we find that clocks do not run quite the same in hot weather as they do in cold. Makers have, indeed, tried to guard against this effect of temperature, by ingenious mechanical contrivances. But these are never quite perfect in their action, and it is best not to test them too severely by exposing the clock to sharp changes of heat and cold. Another thing affecting the going of fine clocks, strange as it may seem, is the variation of barometric pressure. There is a slight but no- ticeable difference in their running when the barometer is high and when it is low. To pre- vent this, some of our best clocks have been en- closed in air-tight cases, so that outside barometric changes may not be felt in the least by the clock itself. But even after all this has been accomplished, and the astronomer is in possession of a clock that may be called a masterpiece of mechanical con- struction, he is not any better off than was the jeweller with his regulator. After all, even the astronomical clock needs to be set, and its error 117 TIME STANDARDS OF THE WORLD must be determined from time to time. A final appeal must then be had to astronomical observations. The clock must be set by the stars and sun. For this purpose the astronomer uses an instrument called a " transit." This is simply a telescope of moderate size, possibly five or six feet long, and firmly attached to an axis at right angles to the tube of the telescope. This axis is supported horizontally in such a way that it points as nearly as may be exactly east and west. The telescope itself being square with the axis, always points in a north-and-south direction. 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 observa- tory. In the field of view of the telescope cer- tain very fine threads are mounted so as to form a little cross. As the telescope is rotated this cross traces out, as it were, a great circle on the sky ; and this great circle is called the astronomi- cal meridian. Now we are in possession of certain star-tables, computed from the combined observations of astronomers in the last 150 years. These tables us TIME STANDARDS OF THE WORLD tell us the exact moment of time when any star is on the meridian. To discover, therefore, whether our clock is right on any given night, it is merely necessary to watch a star with the tele- scope, and note the exact instant by the clock when it reaches the little cross in the field of view. Knowing from the astronomical tables the time when the star ought to have been on the meridian, and having observed the clock time when it is actually there, the difference is, of course, the error of the clock. The result can be checked by observations of other stars, and the slight personal errors of observation can be rendered harmless by taking the mean from sev- eral stars. 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 second. We have not space to enter into the interest- ing details of the methods by which the astro- nomical transit is accurately set in the right position, and how any slight residual error in its setting can be eliminated from our results by certain processes of computation. It must suffice to say that practically all time determinations in 119 TIME STANDARDS OF THE WORLD the observatory depend substantially upon the procedure outlined above. The observatory clock having been once set right by observations of the sky, its error can be re-determined every few days quite easily. Thus even the small irregularities of its nearly perfect mechanism can be prevented from accumulating until they might reach a harmful magnitude. But we obtain in this way only a correct standard of time within the observatory itself. How can this be made available for the general public ? The problem is quite simple with the aid of the electric telegraph. We shall give a brief account of the methods now in use in New York City, and these may be taken as essentially representa- tive of those employed elsewhere. Every day, at noon precisely, an electric signal is sent out by the United States Naval Observa- tory in Washington. The signal is regulated by the standard clock of the observatory, of course taking account of star observations made on the next preceding fine night. This signal is re- ceived in the central New York office of the tele- graph company, where it is used to keep correct TIME STANDARDS OF THE WORLD a very fine clock, which may be called the time standard of the telegraph company. This clock, in turn, has automatic electric connections, by means of which it is made to send out signals over what are called " time wires " that go all over the city. Jewellers, and others who desire correct time, can arrange to have a small electric sounder in their offices connected with the time wires. Thus the ticks of the telegraph com- pany's standard clock are repeated automatically in the jeweller's shop, and used for controlling the exactness of his regulator. This, in brief, is the method by which the astronomer's careful determination of correct time is transferred and distributed to the people at large. Having thus outlined the manner of obtaining and distributing correct time, we shall now consider the question of time differences between different places on the earth. This is a matter which many persons find most perplexing, and yet it is essential- ly quite simple in principle. Travellers, of course, are well acquainted with the fact that their watches often need to be reset when they arrive at their destination. Yet few ever stop to ask the cause. 121 TIME STANDARDS OF THE WORLD Let us consider for a moment our method of measuring time. We go by the sun. If we leave out of account some small irregularities of the sun's motion that are of no consequence for our present purpose, we may lay down this fundamental prin- ciple : When the sun reaches its highest position in the sky it is twelve o'clock or noon. The sun, as everyone knows, rises each morn- ing in the east, slowly goes up higher and higher in the sky, and at last begins to descend again toward the west. But it is clear that as the sun travels from east to west, it must pass over the eastern one of any two cities sooner than the western one. When it reaches its greatest height over a western city it has, therefore, already passed its greatest height over an eastern one. In other words, when it is noon, or twelve o'clock, in the western city, it is already after noon in the "eastern city. This is the simple and evident cause of time differences in different parts of the country. Of any two places the eastern one always has later time than the western. When we consider the matter in this way there is not the slightest diffi- culty in understanding how time differences arise. 122 TIME STANDARDS OF THE WORLD They will, of course, be greatest for places that are very far apart in an east-and-west direction. And this brings us again to the subject of longi- tude, which, as we have already said, plays an important part in all questions relating to time ; for longitude is used to measure the distance in an east-and-west direction between different parts of the earth. 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 Pole. Such a circle could be drawn through any point on the earth. If we imagine a pair of them drawn through two cities, such as New York and London, the longitude difference of these two cities is defined as the angle at the North Pole be- tween the two great circles in question. The size of this angle can be expressed in degrees. If we then wish to know the difference in time between New York and London in hours, we need only divide their longitude difference in degrees by the number 15. In this simple way we can get the time difference of any two places. We merely measure the longitude difference on a map, and 123 TIME STANDARDS OF THE WORLD then divide by 15 to get the time difference. These time differences can sometimes become quite large. Indeed, for two places differing 180 degrees in longitude, the time difference will evi- dently be no less than twelve hours. Most civilized nations have agreed informally to adopt some one city as the fundamental point from which all longitudes are to be counted. Up to the present we have considered only longitude differences ; but when we speak of the longitude of a city we mean its longitude difference from the place chosen by common consent as the ori- gin for measuring longitudes. The town almost universally used for this purpose is Greenwich, near London, England. Here is situated the British Royal Observatory, one of the oldest and most important institutions of its kind in the world. The great longitude circle passing through the centre of the astronomical transit at the Green- wich observatory is the fundamental longitude circle of the earth. 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 described. 124 TIME STANDARDS OF THE WORLD Longitudes are counted both eastward and westward from Greenwich. Thus New York is in 74 degrees west longitude, while Berlin is in 14 degrees east longitude. This has led to a rather curious state of affairs in those parts of the earth the longitudes of which are nearly 180 degrees east or west. There are a number of islands in that part of the world, and if we imagine for a moment one whose longitude is just 180 de- grees, we shall have the following remarkable re- sult as to its time difference from Greenwich. We have seen that of any two places the eastern always has the later time. Now, since our imag- inary island is exactly 180 degrees from Green- wich, we can consider it as being either 1 80 de- grees east or 180 degrees west. But if we call it 1 80 degrees east, its time will be twelve hours later than Greenwich, and if we call it 180 degrees west, its time will be twelve hours earlier than Greenwich. Evidently there will be a difference of just twenty-four hours, or one whole day, be- tween these two possible ways of reckoning its time. This circumstance has actually led to con- siderable confusion in some of the islands of the 125 TIME STANDARDS OF THE WORLD Pacific Ocean. The navigators who discovered the various islands naturally gave them the date which they brought from Europe. And as some of these navigators sailed eastward, around the Cape of Good Hope, and others westward, around Cape Horn, the dates they gave to the several islands differed by just one day. The state of affairs at the present time has been adjusted by a sort of informal agreement. An arbitrary line has been drawn on the map near the iSoth longitude circle, and it has been decided that the islands on the east side of this line shall count their longitudes west from Green- wich, and those west of the line shall count lon- gitude east from Greenwich. Thus Samoa is nearly 180 degrees wes v t of Greenwich, while the Fiji Islands are nearly 180 degrees east. Yet the islands are very near each other, though the ar- bitrary line passes between them. As a result, when it is Sunday in Samoa it is Monday in the Fiji Islands. The arbitrary line described here is sometimes called the International Date-Line. It does not pass very near the Philippine Islands, which are situated in about 120 degrees 126 TIME STANDARDS OF THE WORLD east longitude, and, therefore, use a time about eight hours later than Greenwich. New York, being about 74 degrees west of Greenwich, is about five hours earlier in time. Consequently, as we may remark in passing, Philippine time is about thirteen hours later than New York time. Thus, five o'clock, Sunday morning, May ist, in Manila, would correspond to four o'clock, Saturday afternoon, April 3Oth, in New York. There is another kind of time which we shall ex- plain briefly the so-called " standard," or railroad time, which came into general use in the United States some few years ago, and has since been generally adopted throughout the world. It re- quires 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 hours, minutes, and seconds. Thus a great deal of inconvenience has been caused in the past. For instance, a train might leave New York at a certain hour by New York time. It would then arrive in Buf- falo some hours later by New York time. But it would leave Buffalo by Buffalo time, which is 127 TIME STANDARDS OF THE WORLD quite different. Thus there would be a sort ot jump in the time-table at Buffalo, and it would be a jump of an odd number of minutes. It would be different in different cities, and very hard to remember. Indeed, as each railway usu- ally ran its trains by the time used in the princi- pal city along its line, it might happen that three or four different railroad times would be used in a single city where several roads met. This has all been avoided by introducing the standard time system. According to this the whole coun- try is divided into a series of time zones, fifteen degrees wide, and so arranged that the middle line of each zone falls at a point whose longitude from Greenwich is 60, 75, 90, 105, or 120 de- grees. The times at these middle lines are, therefore, earlier than Greenwich time by an even number of hours. Thus, for instance, the 75~degree line is just five even hours earlier than Greenwich time. All cities simply use the time of the nearest one of these special lines. This does not result in doing away with time differences altogether that would, of course, be impossible in the nature of things but for the 128 TIME STANDARDS OF THE WORLD complicated odd differences in hours and minutes, we have substituted the infinitely simpler series of differences in even hours. 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 unchanged, and no New York timepiece need be consulted to set the watch right on arriving. There can be no doubt that this standard-time system must be consid- ered one of the most important contributions of astronomical science to the convenience of man. Its value has received the widest recognition, and its use has now extended to almost all civil- ized countries France is the only nation of im- portance still remaining outside the time-zone system. In the following table we give the standard time of the various parts of the earth as compared with Greenwich, together with the date of adopting the new time system. It will be noticed that in certain cases even half-hours have been employed to separate the time-zones, in- stead of even hours as used in the United States. 129 TIME STANDARDS OF THE WORLD TABLE OF THE WORLD'S TIME STANDARDS When it is Noon at Greenwich it is In Date of Adopting Standard Time System. Noon Great Britain. Belgium. May, 1892. Holland. May, 1892. Spain. January, 1901. I P.M. Germany. April, 1893. Italy. November, i 893. Denmark. January, 1894. Switzerland. June, 1894. Norway. January, 1895. Austria (railways). 1.30 P.M. Cape Colony. 1892. Orange River Colony. 1892. Transvaal. 1 1892. 2 P.M. Natal. September, 1895. Turkey (railways). Egypt. October, 1900. 8 P.M. West Australia. February, 1895. 9 P.M. Japan. 1896. 9.30 P.M. South Australia. May, 1899. IO P.M. Victoria. February, 1895. New South Wales. February, 1895. Queensland. February, 1895. II P.M. New Zealand. In the United States and Canada it is 4 A.M. by Pacific Time when it is Noon at Greenwich. 5 A.M. ' Mountain " 6 A.M. " Central " Io6 discovery by photography 64 group of 63 photography of, invented by Wolf IO 4 Astronomer, royal J working, description of I5 2 ASTRONOMER'S POLE, THE 184 Astronomy, journalistic 176 practical uses of 112 Atmospheric refraction, explained 193 Axis, of figure of the earth 136 of rotation of the earth 136 polar, of telescope 173 Barnard, discovers satellite of Jupiter 51 Bessel, measures Pleiades 15 Bond, discovers crape ring of Saturn 144 Bradley, observes at Greenwich 219 Brahe, Tycho, his temporary star 40 Bruce, endows polar photography 197 225 INDEX PAGE Campbell, observes Pole-star 18 Cape of Good Hope, observatory, photography at 101 telescope 1 7> 1 74 Capriccio, Galileo's 55 Cassini, shows Saturn's rings to be double H4 Cassiopeia, temporary star in 4 Celestial pole 184 Central sun theory 223 Centre of gravity 217 Chart-room, on ship-board 5 Chronometer, invention of & Circle, meridian, explained 189 Clerk-Maxwell, discusses Saturn's rings 146 Clock, affected by temperature 117 affected by barometric pressure 117 astronomical 115 astronomical, how mounted 1 16 astronomical, its dial 1 16 error of, determined with transit 1 18 jeweller's regulator 114 of telescope 175 Clusters of stars, photography of 98 Columbia University Observatory, latitude observations 139 polar photography 196 Common, his reflecting telescope 32 Confusion of dates, in Pacific Ocean 125 Congress of Astronomers, Paris, 1887 102 Constellations 162 Control, " mouse," for photography 88 Copernican theory of universe 53, 56 demonstration 94 Corkscrew tunnels 220 Crape ring of Saturn 144 Cumulative effect, in photography , 84 Date, confusion of, in Pacific Ocean 125 Date-line, international, explained 126 Development of photograph 81 226 INDEX PAGE Dial, of astronomical clock 116 "Dialogue " of Galileo 53 Differences of time, explained 121 Directions, telescopic measurement of 21 Directory of the heavens 103 Distance, of light-source in photography 83 of stars 94, 106, 1 58 of vSun 67, 97, 106 Donner, polar photography 195 Double telescopes, for photography 86 Earth, motions of its pole 131 rotation of 136, 162, 171, 184 shape of 135 Eclipses, photography of 109 Elkin, measures Pleiades 15 Equatorial telescope, explained 170 Eros, discovered by Witt , . 66, 105 its importance 67 Error of clock, determined by transit 1 18 Exposure, length of, in photography 84 Feldhausen, Herschel's observatory near Capetown 204 Fiji Islands, their date 126 Fixed polar telescope 197 " Following " the stars 88, 173 Four-day cycle of pole-star 24 France, outside time-zone system 129 Fundamental longitude meridian 124 GALILEO 47 and the Church 48 discoveries of 49 observes Saturn 141 Galle, discovers Neptune 61 Gauss, computes first asteroid orbit 60 Gautier, Paris, constructs big telescope 179 Geodetic Association, international , 139 227 INDEX PAGE Geography, maps, astronomical side of 112 Geology, polar motion in 13' Gill, photographs comet 100 Gilliss, at Naval Observatory, Washington 169 Goldsborough, at Naval Observatory, Washington 169 Grande Lunette, Paris, 1900 17> 180 Gravitation 13 in Pleiades 14, 212 law of, Newton's 212 Gravity, centre of 217 Greenwich, origin of longitudes 7 I2 4 time 7 Groombridge, English astronomer I Harrison, inventor of chronometer 8 Head, of heliometer 156 Heidelberg, photography at 104 HELIOMETER 152 head of 156 how used 157 principle of 154 scales of 158 semi-lenses of 155 Helsingfors observatory, polar photography at 195 Henry, measures Pleiades II, 17 Hercules, constellation, solar motion toward 222 Herschel, discovers apex of solar motion 221 discovers Uranus 59, 141 John, the moon hoax 200 Hipparchus, discovers precession 186 early star catalogue 21, 39 invents star magnitudes 91 Huygens, announces rings of Saturn 142 his logogriph 143 Ice-cap, of Earth 131 Index Librorum Prohibitorum 53 International, date-line, explained 126 geodetic association 139 228 INDEX PAGE Inter-stellar motion, in clusters 98 in Pleiades 14 Islands of Pacific, their longitude and time 125 Japan, latitude station in 139 Jewellers' correct time 1 2 1 Journalistic astronomy 176 Jupiter's satellites, discovered by Galileo 50 discovered by Barnard 51 Keeler, observes Saturn's rings 140, 147, 150 photographs nebulas 32 " Keyhole " nebula 205 Lambert, determines longitude of Washington 168 Laplace, discusses Saturn's rings 146 nebular hypothesis 33 stability of solar system 210 Latitude, changes of 133, 138 definition of 134 determining the 6; Leverrier, predicts discovery of Neptune 61, 142 Lick Observatory, Keeler's observations 140 Light, undulatory theory of 19, 148 Light-waves, measuring length of 20, 149 Logogriph, by Huygens 143 Long- exposure photography 85 Longitude, counted East and West 125 determining 6 determining by occultations 167 effect on time differences 123 explained 123 of Washington, first determined 168 Maclear, observes Eta Argus 205 Magnitudes, stellar 91 Manila, its time 127 Maps, astronomical side of H2 229 INDEX PAGE Meridian circle, explained 189 Milky- way, poor in nebulae 33 Minor Planets, see Asteroids. MOON, HOAX 199 motion among stars 163 mountains discovered by Galileo 49 size of, measured 166 Motion of moon 163 MOTIONS of the EARTH'S Pole 131 MOUNTING GREAT TELESCOPES 170 Naked-eye nebulae 28 Naples, Royal Observatory, latitude observations 139 Naval Observatory, Washington, noon signal 120 NAVIGATION i before chronometers 3 use of astronomy in 113 NEBULA 27 Nebula, in Andromeda 28 in Orion 30 " keyhole " 205 Nebular, hypothesis 33 structure in Pleiades 17 Nebulous stars 31 Negative, and positive, in photography 82 Neptune, discovery predicted by Leverrier 61, 142 discovery by Galle 61 Newcomb, fixes apex of solar motion 222 Newton, law of gravitation 212 longitude commission 8 New York, its telegraphic time system 120 Noon Signal, Washington . 1 20 Number, of nebulae 31, 33 of temporary stars , . . . 38 Nutation, explained 188 Occultations 161 explained 165 230 INDEX PACK Occultations, use of 166, 167 Orion nebula 30 Pacific islands, their longitude and time 125 Parallax, solar 67, 106 stellar 94, 106 measured with heliometer 158 Paris, congress of astronomers, 1887 102 exposition of 1900 1 76 Periodic motion of earth's pole 133 Perseus, constellation, temporary star in 46 Philippine Islands, their time 127 Photography, asteroid, invented by Wolf 104 congress of astronomical 102 cumulative effect of light 84 distance of light-source 83 double telescopes for 86 general star catalogue 102 IN ASTRONOMY 8l in discovery of asteroids 64, 104 in solar physics 109 in spectroscopy 108 length of exposure 84 measuring machine, Rutherfurd 93 motion of telescope for. 87 "mouse " control of telescope 88 of eclipses 109 of inter-stellar motion 99 Paris congress, 1877 102 polar 191 Rutherfurd pioneer in 90 star-clusters 98 star-distances measured by 94 summarized no wholesale methods in 103 Piazzi, discovers first asteroid 59 IQ 6 Pitkin, report to House of Representatives 168 Planetary nebulas 3 1 231 INDEX PAGE PLANET OF 1898 58 Planetoids, see Asteroids. Planets known to ancients 58 PLEIADES 10 gravitation among 212 motion among 14, 16, 98 nebular structure 17 number visible 1 1 Polar axis, of telescope 1 73 Polar photography 191 at Helsingfors 195 Pole, celestial 184 of the earth, motions of 131 THE ASTRONOMER'S 184 POLE-STAR , 18 as a binary 25 as a triple 18, 26 change of 187 its four-day cycle 24 motion toward us 24 Positive, and negative, in photography 82 Potsdam, observatory, photographic star-catalogue 103 Practical uses of astronomy 112 Precession, explained 186 Prize, for invention of chronometer 8 Ptolemaic theory of universe 56 Ptolemy, writes concerning Hipparchus 39 Railroad time, explained 127 Refraction, atmospheric, explained 193 " Regulator," the jeweller's clock 114 Ring-nebulae 31 Rings, of Saturn, see Saturn's rings. Roberts, Andromeda nebula 28 Rotation, of Earth 136, 162, 171, 184 of Saturn 150 Royal Astronomer, his duties. 2 Royal Observatory, Greenwich 124 232 INDEX PAGE Greenwich, Bradley's observations 219 Naples, latitude observations 1 39 Rutherfurd, cluster photography 99 invents photographic apparatus 93 pioneer in photography 90 stellar parallax 94 Sagredus, character in Galileo's Dialogue 55 Salusbury, Galileo's translator 50, 54 Salviati, character in Galileo's Dialogue 55 Samoa, its date 126 SATURN'S RINGS 140 analogy to planetoids 147 announced by Huygens 142 observed with spectroscope 147 shown to be double by Cassini. 144 structure and stability 145 Scales, of heliometer 158 Scorpio, constellation, temporary star in 39 Semi-lenses of heliometer 155 Sextant, how used 4 Sicily, latitude station in 139 Sidereus Nuncius, published by Galileo 52 Simplicio, character in Galileo's Dialogue 55 Sirius, brightest star 205 Size of Moon, measured 166 Socie'td de I'Optique 177 Solar parallax, see Sun's distance. physics, by photography 109 system, stability of 210 Spectroscope, its use explained 147 used on pole-star 19 to observe Saturn's rings. 147 Spiral nebulae , 31 Stability, of Saturn's rings 145 of Solar System. . , 210 Standards, time, of the world 1 1 1 table of 130 233 INDEX PAGE ' Standard " time, explained 127 Star-catalogue, general photographic 102 Star-clusters, photography of 98 Star-distances 94, 106 measured with heliometer 158 Rutherfurd 94 Star magnitudes 91 Star-motion, toward us 21 Star-tables, astronomical 118 Stars, variable 42 St. Gothard railway, tunnels 220 Sun, newspaper, the moon hoax 201 SUN-DIAL, How TO MAKE A 69 SUN'S, DESTINATION 210 distance, compared with star distance 97 measured with Eros 67, 106 motion, apex of 221 Sun-spots, discovered by Galileo 49 Systema Saturnium^ Huygens 143 Telescope, clock 175 at Paris Exposition 1 76, 180 double, for photography 86 equatorial, explained 170 first used by Galileo 49 motion of 87 mounting great 1 70 unmoving, for polar photography 197 TEMPORARY STARS 37 in Andromeda nebula 28, 29, 45 in Aquila 40 in Cassiopeia 40 in Perseus 46 in Scorpio 39 their number 38 theory of 42 Time, correct, determined astronomically 113 differences between different places 121 234 INDEX PAGE TIME STANDARDS OF THE WORLD 1 1 1 standards of the World, table of 130 system, in New York 120 zones, explained. 128 Trails, photographic 191 Transit, for determining clock error 1 18 Tycho Brahe, his temporary star 40 Ulugh Beg, early star catalogue 21 Undulatory theory, of light 19, 148 Universe, theories of 34, 53, 56 Uranus, discovered by Herschel 59, 142 Use of occultations 166, 167 Uses of astronomy, practical 1 12 Variable stars 42 in Argo 205 Vega, future pole-star 187 Visibility of stars, in day-time 191 Vision, phenomenon of 20, 149 Washington, its longitude first determined 168 Waves, explained 148 of light 20, 148 Wilkes, at Naval Observatory, Washington 169 Wilkins, imaginary voyage of 208 Witt, discovers Eros 66, 105 Wolf, M., invents asteroid photography 104 measures Pleiades 1 1 World's time standards, table of 130 Yale College, Pleiades measured at 15 Zones, time, explained 128 335 THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO SO CENTS ON THE FOURTH DAY AND TO $I.OO ON THE SEVENTH DAY OVERDUE. MAR 12 193: : ** V .MIL 26 1940 .n i(\A 1 HA MAR 8 Wtl OCT It* t y4& MAR 241947 LD 21-100m-7,'33 70 101613