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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - - - -Title: Practical Talks by an Astronomer - -Author: Harold Jacoby - -Release Date: October 29, 2016 [EBook #53396] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK PRACTICAL TALKS BY AN ASTRONOMER *** - - - - -Produced by Chris Curnow, John Campbell and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - TRANSCRIBER'S NOTE - - Italic text is denoted by _underscores_. - - Obvious typographical errors and punctuation errors have been - corrected after careful comparison with other occurrences within - the text and consultation of external sources. - - More detail can be found at the end of the book. - - - - - PRACTICAL TALKS BY AN ASTRONOMER - -[Illustration: The Moon. First Quarter. - -Photographed by Loewy and Puiseux, February 13, 1894.] - - - - - PRACTICAL TALKS BY AN ASTRONOMER - - BY - HAROLD JACOBY - - ADJUNCT PROFESSOR OF ASTRONOMY IN - COLUMBIA UNIVERSITY - - ILLUSTRATED - - NEW YORK - CHARLES SCRIBNER'S SONS - 1902 - - - - - COPYRIGHT, 1902, BY - CHARLES SCRIBNER'S SONS - - Published, April, 1902 - - TROW DIRECTORY - PRINTING 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 unattractive, because they must aim at -completeness. 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 now and then to a -detached essay on some special 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. - - - - -CONTENTS - - - PAGE - - NAVIGATION AT SEA 1 - - THE PLEIADES 10 - - THE POLE-STAR 18 - - NEBULÆ 27 - - 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 111 - - MOTIONS OF THE EARTH'S POLE 131 - - SATURN'S RINGS 140 - - THE HELIOMETER 152 - - OCCULTATIONS 161 - - MOUNTING GREAT TELESCOPES 170 - - THE ASTRONOMER'S POLE 184 - - THE MOON HOAX 199 - - THE SUN'S DESTINATION 210 - - - - -ILLUSTRATIONS - - - THE MOON. FIRST QUARTER _Frontispiece_ - _Photographed by Loewy and Puiseux, - February 13, 1894._ - - FACING - PAGE - - SPIRAL NEBULA IN CONSTELLATION LEO 26 - _Photographed by Keeler, - February 24, 1900._ - - NEBULA IN ANDROMEDA 28 - _Photographed by Barnard, - November 21, 1892._ - - THE "DUMB-BELL" NEBULA 34 - _Photographed by Keeler, - July 31, 1899._ - - STAR-FIELD IN CONSTELLATION MONOCEROS 84 - _Photographed by Barnard, - February 1, 1894._ - - SOLAR CORONA. TOTAL ECLIPSE 108 - _Photographed by Campbell, - January 22, 1898; Jeur, India._ - - FORTY-INCH TELESCOPE, YERKES OBSERVATORY 170 - - YERKES OBSERVATORY, UNIVERSITY OF CHICAGO 176 - - - - -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 London, to consult, -if possible, the original manuscripts left by one Stephen -Groombridge, an English astronomer of the good old days, who -died in 1832. It was known that they had been filed away about a -generation ago, by the late Sir George Airy, who was Astronomer -Royal of England between the years 1835 and 1881. After a long -search, a large and dusty box was found and opened. It was filled -with documents, of which the topmost was in Sir George's own -handwriting, and began substantially as follows: - - "List of articles within this box. - "No. 1, This list, - "No. 2, etc., etc." - -Astronomical precision can no further go: he had listed even the -list itself. Truly, Airy was rightly styled "prince of precisians." -A worthy Astronomer Royal was he, to act under the royal warrant -of Charles II., who established the office in 1675. 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 stars, in order to find out the so much -desired longitude at sea, for the perfecting the art of navigation." - -The "so much desired longitude at sea" is, indeed, a vastly -important thing to a maritime nation like England. And only in -comparatively recent years has it become possible and easy for -vessels to be navigated with safety and convenience upon long -voyages. The writer was well acquainted with an old sea-captain -of New York, who had commanded one of the earliest transatlantic -steamers, and who died only a few years ago. He had a goodly store -of ocean yarn, fit and ready for the spinning, if he could but -find someone who, like himself, had known and loved the ocean. In -his early sea-going days, only the wealthiest of captains owned -chronometers. This instrument is now considered indispensable in -navigation, but at that time it was a new invention, very rare and -costly. Upon a certain voyage from England to Rio Janeiro, in South -America, the old captain could remember the following odd method of -navigation: The ship was steered by compass to the southward and -westward, more or less, until the skipper's antique quadrant showed -that they had about reached the latitude of Rio. Then they swung -her on a course due west by compass, and away she went for Rio, -relying on the lookout man forward to keep the ship from running -ashore. For after a certain lapse of time, being ignorant of the -longitude, they could not know whether they would "raise" the land -within an hour or in six weeks. We are glad of an opportunity to -put this story on record, 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 -chronometers. - -Anyone who has ever been a passenger on a great transatlantic -liner of to-day knows what an important, imposing personage is -the brass-bound skipper. A very different creature is he on the -deck of his ship from the modest seafaring man we meet on land, -clad for the time being in his shore-going togs. But the captain's -dignity is not all brass buttons and gold braid. He has behind -him the powerful support of a deep, delightful mystery. He it is -who "takes the sun" at noon, and finds out the ship's path at -sea. And in truth, regarded merely as a scientific experiment, -the guiding of a vessel across the unmarked trackless ocean has -few equals within the whole range of human knowledge. It is our -purpose here to explain quite briefly the manner in which this -seeming impossibility is accomplished. We shall not be able to -go sufficiently into details to enable him who reads to run and -navigate a magnificent steamer. But we hope to diminish somewhat -that small part of the captain's vast dignity which depends upon -his mysterious operations with the sextant. - -To begin, then, with the sextant itself. It is nothing but an -instrument with which we can measure how high up the sun is in the -sky. Now, everyone knows that the sun slowly climbs the sky in -the morning, reaches its greatest height at noon, and then slowly -sinks again in the afternoon. The captain simply begins to watch -the sun through the sextant shortly before noon, and keeps at it -until he discovers that the sun is just beginning to descend. That -is the instant of noon on the ship. The captain quickly glances -at the chronometer, or calls out "noon" to an officer who is near -that instrument. And so the error of the chronometer becomes known -then and there without any further astronomical calculations -whatever. Navigators can also find the chronometer error by sextant -observations when the sun is a long way from noon. The methods of -doing this are somewhat less simple than for the noon observation; -they belong to the details of navigation, into which we cannot -enter here. - -Incidentally, the captain also notes with the sextant how high -the sun was in the sky at the noon observation. He has in his -mysterious "chart-room" some printed astronomical tables, -which tell him in what terrestrial latitude the sun will have -precisely that height on that particular day of the year. Thus the -terrestrial latitude becomes known easily enough, and if only the -captain could get his longitude too, he would know just where his -ship was that day at noon. - -We have seen that the sextant observations furnish the error of -the chronometer according to ship's time. In other words, the -captain is in possession of the correct local time in the place -where the ship actually is. Now, if he also had the correct time -at that moment of some well-known place on shore, he would know -the difference in time between that place on shore and the ship. -But every traveller by land or sea is aware that there are always -differences of time between different places on the earth. If a -watch be right on leaving New York, for instance, it will be much -too fast on arriving at Chicago or San Francisco; the farther you -go the larger becomes the error of your watch. In fact, if you -could find out how much your watch had gone into error, you would -in a sense know how far east or west you had travelled. - -Now the captain's chronometer is set to correct "Greenwich time" -on shore before the ship leaves port. His observations having then -told him how much this is wrong on that particular day, and in -that particular spot where the ship is, he knows at once just how -far he has travelled east or west from Greenwich. In other words, -he knows his "longitude from Greenwich," for longitude is nothing -more than distance from Greenwich in an east-and-west direction, -just as latitude is only distance from the equator measured in -a north-and-south direction. Greenwich observatory is usually -selected as the beginning of things for measuring longitudes, -because it is almost the oldest of existing astronomical -establishments, and belongs to the most prominent maritime nation, -England. - -One of the most interesting bits of astronomical history was -enacted in connection with this matter of longitude. From what has -been said, 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 port. Even a very small error of the -chronometer will throw out the longitude a good many miles, 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 sea. - -It was as recently as the year 1736 that the first instrument -capable of keeping anything like accurate time at sea was -successfully completed. It was the work of an English watchmaker -named John Harrison, and is one of the few great improvements in -matters scientific which the world owes to a desire for winning a -money prize. It appears that in 1714 a committee was appointed by -the House of Commons, with no less a person than Sir Isaac Newton -himself as one of its members, to consider the desirability of -offering governmental encouragement for the invention of some means -of finding the longitude at sea. Finally, the British Government -offered a reward of $50,000 for an instrument which would find -the longitude within sixty miles; $75,000, if within forty miles, -and $100,000, if within thirty miles. Harrison's chronometer was -finished in 1736, but he did not receive the final payment of his -prize until 1764. - -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 years. It is a matter of satisfaction that -Harrison lived to receive the money which he had earned. He had the -genius to plan and master intricate mechanical details, but perhaps -he lacked in some degree the ability of tongue and pen to bring -them home to others. This may be the reason he is so little known, -though it was his fortune to contribute so large and essential -a part to the perfection of modern navigation. Let us hope this -brief mention may serve to recall his memory from oblivion even for -a fleeting moment; that we may not have written in vain of that -longitude to which his life was given. - - - - -THE PLEIADES - - -Famed in legend; sung by early minstrels of Persia and Hindustan; - - "--like a swarm of fire-flies tangled in a silver braid"; - -yonder distant misty little cloud of Pleiades has always won -and held the imagination of men. But it was not only for the -inspiration of poets, for quickening fancy into song, that the -seven daughters of Atlas were fixed upon the firmament. The -problems presented by this group of stars to the unobtrusive -scientific investigator are among the most interesting known to -astronomy. Their solution is still very incomplete, 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 -secrets. - -The true student of astronomy is animated by no mere vulgar -curiosity to pry into things hidden. If he seeks the concealed -springs that move the complex visible mechanism of the heavens, he -does so because his imagination is roused by the grandeur of what -he sees; and deep down within him stirs the true love of the artist -for his art. For it is indeed a fine art, that science of astronomy. - -It can have been no mere chance that has massed the Pleiades -from among their fellow stars. Men of ordinary eyesight see but -a half-dozen distinct objects in the cluster; those of acuter -vision can count fourteen; 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 -constructed. With the Paris instrument Wolf in 1876 catalogued 625 -stars in the group; and the searching photographic survey of Henry -in 1887 revealed no less than 2,326 distinct stars within and near -the filmy gauze of nebulous matter always so conspicuous a feature -of the Pleiades. - -The means at our disposal for the study of stellar distances are -but feeble. 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 distance. The most powerful observational machinery, though -directed by the tried skill of experience, has not sufficed to -sound the profounder depths of space. The Pleiad stars are among -those for which no measurement of distance has yet been made, so -that we do not know whether they are all equally far away from -us. We see them projected on the dark background of the celestial -vault; but we cannot tell from actual measurement whether they are -all situated near the same point in space. It may be that some -are immeasurably closer to us than are the great mass of their -companions; possibly we look through the cluster at others far -behind it, clinging, as it were, to the very fringe of the visible -universe. - -Farther on we shall find evidence that something like this really -is the case. 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 cluster. Such a distribution may perhaps remain among the -possibilities, so long as we cannot measure directly the actual -distances of the individual stars. But science never accepts a mere -possibility against which we can marshal strong circumstantial -evidence. We may conclude on general principles that the gathering -of these many objects into a single close assemblage denotes -community of origin and interests. - -The Pleiades then really belong to one another. What is the nature -of their mutual tie? What is their mystery, and can we solve it? -The most obvious theory is, of course, suggested by what we know -to be true within our own solar system. We owe to Newton the -beautiful conception of gravitation, that unique law by means of -which astronomers have been enabled to reduce to perfect order the -seeming tangle of planetary evolutions. The law really amounts, in -effect, to this: All objects suspended within the vacancy of space -attract or pull one another. How they can do this without a visible -connecting link between them is a mystery which may always remain -unsolved. But mystery as it is, we must accept it as an ascertained -fact. It is this pull of gravitation that holds together the sun -and planets, forcing them all to follow out their due and proper -paths, and so to continue throughout an unbroken cycle until the -great survivor, Time, shall be no more. - -This same gravitational attraction must be at work among the -Pleiades. They, too, like ourselves, must have bounds and orbits -set and interwoven, revolutions and gyrations far more complex -than the solar system knows. The visual discovery of such motion -of rotation among the Pleiades may be called one of the pressing -problems of astronomy to-day. We feel sure that the time is ripe, -and that the discovery is actually being made at the present -moment: for a generation of men is not too great a period to call a -moment, when we have to deal with cosmic time. - -It is indeed the lack of observations extending through sufficient -centuries that stays our hand from grasping the coveted result. The -Pleiades are so far from us that we cannot be sure of changes among -them. Magnitudes are always relative. It matters not how large the -actual movements may be; if they are extremely small in comparison -with our distance, they must shrink to nothingness in our eyes. -Trembling on the verge of invisibility, elusive, they are in that -borderland where science as yet but feels her way, though certain -that the way is there. - -The foundations of exact modern knowledge of the group were laid -by Bessel about 1840. With the modesty characteristic of the -great, he says quite simply that he has made a number of measures -of the Pleiades, thinking that the time may come when astronomers -will be able to find some evidence of motion. In this unassuming -way he prefaces what is still the classic model of precision and -thoroughness in work of this kind. Bessel cleared the ground for a -study of inter-stellar motion within the close star-clusters; and -it is probable that only by such study may we hope to demonstrate -the universality of the law of gravitation in cosmic space. - -Bessel's acuteness in forecasting the direction of coming research -was amply verified by the work of Elkin in 1885 at Yale College. -Provided with a more modern instrument, but similar to Bessel's, -Elkin was able to repeat his observations with a slight increase -of precision. Motions in the interval of forty-five years, -sufficiently great to hint at coming possibilities, were shown -conclusively to exist. Six stars at all events have been fairly -excluded from the group on account of their peculiar motions shown -by Elkin's research. It is possible that they are merely seen in -the background through the interstices of the cluster itself, -or they may be suspended between us and the Pleiades, in either -case having no real connection with the group. Finally, these -observations make it reasonably certain that many of the remaining -mass of stars really constitute a unit aggregation in space. -Astronomers of a coming generation will again repeat the Besselian -work. 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 direction. 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 imposes. - -These will doubtless be of a kind not even suggested by the -lesser complexities of our solar system. 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 mathematician. There is an extraordinary nebulous -condensation that seems to pervade the entire space occupied by -the stellar constituents of the group. The stars are swimming in a -veritable sea of luminous cloud. There are filmy tenuous places, -and again condensing whirls of material doubtless still in the -gaseous or plastic stage. Most noticeable of all are certain almost -straight lines of nebula that connect series of stars. In one case, -shown upon a photograph made by Henry at Paris, six stars are -strung out upon such a hazy line. We might give play to fancy, and -see in this the result of some vast eruption of gaseous matter that -has already begun to solidify here and there into stellar nuclei. -But sound science gives not too great freedom to mere speculative -theories. Her duty has been found in quiet research, and her -greatest rewards have flowed from imaginative speculation, only -when tempered by pure reason. - - - - -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 -star. Centuries and centuries ago, when astronomy, that venerable -ancient among the sciences, was but an infant, the pole-star must -have been considered the very oldest of observed heavenly bodies. -In the beginning it was the only sure guide of the navigator at -night, just as to this day it is the foundation-stone for all -observational stellar astronomy of precision. 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 -night. So it is indeed strange that we should find out something -altogether new about it after all these ages of study. - -But the importance of the discovery rests upon a surer foundation -than this. The method by which it has been made is almost a new -one in the science. A generation ago, men thought the "perfect -science," for so we love to call astronomy, could advance only by -increasing a little the exact precision of observation. The citadel -of perfect truth might be more closely invested; the forces of -science might push forward step by step; the machinery of research -might be strengthened, but that a new engine of investigation -would be discovered capable of penetrating where no telescope can -ever reach, this, indeed, seemed far beyond the liveliest hope -of science. Even the discoverer of the spectroscope could never -have dreamed of its possibilities, could never have foreseen its -successes, its triumphs. - -The very name of this instrument suggests mystery to the popular -mind. It is set down at once among the things too difficult, too -intricate, too abstruse to understand. Yet in its essentials there -is nothing about the spectroscope that cannot be made clear in a -few words. Even the modern "undulatory theory" of light itself is -terrible only in the length of its name. Anyone who has seen the -waves of ocean roll, roll, and ever again roll in upon the shore, -can form a very good notion of how light moves. 'Tis just such -a series of rolling waves; started perhaps from some brilliant -constellation far out upon the confining bounds of the visible -universe, or perhaps coming from a humble light upon the student's -table; yet it is never anything but a succession of rolling waves. -Only, unlike the waves of the sea, light waves are all excessively -small. We should call one whose length was a twenty-thousandth of -an inch a big one! - -Now the human eye possesses the property of receiving and -understanding these little waves. The process is an unconscious -one. Let but a set of these tiny waves roll up, as it were, out -of the vast ocean of space and impinge upon the eye, and all the -phenomena of light and color become what we call "visible." We see -the light. - -And how does all this find an application in astronomy? Not -to enter too much into technical details, we may say that the -spectroscope is an instrument which enables us to measure the -length of these light waves, though their length is so exceedingly -small. 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 unaided. Telescope, microscope, and spectroscope are -essential now to him who would penetrate any of Nature's secrets. -But measurements with a telescope, like eye observations, are -limited strictly to determining the directions in which we see -the heavenly bodies. Ever since the beginning of things, when -old Hipparchus and Ulugh Beg made the first rude but successful -attempts to catalogue the stars, the eye and telescope have been -able to measure only such directions. We aim the telescope at a -star, and record the direction in which it was pointed. Distances -in astronomy can never be measured directly. All that we know of -them has been obtained by calculations based upon the Newtonian law -of gravitation and observations of directions. - -Now the spectroscope seems to offer a sort of exception to this -rule. Suppose we can measure the wave-lengths of the light sent us -from a star. Suppose again that the star is itself moving swiftly -toward us through space, while continually setting in motion the -waves of light that are ultimately to reach the waiting astronomer. -Evidently the light waves will be crowded together somewhat on -account of the star's motion. More waves per second will reach us -than would be received from a star at rest. It is as though the -light waves were compressed or shortened a little. And if the star -is leaving us, instead of coming nearer, opposite effects will -occur. 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 us. And by -a simple process of calculation this stellar motion can be obtained -in miles per second. Thus we can now actually measure directly, in -a certain sense, linear speed in stellar space, though we are still -without the means of getting directly at stellar distances. - -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 observation. What we learn through the -spectroscope comes from a study of the waves themselves, and it -is of no consequence how far they have travelled, or how long -they have been a-coming. For it must not be supposed that these -waves consume no time in passing from a distant star to our own -solar system. It is true that they move exceeding fast; certainly -180,000 miles per second may be called rapid motion. But if -this cosmic velocity of light is tremendous, so also are cosmic -distances correspondingly vast. Light needs to move quickly coming -from a star, for even at the rate of motion we have mentioned it -requires many years to reach us from some of the more distant -constellations. It has been well said that an observer on some -far-away star, if endowed with the power to see at any distance, -however great, might at this moment be looking on the Crusaders -proceeding from Europe against the Saracen at Jerusalem. For it is -quite possible that not until now has the light which would make -the earth visible had time to reach him. Yet distant as such an -observer might be, light from the star on which he stood could be -measured in the spectroscope, and would infallibly tell us whether -the earth and star are approaching in space or gradually drawing -farther asunder. - -The pole-star is not one of the more distant stellar systems. We do -not know how far it is from us very exactly, but certainly not less -than forty or fifty years are necessary for its light to reach us. -The star might have gone out of existence twenty years ago, and we -not yet know of it, for we would still be receiving the light which -began its long journey to us about 1850 or 1860. But no matter -what may be its distance, Campbell found by careful observations, -made in the latter part of 1896, that the pole-star was then -approaching the earth at the rate of about twelve miles per second. -So far there was nothing especially remarkable. But in August and -September of the present year twenty-six careful determinations -were made, and these showed that now the rate of approach varied -between about five and nine miles per second. More astonishing -still, there was a uniform period in the changes of velocity. In -about four days the rate of motion changed from about five to -nine miles and back again. And this variation kept on with great -regularity. Every successive period of four days saw a complete -cycle of velocity change forward and back between the same limits. -There can be but one reasonable explanation. This star must be a -double, or "binary" star. The two components, under the influence -of powerful mutual gravitational attraction, must be revolving -in a mighty orbit. Yet this vast orbit, as a whole, with the two -great stars in it, must be approaching our part of the universe -all the time. For the spectroscope shows the velocity of approach -to increase and diminish, indeed, but it is always present. Here, -then, is this great stellar system, having a four-day revolution of -its own, and yet swinging rapidly through space in our direction. -Nor is this all. One of the component stars must be nearly or -quite dark; else its presence would infallibly be detected by our -instruments. - -And now we come to the most astonishing thing of all. How comes it -that the average rate of approach of the "four-day system," as a -whole, changed between 1896 and 1899? In 1896 only this velocity -of the whole system was determined, the four-day period remaining -undiscovered until the more numerous observations of 1899. -But even without considering the four-day period, the changing -velocity of the entire system offers one of those problems that -exact science can treat only by the help of the imagination. There -must be some other great centre of attraction, some cosmic giant, -holding the visible double pole-star under its control. Thus, that -which we see, and call the pole-star, is in reality threading its -path about the third and greatest member of the system, itself -situated in space, we know not where. - -[Illustration: Spiral Nebula in Constellation Leo. - -Photographed by Keeler, February 24, 1900. - -Exposure, three hours, fifty minutes.] - - - - -NEBULÆ - - -Scattered about here and there among the stars are certain patches -of faint luminosity called by astronomers Nebulæ. These "little -clouds" of filmy light are among the most fascinating of all the -kaleidoscopic phenomena of the heavens; 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 made. All our knowledge of Nature leads -us to expect in her finished work the result of a series of gradual -processes of development. Highly organized phenomena such as those -existing in our solar system did not spring into perfection in -an instant. Influential forces, easy to imagine, but difficult -to define, must have directed the slow, sure transformation of -elemental matter into sun and planets, things and men. 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 science. - -Our knowledge of the nebulæ is of two kinds--that which has -been ascertained from observation as to their appearance, size, -distribution, and distance; and that which is based upon hypotheses -and theoretical reasoning about the condensation of stellar systems -out of nebular masses. It so happens that our observational -material has received a very important addition quite recently -through the application of photography to the delineation of -nebulæ, and this we shall describe farther on. - -Two nebulæ only are visible to the unaided eye. The brighter -of these is in the constellation Andromeda; it is of oval or -elliptical shape, and has a distinct central condensation or -nucleus. Upon a photograph by Roberts it appears to have several -concentric rings surrounding the nebula proper, 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 disk. Very recent photographs of this nebula, -made with the three-foot reflecting telescope of the Lick -Observatory, bring out the fact that it is really spiral in form, -and that the outlying nebulous rings are only parts of the spires -in a great cosmic whorl. - -[Illustration: Nebula in Andromeda. - -Lower object in the photograph is a Comet. - -Photographed by Barnard, November 21, 1892.] - -This Andromeda nebula is the one in which the temporary star of -1885 appeared. It blazed up quite suddenly near the apparent -centre of the nebula, and continued in view for six months, fading -finally beyond the reach of our most powerful telescopes. There -can be little doubt that the star was actually in the nebula, -and not merely seen through it, though in reality situated in -the extreme outlying part of space at a distance immeasurably -greater than that separating us from the nebula itself. Such an -accidental superposition of nebula and star might even be due to -sudden incandescence of a new star between us and the nebula. In -such a case we should see the star projected upon the surface of -the nebula, so that the superposition would be identical with that -actually observed. Therefore, while it is, indeed, possible that -the star may have been either far behind the nebula or in front of -it, we must accept as more probable the supposition that there was -a real connection between the two. In that case there is little -doubt that we have actually observed one of those cataclysms that -mark successive steps of cosmic evolution. We have no thoroughly -satisfactory theory to account for such an explosive catastrophe -within the body of the nebula itself. - -The other naked-eye nebula is in the constellation Orion. In -the telescope it is a more striking object, perhaps, than the -Andromeda nebula; for it has no well-defined geometrical form, -but consists of an immense odd-shaped mass of light enclosing and -surrounding a number of stars. It is unquestionably of a very -complicated structure, and is, therefore, less easily studied and -explained than the nebulæ of simpler form. There is no doubt that -the Orion nebula is composed of luminous gas, and is not merely a -cluster of small stars too numerous and too near together to be -separated from each other, even in our most powerful telescopes. -It was, indeed, supposed, until about forty years ago, that all -the nebulæ are simply irresolvable star-clusters; but we now have -indisputable evidence, derived from the spectroscope, that many -nebulæ are composed of true gases, similar to those with which -we experiment in chemical laboratories. This spectroscopic proof -of the gaseous character of nebulæ is one of the most important -discoveries contributed by that instrument to our small stock of -facts concerning the structure of the sidereal universe. - -Coming now to the smaller nebulæ, we find a great diversity of -form and appearance. Some are ring-shaped, perhaps having a -less brilliant nebulosity within the ring. Many show a central -condensation of disk-like appearance (planetary nebulæ), or have -simply a star at the centre (nebulous stars). Altogether about -ten thousand such objects have been catalogued by successive -generations of astronomers since the invention of the telescope, -and most of these have been reported as oval in form. Now we have -already referred to the important addition to our knowledge of -the nebulæ obtained by recent photographic observations; and this -addition consists in the discovery that most of these oval nebulæ -are in reality spirals. Indeed, it appears that the spiral type is -the normal type, and that nebulæ of irregular or other forms are -exceptions to the general rule. Even the great Andromeda nebula, as -we have seen, is now recognized as a spiral. - -The instrument with which its convolute structure was discovered -is a three-foot reflecting telescope, made by Common of England, -and now mounted at the Lick Observatory, in California. The late -Professor Keeler devoted much of his time to photographing nebulæ -during the last year or two. He was able to establish the important -fact just mentioned, that most nebulæ formerly thought to be mere -ovals, 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 size, and with an exposure to the feeble nebular -light extending through three or four consecutive hours. - -Many of the spirals have more than a single volute. It is as though -one were to attach a number of very flexible rods to an axle, -like spokes of a wheel without a rim and then revolve the axle -rapidly. The flexible rods would bend under the rapid rotation, and -form a series of spiral curves not unlike many of these nebulæ. -Indeed, it is impossible to escape the conviction that these great -celestial whorls are whirling around an axis. And it is most -important in the study of the growth of worlds, to recognize that -the type specimen is a revolving spiral. Therefore, the rotating -flattened globe of incandescent matter postulated by Laplace's -nebular hypothesis would make of our solar system an exceptional -world, and not a type of stellar evolution in general. - -Keeler's photographs have taught us one thing more. Scarcely is -there a single one of his negatives that does not show nebulæ -previously uncatalogued. It is estimated that if this process of -photography could be extended so as to cover the entire sky, the -whole number of nebulæ would add up to the stupendous total of -120,000; and of these the great majority would be spiral. - -When we approach the question of the distribution of nebulæ in -different parts of the sky, as shown by their catalogued positions, -we are met by a curious fact. It appears that the region in the -neighborhood of the Milky Way is especially poor in nebulæ, -whereas these objects seem to cluster in much larger numbers about -those points in the sky that are farthest from the Milky Way. -But we know that the Milky Way is richer in stars than any other -part of the sky, since it is, in fact, made up of stellar bodies -clustered so closely that it is wellnigh impossible to see between -them in the denser portions. Now, it cannot be the result of chance -that the stars should tend to congregate in the Milky Way, while -the nebulæ tend to seek a position as far from it as possible. -Whatever may be the cause, we must conclude that the sidereal -system, as we see it, is in general constructed upon a single plan, -and does not consist of a series of universes scattered at random -throughout space. If we are to suppose that nebulæ turn into stars -as a result of condensation or any other change, then it is not -astonishing to find a minimum of nebulæ where there is a maximum of -stars, since the nebulæ will have been consumed, as it were, in the -formation of the stars. - -[Illustration: The "Dumb-Bell" Nebula. - -Photographed by Keeler, July 31, 1899. - -Exposure, three hours.] - -It is never advisable to push philosophical speculation very -far when supported by too slender a basis of fact. But if we -are to regard the visible universe as made up on the whole of a -single system of bodies, we may well ask one or two questions to -be answered by speculative theory. We have said the stars are not -uniformly distributed in space. Their concentration in the Milky -Way, forming a narrow band dividing the sky into two very nearly -equal parts, must be due to their being actually massed in a -thin disk or ring of space within which our solar system is also -situated. This thin disk projected upon the sky would then appear -as the narrow star-band of the Milky Way. Now, suppose this disk -has an axis perpendicular to itself, and let us imagine a rotation -of the whole sidereal system about that axis. Then the fact that -the visible nebulæ are congregated far from the Milky Way means -that they are actually near the imaginary axis. - -Possibly the diminished velocity of motion near the axis may have -something to do with the presence of the nebulæ there. Possibly -the nebulæ themselves have axes perpendicular to the plane of -the Milky Way. If so, we should see the spiral nebulæ near the -Milky Way edgewise, and those far from it without foreshortening. -Thus, the paucity of nebulæ near the Milky Way may be due in -part to the increased difficulty of seeing them when looked at -edgewise. Indeed, there is no limit to the possibilities of -hypothetical reasoning about the nebular structure of our universe; -unfortunately, the whole question must be placed for the present -among those intensely interesting cosmic problems awaiting -elucidation, let us hope, in this new century. - - - - -TEMPORARY STARS - - -Nothing can be more erroneous than to suppose that the stellar -multitude has continued unchanged throughout all generations of -men. "Eternal fires" poets have called the stars; yet they burn -like any little conflagration on the earth; now flashing with -energy, brilliant, incandescent, and again sinking into the dull -glow of smouldering half-burned ashes. It is even probable that -space contains many darkened orbs, stars that may have risen in -constellations to adorn the skies of prehistoric time--now cold, -unseen, unknown. So far from dealing with an unvarying universe, -it is safe to say that sidereal astronomy can advance only by the -discovery of change. Observational science watches with untiring -industry, and night hides few celestial events from the ardent -scrutiny of astronomers. Old theories are tested and newer ones -often perfected by the detection of some slight and previously -unsuspected alteration upon the face of the sky. The interpretation -of such changes is the most difficult task of science; it has taxed -the acutest intellects among men throughout all time. - -If, then, changes can be seen among the stars, what are we to think -of the most important change of all, the blazing into life of a -new stellar system? Fifteen times since men began to write their -records of the skies has the birth of a star been seen. Surely -we may use this term when we speak of the sudden appearance of a -brilliant luminary where nothing visible existed before. 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 matter. It is old material becoming suddenly -luminous for some hidden reason. In fact, whenever a new object of -great brilliancy has been discovered, it has been found to lose its -light again quite soon, ending either in total extinction or at -least in comparative darkness. It is for this reason that the name -"temporary star" has been applied to cases of this kind. - -The first authenticated instance dates from the year 134 B.C., -when a new star appeared in the constellation Scorpio. It was this -star that led Hipparchus to construct his stellar catalogue, the -first ever made. It occurred to him, of course, that there could -be but one way to make sure in the future that any given object -discovered in the sky was new; it was necessary to make a complete -list of everything visible in his day. 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 appeared. -This work of Hipparchus became the foundation of sidereal study, -and led to most important discoveries of various kinds. - -But no records remain concerning his new star except the bare fact -of its appearance in Scorpio. Hipparchus's published works are all -lost. We do not even know the exact place of his birth, and as for -those two dates of entry and exit that history attaches to great -names--we have them not. Yet he was easily the first astronomer of -antiquity, one of the first of all time; and we know of him only -from the writings of Ptolemy, who lived three hundred years after -him. - -More than five centuries elapsed before another temporary star was -entered in the records of astronomy. This happened in the year 389 -A.D., when a star appeared in Aquila; and of this one also we know -nothing further. But about twelve centuries later, in November, -1572, a new and brilliant object was found in the constellation -Cassiopeia. It is known as Tycho's star, since it was the means -of winning for astronomy a man who will always take high rank in -her annals, Tycho Brahe, of Denmark. When he first saw this star, -it was already very bright, equalling even Venus at her best; and -he continued a careful series of observations for sixteen months, -when it faded finally from his view. The position of the new star -was measured with reference to other stars in the constellation -Cassiopeia, and the results of Tycho's observations were finally -published by him in the year 1573. It appears that much urging on -the part of friends was necessary to induce him to consent to this -publication, not because of a modest reluctance to rush into print, -but for the reason that he considered it undignified for a nobleman -of Denmark to be the author of a book! - -An important question in cosmic astronomy is opened by Tycho's -star. Did it really disappear from the heavens when he saw it -no more, or had its lustre simply been reduced below the visual -power of the unaided eye? Unfortunately, Tycho's observations of -the star's position in the constellation were necessarily crude. -He possessed no instruments of precision such as we now have -at our disposal, and so his work gives us only a rather rough -approximation of the true place of the star. A small circle might -be imagined on the sky of a size comparable with the possible -errors of Tycho's observations. We could then say with certainty -that his star must have been situated somewhere within that little -circle, but it is impossible to know exactly where. - -It happens that our modern telescopes reveal the existence of -several faint stars within the space covered by such a circle. -Any one of these would have been too small for Tycho to see, and, -therefore, any one of them may be his once brilliant luminary -reduced to a state of permanent or temporary semi-darkness. These -considerations are, indeed, of great importance in explaining the -phenomena of temporary stars. If Tycho had been able to leave us a -more exact determination of his star's place in the sky, and even -if our most powerful instruments could not show anything in that -place to-day, we might nevertheless theorize on the supposition -that the object still exists, but has reached a condition almost -entirely dark. - -Indeed, the latest theory classes temporary stars among those known -as variable. For many stars are known to undergo quite decided -changes in brilliancy; possibly inconstancy of light is the rule -rather than the exception. But while such changes, when they -exist, are too small to be perceptible in most cases, there is -certainly a large number of observable variables, subject to easily -measurable alterations of light. Astronomers prefer to see in the -phenomena of temporary stars simple cases of variation in which the -increase of light is sudden, and followed by a gradual diminution. -Possibly there is then a long period of comparative or even -complete darkness, to be followed as before by a sudden blazing up -and extinction. No temporary star, however, has been observed to -reappear in the same celestial place where once had glowed its -sudden outburst. But cases are not wanting where incandescence has -been both preceded and followed by a continued existence, visible -though not brilliant. - -For such cases as these it is necessary to come down to modern -records. We cannot be sure that some faint star has been -temporarily brilliant, unless we actually see the conflagration -itself, 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 precision. But no one has -ever seen the smouldering fires break out. Temporary stars have -always been first noticed only after having been active for hours -if not for days. So we must perforce fall back on instrumental -identification by determinations of the star's exact position upon -the celestial vault. - -Some time between May 10th and 12th in the year 1866 the ninth star -in the list of known "temporaries" appeared. It possessed very -great light-giving power, being surpassed in brilliancy by only -about a score of stars in all the heavens. It retained a maximum -luminosity only three or four days, and in less than two months -had diminished to a point somewhere between the ninth and tenth -"magnitudes." In other words, from a conspicuous star, visible to -the naked eye, it had passed beyond the power of anything less than -a good telescope. Fortunately, we had excellent star-catalogues -before 1866. These were at once searched, and it was possible to -settle quite definitely that a star of about the ninth or tenth -magnitude had really existed before 1866 at precisely the same -point occupied by the new one. Needless to say, observations were -made of the new star itself, and afterward compared with later -observations of the faint one that still occupies its place. These -render quite certain the identity of the temporary bright star with -the faint ones that preceded and followed it. - -Such results, on the one hand, offer an excellent vindication -of the painstaking labor expended on the construction of -star-catalogues, and, on the other, serve to elucidate the mystery -of temporary stars. Nothing can be more plausible than to explain -by analogy those cases in which no previous or subsequent existence -has been observed. It is merely necessary to suppose that, instead -of varying from the ninth or tenth magnitude, other temporary -objects have begun and ended with the twentieth; for the twentieth -magnitude would be beyond the power of our best instruments. - -Nor is the star of 1866 an isolated instance. Ten years later, in -1876, a temporary star blazed up to about the second magnitude, and -returned to invisibility, so far as the naked eye is concerned, -within a month, having retained its greatest brilliancy only one -or two days. This star is still visible as a tiny point of light, -estimated to be of the fifteenth magnitude. Whether it existed -prior to its sudden outburst can never be known, because we do not -possess catalogues including the generality of stars as faint as -this one must have been. But at all events, the continued existence -of the object helps to place the temporary stars in the class of -variables. - -The next star, already mentioned under "nebula," was first seen -in 1885. It was in one respect the most remarkable of all, for -it appeared almost in the centre of the great nebula in the -constellation Andromeda. It was never very bright, reaching only -the sixth magnitude or thereabouts, was observed during a period of -only six months, and at the end of that time had faded beyond the -reach of our most powerful glasses. It is a most impressive fact -that this event occurred within the nebula. Whatever may be the -nature of the explosive catastrophe to which the temporary stars -owe their origin, we can now say with certainty that not even those -vast elemental luminous clouds men call nebulæ are free from danger. - -The last outburst on our records was first noticed February 22, -1901. The star appeared in the constellation Perseus, and soon -reached the first magnitude, surpassing almost every other star -in the sky. It has been especially remarkable in that it has -become surrounded by a nebulous mass in which are several bright -condensations or nuclei; and these seem to be in very rapid motion. -The star is still under observation (January, 1902). - - - - -GALILEO - - -Among the figures that stand out sharply upon the dim background -of old-time science, there is none that excites a keener interest -than Galileo. Most people know him only as a distinguished man -of learning; one who carried on a vigorous controversy with the -Church on matters scientific. It requires some little study, some -careful reading between the lines of astronomical history, to gain -acquaintance with the man himself. He had a brilliant, incisive -wit; was a genuine humorist; knew well and loved the amusing side -of things; and could not often forego a sarcastic pleasantry, or -deny himself the pleasure of argument. Yet it is more than doubtful -if he ever intended impertinence, or gave willingly any cause of -quarrel to the Church. - -His acute understanding must have seen that there exists no real -conflict between science and religion; for time, in passing, has -made common knowledge of this truth, as it has of many things once -hidden. When we consider events that occurred three centuries -ago, it is easy to replace excited argument with cool judgment; -to remember that those were days of violence and cruelty; that -public ignorance was of a density difficult to imagine to-day; -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 least. Charlatans, -unbalanced theorists, purveyors of scientific marvels, were all -liable to be passed upon definitely by the Church, not in a spirit -of impertinent interference, but simply as part of her regular -duties. - -If the Church's judgment in such matters was sometimes erroneous; -if her interference now and again was cruel, the cause must be -sought in the manners and customs of the time, when persecution -rioted in company with ignorance, and violence was the law. Perhaps -even to-day it would not be amiss to have a modern scientific board -pass authoritatively upon novel discoveries and inventions, so as -to protect the public against impostors as the Church tried to do -of old. - -Galileo was born at Pisa in 1564, and his long life lasted -until 1642, the very year of Newton's birth. His most important -scientific discoveries may be summed up in a few words; he was the -first to use a telescope for examining the heavenly bodies; he -discovered mountains on the moon; the satellites of Jupiter; the -peculiar appearance of Saturn which Huygens afterward explained as -a ring surrounding the ball of the planet; and, finally, he found -black spots on the sun's disk. These discoveries, together with his -remarkable researches in mechanical science, constitute Galileo's -claim to immortality as an investigator. But, as we have said, it -is not our intention to consider his work as a series of scientific -discoveries. We shall take a more interesting point of view, and -deal with him rather as a human being who had contracted the habit -of making scientific researches. - -What must have been his feelings when he first found with his "new" -telescope the satellites of Jupiter? They were seen on the night -of January 7, 1610. He had already viewed the planet through his -earlier and less powerful glass, and was aware that it possessed -a round disk like the moon, only smaller. Now he saw also three -objects that he took to be little stars near the planet. But on the -following night, as he says, "drawn by what fate I know not," the -tube was again turned upon the planet. The three small stars had -changed their positions, and were now all situated to the west of -Jupiter, whereas on the previous night two had been on the eastern -side. He could not explain this phenomenon, but he recognized that -there was something peculiar at work. Long afterward, in one of -his later works, translated into quaint old English by Salusbury, -he declared that "one sole experiment sufficeth to batter to the -ground a thousand probable Arguments." This was already the guiding -principle of his scientific activity, a principle of incomparable -importance, and generally credited to Bacon. Needless to say, -Jupiter was now examined every night. - -The 9th was cloudy, but on the 10th he again saw his little stars, -their number now reduced to two. He guessed that the third was -behind the planet's disk. The position of the two visible ones was -altogether different from either of the previous observations. On -the 11th he became sure that what he saw was really a series of -satellites accompanying Jupiter on his journey through space, and -at the same time revolving around him. On the 12th, at 3 A.M., -he actually saw one of the small objects emerge from behind the -planet; and on the 13th he finally saw four satellites. Two hundred -and eighty-two years were destined to pass away before any human -eye should see a fifth. It was Barnard in 1892 who followed Galileo. - -To understand the effect of this discovery upon Galileo requires -a person who has himself watched the stars, not, as a dilettante, -seeking recreation or amusement, but with that deep reverence -that comes only to him who feels--nay, knows--that in the moment -of observation just passed he too has added his mite to the great -fund of human knowledge. Galileo's mummied forefinger still points -toward the stars from its little pedestal of wood in the _Museo_ -at Florence, a sign to all men that he is unforgotten. But Galileo -knew on that 11th of January, 1610, that the memory of him would -never fade; that the very music of the spheres would thenceforward -be attuned to a truer note, if any would but hearken to the Jovian -harmony. For he recognized at once that the visible revolution of -these moons around Jupiter, while that planet was himself visibly -travelling through space, must deal its death-blow to the old -Ptolemaic system of the universe. Here was a great planet, the -centre of a system of satellites, and yet not the centre of the -universe. Surely, then, the earth, too, might be a mere planet like -Jupiter, and not the supposed motionless centre of all things. - -The satellite discovery was published in 1610 in a little book -called "Sidereus Nuncius," usually translated "The Sidereal -Messenger." It seems to us, however, that the word "messenger" is -not strong enough; surely in Papal Italy a _nuncius_ was more than -a mere messenger. He was clothed with the very highest authority, -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 science. At all events, the book made him at once a -great reputation and numerous enemies. - -But it was not until 1616 that the Holy Office (Inquisition) issued -an edict ordering Galileo to abandon his opinion that the earth -moved, and at the same time placed Copernicus's _De Revolutionibus_ -and two other books advocating that doctrine on the "Index Librorum -Prohibitorum," or list of books forbidden by the Church. These -volumes remained in subsequent editions of the "Index" down to -1821, but they no longer appear in the edition in force to-day. - -Galileo's most characteristic work is entitled the "Dialogue on the -Two Chief Systems of the World." It was not published until 1632, -although the idea of the book was conceived many years earlier. -In it he gave full play to his extraordinary powers as a true -humorist, a _fine lame_ among controversialists, and a genuine man -of science, valuing naked truth above all other things. As may -be imagined, it was no small matter to obtain the authorities' -consent to this publication. Galileo was already known to hold -heretical opinions, and it was suspected that he had not laid them -aside when commanded to do so by the edict of 1616. But perhaps -Galileo's introduction to the "Dialogue" secured the censor's -_imprimatur_; it is even suspected that the Roman authorities -helped in the preparation of this introduction. Fortunately, we -have a delightful contemporary translation into English, by Thomas -Salusbury, printed at London by Leybourne in 1661. We have already -quoted from this translation, and now add from the same work part -of Galileo's masterly preface to the "Dialogue": - -"Judicious reader, there was published some years since in _Rome_ -a salutiferous Edict, that, for the obviating of the dangerous -Scandals of the Present Age, imposed a reasonable Silence upon the -Pythagorean (Copernican) opinion of the Mobility of the Earth. -There want not such as unadvisedly affirm, that the Decree was not -the production of a sober Scrutiny, but of an ill-formed passion; -and one may hear some mutter that Consultors altogether ignorant -of Astronomical observations ought not to clipp the wings of -speculative wits with rash prohibitions." - -Galileo first states his own views, and then pretends that he will -oppose them. He goes on to say that he believes in the earth's -immobility, and takes "the contrary only for a mathematical -_Capriccio_," as he calls it; something to be considered, because -possessing an academical interest, but on no account having a real -existence. Of course any one (even a censor) ought to be able to -see that it is the Capriccio, and not its opposite, that Galileo -really advocates. Three persons appear in the "Dialogue": Salviati, -who believes in the Copernican system; Simplicio, of suggestive -name, who thinks the earth cannot move; and, finally, Sagredus, -a neutral gentleman of humorous propensities, who usually begins -by opposing Salviati, but ends by being convinced. He then helps -to punish poor Simplicio, who is one of those persons apparently -incapable of comprehending a reasonable argument. Here is an -interesting specimen of the "Dialogue" taken from Salusbury's -translation: Salviati refers to the argument, then well known, that -the earth cannot rotate on its axis, "because of the impossibility -of its moving long without wearinesse." Sagredus replies: "There -are some kinds of animals which refresh themselves after wearinesse -by rowling on the earth; and that therefore there is no need -to fear that the Terrestrial Globe should tire, nay, it may be -reasonably affirmed that it enjoyeth a perpetual and most tranquil -repose, keeping itself in an eternal rowling." Salviati's comment -on this sally is, "You are too tart and satyrical, Sagredus." - -There is no doubt that the "Dialogue" finished the Ptolemaic -theory, and made that of Copernicus the only possible one. At -all events, it brought about the well-known attack upon Galileo -from the authorities of the Holy Office. We shall not recount -the often-told tale of his recantation. He was convicted (very -rightly) of being a Copernican, and was forced to abjure that -doctrine. Galileo's life may be summed up as one of those through -which the world has been made richer. A clean-cutting analytic -wit, never becoming dull: heated again and again in the fierce -blaze of controversy, it was allowed to cool only that it might -acquire a finer temper, to pierce with fatal certainty the smallest -imperfections in the armor of his adversaries. - - - - -THE PLANET OF 1898 - - -The discovery of a new and important planet usually receives -more immediate popular attention and applause than any other -astronomical event. 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 space. They are -wont to point out that this solar system, small and insignificant -as a whole in comparison with many of the stellar worlds, is, -nevertheless, made up of a large number of constituent planets; and -these in turn are often accompanied with still smaller satellites, -or moons. Thus does Nature provide worlds within worlds, 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 planets. The -ancients were acquainted with only five of the bodies now counted -as planets, viz.: Mercury, Venus, Mars, Jupiter, and Saturn. The -dates of their discovery are lost in antiquity. To these Uranus was -added in 1781 by a brilliant effort of the elder Herschel. We are -told that intense popular excitement followed the announcement of -Herschel's first observation: he was knighted and otherwise honored -by the English King, and was enabled to lay a secure foundation for -the future distinguished astronomical reputation of his family. - -Herschel's discovery quickened the restless activity of -astronomers. Persistent efforts were made to sift the heavens -more and more closely, with the strengthened hope of adding still -further to our planetary knowledge. An association of twenty-four -enthusiastic German astronomers was formed for the express purpose -of hunting planets. But it fell to the lot of an Italian, Piazzi, -of Palermo, to find the first of that series of small bodies now -known as the asteroids or minor planets. He made the discovery at -the very beginning of our century, January 1, 1801. - -But news travelled slowly in those days, and it was not until -nearly April that the German observers heard from Piazzi. In the -meantime, he had himself been prevented by illness from continuing -his observations. Unfortunately, the planet had by this time moved -so near the sun, on account of its own motions and those of the -earth, that it could no longer be observed. The bright light of -the sun made observations of the new body impossible; and it was -feared that, owing to lack of knowledge of the planet's orbit, -astronomers would be unable to trace it. So there seemed, indeed, -to be danger of an almost irreparable loss to science. But in -scientific, as in other human emergencies, someone always appears -at the proper moment. A very young mathematician at Göttingen, -named Gauss, attacked the problem, and was able to devise a method -of predicting the future course of the planet on the sky, using -only the few observations made by Piazzi himself. Up to that time -no one had attempted to compute a planetary orbit, unless he had -at his disposal a series of observations extending throughout the -whole period of the planet's revolution around the sun. But the -Piazzi planet offered a new problem in astronomy. It had become -imperatively necessary to obtain an orbit from a few observations -made at nearly the same date. Gauss's work was signally triumphant, -for the planet was actually found in the position predicted by him, -as soon as a change in the relative places of the planet and earth -permitted suitable observations to be made. - -But after all, Piazzi's planet belongs to a class of quite small -bodies, and is by no means as interesting as Herschel's discovery, -Uranus. Yet even this must be relegated to second rank among -planetary discoveries. On September 23, 1846, the telescope of the -Berlin Observatory was directed to a certain point on the sky for a -very special reason. Galle, the astronomer of Berlin, had received -a letter from Leverrier, of Paris, telling him that if he would -look in a certain direction he would detect a new and large planet. - -Leverrier's information was based upon a mathematical calculation. -Seated in his study, with no instruments but pen and paper, he -had slowly figured out the history of a world as yet unseen. -Tiny discrepancies existed in the observed motions of Herschel's -planet Uranus. No man had explained their cause. To Leverrier's -acute understanding they slowly shaped themselves into the -possible effects of attraction emanating from some unknown planet -exterior to Uranus. Was it conceivable that these slight tremulous -imperfections in the motion of a planet could be explained in this -way? Leverrier was able to say confidently, "Yes." But we may rest -assured that Galle had but small hopes that upon his eye first, of -all the myriad eyes of men, would fall a ray of the new planet's -light. Careful and methodical, he would neglect no chance of -advancing his beloved science. He would look. - -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 planet. To his -trained eye it was certainly recognizable at once. And then the -good news was sent on to Paris. We can imagine Leverrier, the cool -calculator, saying to himself: "Of course he found it. It was a -mathematical certainty." Nevertheless, his satisfaction must have -been of the keenest. No triumphs give a pleasure higher than those -of the intellect. Let no one imagine that men who make researches -in the domain of pure science are under-paid. They find their -reward in pleasure that is beyond any price. - -The Leverrier planet was found to be the last of the so-called -major planets, so far as we can say in the present state of -science. It received the name Neptune. Observers have found no -other member of the solar system comparable in size with such -bodies as Uranus and Neptune. More than one eager mathematician has -tried to repeat Leverrier's achievement, but the supposed planet -was not found. It has been said that figures never lie; yet such is -the case only when the computations are correctly made. 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 craft. It requires the test of time to affix to any man's work -the stamp of true genius. - -While, then, we have found no more large planets, quite a group of -companions to Piazzi's little one have been discovered. They are -all small, probably never exceeding about 400 miles in diameter. -All travel around the sun in orbits that lie wholly within that -of Jupiter and are exterior to that of Mars. The introduction of -astronomical photography has given a tremendous impetus to the -discovery of these minor planets, as they are called. It is quite -interesting to examine the photographic process by which such -discoveries are made possible and even easy. The matter will not -be difficult to understand if we remember that all the planets are -continually changing their places among the other stars. For the -planets travel around the sun at a comparatively small distance. -The great majority of the stars, on the contrary, are separated -from the sun by an almost immeasurable space. As a result, they do -not seem to move at all among themselves, and so we call them fixed -stars: they may, indeed, be in motion, but their great distance -prevents our detecting it in a short period of time. - -Now, stellar photographs are made in much the same way as ordinary -portraits. Only, instead of using a simple camera, the astronomer -exposes his photographic plate at the eye-end of a telescope. The -sensitive surface of the plate is substituted for the human eye. We -then find on the picture a little dot corresponding to every star -within the photographed region of the sky. But, as everyone knows, -the turning of the earth on its axis makes the whole heavens, -including the sun, moon, and stars, rise and set every day. So the -stars, when we photograph them, are sure to be either climbing up -in the eastern sky or else slowly creeping down in the western. And -that makes astronomical photography very different from ordinary -portrait work. - -The stars correspond to the sitter, but they don't sit still. -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 clock. The arrangement is so adjusted that the -telescope, once aimed at the proper object in the sky, will move -so as to remain pointed exactly the same during the whole time of -the photographic exposure. Thus, while the light of any star is -acting on the plate, such action will be continuous at a single -point. Consequently, the finished picture will show the star as a -little dot; while without this arrangement, the star would trail -out into a line instead of a dot. Now we have seen that the planets -are all moving slowly among the fixed stars. So if we make a star -photograph in a part of the sky where a planet happens to be, the -planet will make a short line on the plate; whereas, if the planet -remained quite unmoved relatively to the stars it would give a dot -like the star dots. The presence of a line, therefore, at once -indicates a planet. - -This method of planet-hunting has proved most useful. More than 400 -small planets similar to Piazzi's have been found, though never -another one like Uranus and Neptune. As we have said, all these -little bodies lie between Mars and Jupiter. They evidently belong -to a group or family, and many astronomers have been led to believe -that they are but fragments of a former large planet. - -In August, 1898, however, one was found by Witt, of Berlin, which -will probably occupy a very prominent place in the annals of -astronomy. For this planet goes well within the orbit of Mars, -and this will bring it at times very close to the earth. In fact, -when the motions of the new planet and the earth combine to bring -them to their positions of greatest proximity, the new planet will -approach us closer than any other celestial body except our own -moon. Witt named his new planet Eros. Its size, though small, may -prove to be sufficient to bring it within the possibilities of -naked-eye observation at the time of closest approach to the earth. - -To astronomers the great importance of this new planet is due to -the following circumstance: For certain reasons too technical to be -stated here in detail, the distance from the earth to any planet -can be determined with a degree of precision which is greatest for -planets that are near us. Thus in time we shall learn the distance -of Eros more accurately than we know any other celestial distance. -From this, by a process of calculation, the solar distance from the -earth is determinable. But the distance from earth to sun is the -fundamental astronomical unit of measure; so that Witt's discovery, -through its effect on the unit of measure, will doubtless -influence every part of the science of astronomy. 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 1898. - - - - -HOW TO MAKE A SUN-DIAL[A] - - -Long before clocks and watches had been invented, people began to -measure time with sun-dials. Nowadays, when almost everyone has a -watch in his pocket, and can have a clock, too, on the mantel-piece -of every room in the house, the sun-dial has ceased to be needed -in ordinary life. But it is still just as interesting as ever to -anyone who would like to have the means of getting time direct -from the sun, the great hour-hand or timekeeper of the sky. Any -person who is handy with tools can make a sun-dial quite easily, by -following the directions given below. - -In the first place, you must know that the sun-dial gives the time -by means of the sun's shadow. If you stick a walking-cane up in the -sand on a bright, sunshiny day, the cane has a long shadow that -looks like a dark line on the ground. Now if you watch this shadow -carefully, you will see that it does not stay in the same place all -day. Slowly but surely, as the sun climbs up in the sky, the shadow -creeps around the cane. You can see quite easily that if the cane -were fastened in a board floor, and if we could mark on the floor -the places where the shadow was at different hours of the day, we -could make the shadow tell us the time just like the hour-hand of a -clock. A sun-dial is just such an arrangement as this, and I will -show you how to mark the shadow places exactly, so as to tell the -right time without any trouble whenever the sun shines. - -If you were to watch very carefully such an arrangement as a cane -standing in a board floor, you would not find the creeping shadow -in just the same place at the same time every day. If you marked -the place of the shadow at exactly ten o'clock by your watch some -morning, and then went back another day at ten, you would not find -the shadow on the old mark. It would not get very far from it in -a day or two, but in a month or so it would be quite a distance -away. Now, of course, a sun-dial would be of no use if it did not -tell the time correctly every day; and in fact, it is not easy to -make a dial when the shadow is cast by a stick standing straight -up. 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 amount, as I will explain in a moment. Of course, we should -not really use the floor for our sun-dial. It is much better to -mark out the hour-lines, as they are called, on a smooth piece of -ordinary white board, and then, after the dial is finished, it can -be screwed down to a piazza floor or railing, or it can be fastened -on a window-sill. It ought to be put in a place where the sun can -get at it most of the time, because, of course, you cannot use the -sun-dial when the sun is not shining on it. If the dial is set on a -window-sill (of a city house, for instance) you must choose a south -window if you can, so as to get the sun nearly all day. If you have -to take an east window, you can use the dial in the morning only, -and in a west window only in the afternoon. Sometimes it is best -not to try to fasten the dial to its support with screws, but just -to mark its place, and then set it out whenever you want to use -it. For if the dial is made of wood, and not painted, it might be -injured by rain or snow in bad weather if left out on a window-sill -or piazza. - -[Illustration: Fig. 1.] - -It is not quite easy to fasten a little stick to a board so that it -will lean over just right. So it is better not to use a stick or -a cane in the way I have described, but instead to use a piece of -board cut to just the right shape. - -Fig. 1 shows what a sun-dial should look like. The lines to show -the shadow's place at the different hours of the day will be marked -on the board ABCD, and this will be put flat on the window-sill -or piazza floor. The three-cornered piece of board _abc_ is -fastened to the bottom-board ABCD by screws going through ABCD -from underneath. The edge _ab_ of the three-cornered board _abc_ -then takes the place of the leaning stick or cane, and the time -is marked by the shadow cast by the edge _ab_. Of course, it is -important that this edge should be straight and perfectly flat and -even. If you are handy with tools, you can make it quite easily, -but if not, you can mark the right shape on a piece of paper very -carefully, and take it to a carpenter, who can cut the board -according to the pattern you have marked on the paper. - -[Illustration: Fig. 2.] - -Now I must tell you how to draw the shape of the three-cornered -board _abc_. Fig. 2 shows how it is done. The side _ac_ should -always be just five inches long. The side _bc_ is drawn at right -angles to _ac_, which you can do with an ordinary carpenter's -square. The length of _bc_ depends on the place for which the dial -is made. The following table gives the length of _bc_ for various -places in the United States, and, after you have marked out the -length of _bc_, it is only necessary to complete the three-cornered -piece by drawing the side _ab_ from _a_ to _b_. - - -TABLE SHOWING THE LENGTH OF THE SIDE _bc_. - - -------------------------- - Place. _b c_ - Inches. - -------------------------- - Albany 4-11/16 - Baltimore 4-1/16 - Boston 4-1/2 - Buffalo 4-11/16 - Charleston 3-1/4 - Chicago 4-1/2 - Cincinnati 4-1/16 - Cleveland 4-1/2 - Denver 4-3/16 - Detroit 4-1/2 - Indianapolis 4-1/16 - Kansas City 3-15/16 - Louisville 3-15/16 - Milwaukee 3-11/16 - New Orleans 2-7/8 - New York 4-3/8 - Omaha 4-3/8 - Philadelphia 4-3/16 - Pittsburg 4-3/8 - Portland, Me 4-13/16 - Richmond 3-15/16 - Rochester 4-11/16 - San Diego 3-1/4 - San Francisco 3-15/16 - Savannah 3-1/8 - St. Louis 3-15/16 - St. Paul 5 - Seattle 5-9/16 - Washington, D. C. 4-1/16 - -------------------------- - -If you wish to make a dial for a place not given in the table, -it will be near enough to use the distance _bc_ as given for the -place nearest to you. But in selecting the nearest place from the -table, please remember to take that one of the cities mentioned -which is nearest to you in a north-and-south direction. It does -not matter how far away the place is in an east-and-west direction. -So, instead of taking the place that is nearest to you on the map -in a straight line, take the place to which you could travel by -going principally east or west, and very little north or south. The -figure drawn is about the right shape for New York. The board used -for the three-cornered piece should be about one-half inch thick. -But if you are making a window-sill dial, you may prefer to have it -smaller than I have described. You can easily have it half as big -by making all the sizes and lines in half-inches where the table -calls for inches. - -After you have marked out the dimensions for the three-cornered -piece that is to throw the shadow, you can prepare the dial itself, -with the lines that mark the place of the shadow for every hour -of the day. This you can do in the manner shown in Fig. 3. Just -as in the case of the three-cornered piece, you can draw the dial -with a pencil directly on a smooth piece of white board, about -three-quarters of an inch thick, or you can mark it out on a paper -pattern and transfer it afterward to the board. Perhaps it will be -as well to begin by drawing on paper, as any mistakes can then be -corrected before you commence to mark your wood. - -[Illustration: Fig. 3.] - -In the first place you must draw a couple of lines MN and M′N′, -eight inches long, and just far enough apart to fit the edge of -your three-cornered shadow-piece. You will remember I told you -to make that one-half inch thick, so your two lines will also be -one-half inch apart. Now draw the two lines NO and N′O′ square -with MN and M′N′, and make the distances NO and N′O′ just five -inches each. The lines OK, O′K′, and the other lines forming the -outer border of the dial, are then drawn just as shown, OK and O′K′ -being just eight inches long, the same as MN and M′N′. The lower -lines in the figure, which are not very important, are to complete -the squares. You must mark the lines NO and N′O′ with the figures -VI, these being the lines reached by the shadow at six o'clock in -the morning and evening. The points where the VII, VIII, and other -hour-lines cut the lines OK, O′K′, MK, and M′K′ can be found from -the table on page 78. - -In using the table you will notice that the line IX falls sometimes -on one side of the corner K, and sometimes on the other. Thus for -Albany the line passes seven and seven-sixteenth inches from O, -while for Charleston it passes four and three-eighth inches from M. -For Baltimore it passes exactly through the corner K. - -TABLE SHOWING HOW TO MARK THE HOUR-LINES. - - -----------------+-----------------------------+------------------------ - | Distance from O to the line | Distance from M to the - | marked | line marked - PLACE. +---------+---------+---------+-------+--------+------- - | VII. | VIII. | IX. | IX. | X. | XI. - -----------------+---------+---------+---------+-------+--------+------- - | Inches. | Inches. | Inches. |Inches.|Inches. |Inches. - Albany | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16 - Baltimore | 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16 - Boston | 2 | 4-5/16 | 7-7/16 | | 3-1/16 | 1-7/16 - Buffalo | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16 - Charleston | 2-7/16 | 5-3/8 | | 4-3/8 | 2-1/2 | 1-1/8 - Chicago | 2 | 4-5/16 | 7-7/16 | | 3-1/16 | 1-7/16 - Cincinnati | 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16 - Cleveland | 2 | 4-5/16 | 7-7/16 | -- | 3-1/16 | 1-7/16 - Denver | 2-1/8 | 4-1/2 | 7-11/16 | | 2-7/8 | 1-7/16 - Detroit | 2 | 4-5/16 | 7-7/16 | | 3-1/16 | 1-7/16 - Indianapolis | 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16 - Kansas City | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16 - Louisville | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16 - Milwaukee | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16 - New Orleans | 2-11/16 | 5-3/4 | | 4-1/16| 2-5/16 | 1-1/8 - New York | 2 | 4-5/16 | 7-11/16 | | 3-1/16 | 1-7/16 - Omaha | 2 | 4-5/16 | 7-11/16 | | 3-1/16 | 1-7/16 - Philadelphia | 2-1/8 | 4-1/2 | 7-11/16 | | 2-7/8 | 1-7/16 - Pittsburg | 2 | 4-5/16 | 7-11/16 | | 3-1/16 | 1-7/16 - Portland, Me | 1-15/16 | 4-3/16 | 7-1/8 | | 3-3/16 | 1-1/2 - Richmond | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16 - Rochester | 1-15/16 | 4-3/16 | 7-7/16 | | 3-1/16 | 1-7/16 - San Diego | 2-7/16 | 5-3/8 | | 4-3/8 | 2-1/2 | 1-1/8 - San Francisco | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16 - Savannah | 2-9/16 | 5-9/16 | | 4-1/4 | 2-1/2 | 1-1/8 - St. Louis | 2-1/4 | 4-11/16 | 8 | | 2-7/8 | 1-5/16 - St. Paul | 1-15/16 | 4-1/16 | 7-1/8 | | 3-3/16 | 1-1/2 - Seattle | 1-13/16 | 3-15/16 | 6-5/8 | | 3-3/8 | 1-1/2 - Washington, D. C.| 2-1/8 | 4-11/16 | 8 | | 2-7/8 | 1-7/16 - -----------------+---------+---------+---------+-------+--------+------- - -The distance for the line marked V from O′ is just the same as the -distance from O to VII. Similarly, IV corresponds to VIII, III to -IX, II to X, and I to XI. 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 -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 carefully, you must fasten -the three-cornered shadow-piece to it in such a way that the whole -instrument will look like Fig. 1. 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. 1 and 3) and -into the edge _ac_ (Fig. 2) of the three-cornered piece. The whole -instrument will then look something like Fig. 1. - -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 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 error 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. - - -FOOTNOTE: - -[A] This chapter is especially intended for boys and girls and -others who like to make things with carpenters' tools. - - - - -PHOTOGRAPHY IN ASTRONOMY - - -New highways of science have been monumented 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 photography do -not recount the achievements of extraordinary 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 invisible change takes place. -It is then merely necessary to plunge the plate into a properly -prepared chemical bath, and the gradual process of 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 photographic 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 Nature, -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 copying 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 photograph; -technically, it is known as a "positive." - -One of the remarkable things about the sensitive plate is its -complete indifference to the distance 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 photographer's table. This quality makes its use especially -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 advantage over the human -eye. If the conditions of weather and atmosphere are favorable, -an observer 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 diagrams. Occasional -moments of especially undisturbed 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 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. - -[Illustration: Star-Field in Constellation Monoceros. - -Photographed by Barnard, February 1, 1894. - -Exposure, three hours.] - -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 becomes 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 imagination. Scientific men are not given to fancies; nor -should they be. But the first long-exposure photograph must have -been an exciting thing. 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 -experimenter 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 astronomer must stop before even the faintest streak of dawn -begins to redden the eastern sky. Moreover, making astronomical -negatives requires excessively 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 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 matter; -for the slightest disturbance of the plate before the second or -subsequent portions of the exposure 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 looking through the visual glass, can watch -objects in the heavens even while they are being photographed. The -visual half of the instrument is provided with a pair of very fine -cross-wires movable at will in the field of view. These can be -made to bisect some little star exactly, before beginning the first -night's work. Afterward, everything about the instrument having -been left unchanged, 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. Consequently, this motion has to be -counteracted by shifting the telescope so as to follow the stars. -This can be accomplished accurately and automatically by means -of clock-work mechanism. Such contrivances have already been -applied in the past to visual telescopes, because even then they -facilitated the observer's work. They save him the trouble of -turning his instrument every few minutes, and allow him to give his -undivided attention to the actual business of observation. - -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 -instrument 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 mechanical device sometimes called -a "mouse-control." He need only touch an electric button, 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 substituted, 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 made to run slower temporarily. Thus by watching -the cross-wires continuously, and manipulating his two electric -buttons, the photographic astronomer can compel his telescope -to follow exactly the object under observation, and he can make -certain of obtaining a perfect negative. - -These long-exposure plates are intended especially for what may be -called descriptive astronomy. 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 -beyond the confines of the old visible universe. Extremely faint -objects can be recorded, even down to their minutest details, with -a fidelity unknown to older visual methods. But at present we -intend to consider principally applications of photography in the -astronomy of measurement, 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 precise measurement, with all that the word precision -implies in celestial science. - -Taking up first the photography of stars, we must begin by -mentioning the work of Rutherfurd at New York. 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 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. -Consequently, 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 difficulties in the way of photographing faint stars -in Rutherfurd's time. Nor did he possess the modern ingenious -device of a combined visual and photographic instrument. He had no -electric controlling apparatus. In fact, the younger generation of -astronomers can form no adequate idea of the patience and personal -skill Rutherfurd 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 imagination 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 scientific system, which has -been made to conform 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 1, and the denominator 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 ⅖. The third magnitude, in turn, may -be compared with the second by the same fraction ⅖; and so the -classification is extended to magnitudes below those visible to the -unaided eye. Each magnitude 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 photographs. 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 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 photography, 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 accredited -methods of direct eye-observation. - -And photographs are far superior in ease of manipulation. -Convenient day-observing under the microscope in a comfortable -astronomical laboratory 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. - -Rutherfurd's stellar work extended down to about 1877, and -included especially parallax determinations 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 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 observation is very far away, in -comparison with the distance 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 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 -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 astronomers 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 mentioned, 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 examination of the photographs under the microscope -will then enable us to measure the slightest displacement of the -parallax star. From these measures, by a process of calculation, -astronomers can then obtain the star's distance. It will not -become known in miles; we shall only ascertain 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 separating us from the -star: and the subsequent evaluation of this distance "earth to sun" -in miles is another important problem in whose solution 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 astronomy 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 impose a limit on -numbers. Usually only two can 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 "parallax 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 number of stars condensed into one small spot -upon the heavens means something more than chance aggregation. -The Pleiades group (page 10) contains thousands of massive stars, -doubtless held together by the force of their mutual gravitational -attraction. If this be true, there must be complex orbital motion -in the cluster; and, as time goes on, we should actually see the -separate components change their relative positions, as it were, -before our eyes. 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 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 surpassingly -accurate picture of a star-cluster, we can subject it easily to -precise microscopic measurement. The same operations repeated at a -later date will enable us to compare the two series of measures, -and thus ascertain the motions 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 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 facilities, -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 -photography 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 himself forward in any way, and disliking to rush into -print, Rutherfurd had given but little publicity to his work. This -peculiarity has, doubtless, delayed his just reputation; but he -will lose nothing in the end from a brief postponement. Gill must, -however, 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 ideas. - -However this may be, the comet was photographed with the help of -a local portrait photographer named Allis. When Gill and Allis -fastened a simple portrait camera belonging to the latter upon the -tube of one of the Cape telescopes, and pointed it at the great -comet, they little thought the experiment would lead to one of the -greatest astronomical works ever attempted by men. Yet this was -destined to occur. The negative they obtained showed an excellent -picture of the comet; but what was more important for the future of -sidereal astronomy, it was also quite thickly dotted with little -black points corresponding to stars. The extraordinary ease with -which the whole heavens could be thus charted photographically -was brought home to Gill as never before. It was this comet -picture that interested him in the application of photography -to star-charting; and without his interest the now famous -astro-photographic 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 observatory to be photographed. A -series of telescopes 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 particularly 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 under way. One of the participating -observatories, that at Potsdam, Germany, has published the first -volume of its part of the catalogue. It is estimated that this -observatory alone will require twenty quarto volumes to contain -merely the final results of its work on the catalogue. Altogether -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 characteristic of photography. The great -catalogue is, perhaps, the best illustration of this tendency; but -of scarcely smaller interest, though less important in reality, is -the photographic method of 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 discovering unknown members -of this group was painfully 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 region -of the sky was made with an exposure lasting 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 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 exposed. This motion would be faithfully reproduced -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 ascertain whether the -object is a planet already known or a genuine new discovery. Wolf, -and others using his method in recent years, have made immense -additions to our catalogue of asteroids. Indeed, the matter was -beginning to lose interest 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, discovered the small body -that bears the number "433" in the list of minor planets, and has -received the name Eros. Its important peculiarity consists in the -exceptional position of the orbit. While all the other asteroids -are farther 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 neighbor; and this fact alone makes it the -most interesting of all the minor planets. The nineteenth century -was opened by Piazzi's well-known discovery 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 certain 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 fundamental -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. - -We shall not here enter into the somewhat intricate details of -this subject, however interesting they may be. The problem offers -difficulties 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 precisely 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 sufficient size. The best we can do is to use the distance -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 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 observational determinations of the planetary orbital elements. - -Photography is by no means limited to investigations in the older -departments of astronomical observation. Its powerful arm has -been stretched out to grasp as well the newer instruments of -spectroscopic study. Here the sensitive 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 rendered -practicable series of observations extensive enough in numbers to -furnish statistical information 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 approach or recession has become measurable, -and has been evaluated in the case of many individual stars. (See -page 21.) - -[Illustration: Solar Corona. Total Eclipse. - -Photographed by Campbell, January 22, 1898; Jeur, India.] - -The subject of solar physics has become a veritable department of -astronomy in the hands of photographic investigators. Ingenious -spectro-photographic methods have been devised, whereby 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 corona, 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 -hurried excitement have comparatively little value. Half a dozen -persons might make drawings of the corona during the same eclipse, -yet they 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 discussed -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 revolutionized. There is to-day scarcely an instrument 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 photography; and the end is not yet. Valuable as are -the achievements already consummated, photography is richest in -its promise for the future. Astronomy has been called the "perfect -science"; 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 title. - - - - -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 astronomy 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 endless -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 latest 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 ever been a labor of love to the greatest -minds; their solution has been counted justly among man's loftiest -achievements. - -And yet of all the difficult and abstruse sciences, astronomy is, -perhaps, the one that comes into the ordinary practical daily -life of the people more definitely and frequently than any other. -There exist at least three things we owe to astronomy that must -be regarded as quite indispensable, from a purely practical point -of view. In the first place, let us consider the maps in a work -on geography. How many people ever think to ask how these maps -are made? 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 surface. Even the locations of towns and rivers might -be marked in this way. But one of the most important things of all -could not be added without the aid of astronomical observations. -The latitude and longitude lines, which are essential to show the -relation of the map to the rest of the earth, we owe to astronomy. -The longitude lines, particularly, as we shall see farther on, play -a most important part in the subject of time. - -The second indispensable application of astronomy to ordinary -business affairs relates to the subject of navigation. How do ships -find their way across the ocean? There are no permanent marks on -the sea, as there are on the land, by which the navigator can guide -his course. Nevertheless, seamen know their path over the trackless -ocean with a certainty as unerring as would be possible on shore; -and it is all done by the help of astronomy. The navigator's -observations of the sun are astronomical observations; 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 -tables. Indeed, it is not too much to say that without astronomy -there could be no safe ocean navigation. - -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 purposes. It is to this practical use of -astronomical science that we would direct particular attention. Few -persons ever think of the complicated machinery that must be put -in motion in order to set a clock. A man forgets some evening 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 persons, perhaps, visit the jeweller's -and take the time from his "regulator." But the regulator itself -needs to be regulated. After all, it is nothing 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 jeweller'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 principal -duty of the astronomer has been the furnishing of accurate time -to the people. We shall not here enter into a detailed account, -however interesting 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 person an astronomical clock -is a complicated and elaborate affair, giving the date, day of the -week, phases of the moon, and other miscellaneous information. But -in reality the astronomer wants none of these things. 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 hands. No -expense is spared in making the standard clock for an observatory. -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-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 complications 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 necessary 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 accurately 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 foundation. Moreover, this pier is best placed underground -in a cellar, so that the temperature of the 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 noticeable difference in their running when the barometer is -high and when it is low. To prevent this, some of our best clocks -have been enclosed 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 construction, 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 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 observatory. In the field of view of the telescope certain -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 astronomical -meridian. - -Now we are in possession of certain star-tables, computed from -the combined observations of astronomers in the last 150 years. -These tables 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 telescope, 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 several 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 interesting details of the -methods by which the astronomical 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 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 representative of those employed elsewhere. - -Every day, at noon precisely, an electric signal is sent out by -the United States Naval Observatory 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 received in the central New York office -of the telegraph company, where it is used to keep correct 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 company'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 essentially -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. - -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 principle: When the sun -reaches its highest position in the sky it is twelve o'clock or -noon. - -The sun, as everyone knows, rises each morning 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 -difficulty in understanding how time differences arise. 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 -longitude, 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 -between 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 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 evidently 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 origin 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 Greenwich 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. - -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 degrees, we shall have the following -remarkable result 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 imaginary island is exactly 180 degrees -from Greenwich, we can consider it as being either 180 degrees east -or 180 degrees west. But if we call it 180 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, between these two possible ways of reckoning its -time. This circumstance has actually led to considerable confusion -in some of the islands of the 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 180th longitude circle, and it has been decided -that the islands on the east side of this line shall count their -longitudes west from Greenwich, and those west of the line shall -count longitude east from Greenwich. Thus Samoa is nearly 180 -degrees west of Greenwich, while the Fiji Islands are nearly 180 -degrees east. Yet the islands are very near each other, though the -arbitrary 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 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 1st, in Manila, would correspond to four -o'clock, Saturday afternoon, April 30th, in New York. - -There is another kind of time which we shall explain 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 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 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 Buffalo some hours later by -New York time. But it would leave Buffalo by Buffalo time, which -is quite different. Thus there would be a sort of 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 usually ran its trains by the -time used in the principal 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 -country 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 -degrees. 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 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 -considered 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 civilized countries--France is the only -nation of importance 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, instead of even hours as used in the United States. - - -TABLE OF THE WORLD'S TIME STANDARDS - - ----------------+---------------------++------------------ - When it is Noon | || Date of Adopting - at Greenwich | In || Standard Time - it is | || System. - ----------------+---------------------++------------------ - Noon | Great Britain. || - | Belgium. || May, 1892. - | Holland. || May, 1892. - | Spain. || January, 1901. - 1 P.M. | Germany. || April, 1893. - | Italy. || November, 1893. - | Denmark. || January, 1894. - | Switzerland. || June, 1894. - | Norway. || January, 1895. - | Austria (railways). || - 1.30 P.M. | Cape Colony. || 1892. - | Orange River Colony.|| 1892. - | Transvaal. || 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. - 10 P.M. | Victoria. || February, 1895. - | New South Wales. || February, 1895. - | Queensland. || February, 1895. - 11 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 " " " " " " - 7 A.M. " Eastern " " " " " " - 8 A.M. " 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 ice. 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 earth. If we are to regard the pole as very near the -point of greatest cold, it becomes a matter of much interest to -examine whether the pole has always occupied its present position, -or whether it has been subject to slow changes of place upon -the earth's surface. Therefore, 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 motion. - -Now we may say at once that astronomical research has not as yet -revealed the evidence thus expected. Astronomy has been unable to -come to the rescue of geological theory. From about the year 1750, -which saw the beginning of precise observation in the modern sense, -down to very recent times, astronomers were compelled to deny the -possibility of any appreciable motion of the pole. Observational -processes, it is true, furnished slightly divergent pole positions -from time to time. 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 eye. - -But in the last few years improved methods of observation, coupled -with extreme diligence in their application by astronomers -generally, have brought to light a certain small motion of the pole -which had never before been demonstrated in a reliable way. This -motion, it is true, is not of the character demanded by geological -theory, for the geologists had been led to expect a motion which -would be continuous in the same direction, no matter how slow might -be its annual amount; for the vast extent of geologic time would -give even the slowest of motions an opportunity to produce large -effects, provided its results could be continuously cumulative. -Given time enough, and the pole might move anywhere on the earth, -no matter how slow might be its tortoise speed. - -But the small motion we have discovered is neither cumulative nor -continuous in one direction. It is what we call a periodic motion, -the pole swinging now to one side, and now to the other, of its -mean or average position. Thus this new discovery cannot be said -to unravel the mysterious puzzle of the geologists. Yet it is not -without the keenest interest, even from their point of view; for -the proof of any form of motion in a pole previously supposed to be -absolutely at rest may mean everything. No man can say what results -will be revealed by the further observations now being continued -with great diligence. - -In the first place, 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 latitudes. Let us imagine a pair of straight lines -passing through the centre of the earth and terminating, one at the -observer's station on the earth's surface, and the other at that -point of the equator which is nearest the observer. Then, according -to the ordinary definition of latitude, the angle between these two -imaginary lines is called the latitude of the point of observation. -Now we know, of course, that the equator is everywhere just 90 -degrees from the pole. Consequently, if the pole is subject to any -motion at all, the equator must also partake of the motion. - -Thus the angle between our two imaginary lines will be affected -directly by polar movement, and the latitude obtained by -astronomical observation will be subject to quite similar changes. -To clear up the whole question, so far as this can be done by -the gathering of observational evidence, it is only necessary to -keep up a continual series of latitude determinations at several -observatories. These determinations should show small variations -similar in magnitude to the wabblings of the pole. - -Let us now consider for a moment what is meant by the axis of the -earth. It has long been known that the planet has in general the -shape of a ball or sphere. That this is so can be seen at once -from the way ships at sea disappear at the horizon. As they go -farther and farther from us, we first lose sight of the hull, and -then slowly and gradually the spars and sails seem to sink down -into the ocean. This proves that the earth's surface is curved. -That it is more or less like a sphere is evident from the fact that -it always casts a round shadow in eclipses. Sometimes the earth -passes between the sun and eclipsed moon. Then we see the earth's -black shadow projected on the moon, which would otherwise be quite -bright. This shadow has been observed in a very large number of -such eclipses, and it has always been found to have a circular edge. - -While, therefore, the earth is nearly a round ball, it must not be -supposed that it is exactly spherical in form. We may disregard -the small irregularities of its surface, for even the greatest -mountains are insignificant in height when compared with the -entire diameter of the earth itself. But even leaving these out of -account, the earth is not perfectly spherical. We can describe it -best as a flattened sphere. It is as though one were to press a -round rubber ball between two smooth boards. It would be flattened -at the top and bottom and bulged out in the middle. This is the -shape of the earth. It is flattened at the poles and bulges out -near the equator. 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 figure; and the two -points where this axis meets the surface are called the poles of -figure. - -But the earth has another axis, called the axis of rotation. This -is the one about which the planet turns once in a day, giving -rise to the well-known phenomena called the rising and setting of -sun, moon, and stars. For these motions of the heavenly bodies -are really only apparent ones, caused by an actual motion of the -observer on the earth. The observer turns with the earth on its -axis, and is thus carried past the sun and stars. - -This daily turning of the earth, then, takes place about the axis -of rotation. Now, it so happens that all kinds of astronomical -observations for the determination of latitude lead to values based -on the rotation axis of the earth, and not on its axis of figure. -We have seen how the earth's equator, from which we count our -latitudes, is everywhere 90 degrees distant from the pole. But this -pole is the pole of rotation, or the point at which the rotation -axis pierces the earth's surface. It is not the pole of figure. - -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 other. These -two poles are actually very near together; indeed, it was supposed -for a very long time that they were absolutely coincident, so that -there could not be any variations of latitude. But it now appears -that they are separated slightly. - -Strange to say, one of them is revolving about the other in a -little curve. The pole of figure is travelling around the pole of -rotation. The distance between them varies a little, never becoming -greater than about fifty feet, and it takes about fourteen months -to complete a revolution. There are some slight irregularities in -the motion, but, in the main, it takes place in the manner here -stated. In consequence of this rotation of the one pole about the -other, the pole of figure is now on one side of the rotation pole -and now on the opposite side, but it never travels continuously -in one direction. Thus, as we have already seen, the sort of -continuous motion required to explain the observed geological -phenomena has not yet been found by astronomers. - -Observations for the study of latitude variations have been made -very extensively within recent years both in Europe and the United -States. It has been found practically most advantageous to carry -out simultaneous series of observations at two observatories -situated in widely different parts of the earth, but having very -nearly the same latitude. It is then possible to employ the same -stars for observation in both places, whereas it would be necessary -to use different sets of stars if there were much difference in the -latitudes. - -There is a special advantage in using the same stars in both -places. 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 observed; for, strange -as it may seem, our star-catalogues do not contain absolutely -accurate numbers. Like all other data depending on fallible -human observation, they are affected with small errors. But if -we can determine simply the difference in latitude of the two -observatories, we can discover from its variation the path in -which the pole is moving. If, for instance, the observatories are -separated by one-quarter the circumference of the globe, the pole -will be moving directly toward one of them, when it is not changing -its distance from the other one at all. - -This method was used for seven years with good effect at the -observatories of Columbia University in New York, and the Royal -Observatory at Naples, Italy. For obtaining its most complete -advantages it is, of course, better to establish several observing -stations on about the same parallel of latitude. This was done -in 1899 by the International Geodetic Association. Two stations -are in the United States, one in Japan, and one in Sicily. We -can, therefore, hope confidently that our knowledge as to the -puzzling problem of polar motion will soon receive very material -advancement. - - - - -SATURN'S RINGS - - -The death of James E. Keeler, Director of the Lick Observatory, in -California (p. 32), recalls to mind one of the most interesting -and significant of later advances in astronomical science. -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 Saturn. His results have not -yet been made sufficiently accessible to the public at large, nor -have they been generally valued at their true worth. We consider -this work of Keeler's interesting, because the problem of the rings -has been a classic one for many generations; and we have been -particular, also, to call it significant, because it is pregnant -with the possibilities of newer methods of spectroscopic research, -applied in the older departments of observational astronomy. - -The troubles of astronomers with the rings began with the invention -of the telescope itself. They date back to 1610, when Galileo -first turned his new instrument to the heavens (p. 49). It may -be imagined easily that the bright planet Saturn was among the -very first objects scrutinized by him. His "powerful" instrument -magnified only about thirty times, and was, doubtless, much -inferior to our pocket telescopes of to-day. But it showed, at -all events, that something was wrong with Saturn. Galileo put it, -"_Ultimam planet am tergeminam observavi_" ("I have observed the -furthest planet to be triple"). - -It is easy to understand now how Galileo's eyes deceived him. For -a round luminous ball like Saturn, surrounded by a thin flat ring -seen nearly edgewise, really looks as if it had two little attached -appendages. Strange, indeed, it is to-day to read a scientific -book so old that the planet Saturn could be called the "furthest" -planet. But it was the outermost known in Galileo's day, and -for nearly two centuries afterward. Not until 1781 did William -Herschel discover Uranus (p. 59); and Neptune was not disclosed by -the marvellous mathematical perception of Le Verrier until 1846 (p. -61). - -Galileo's further observations of Saturn bothered him more and -more. The planet's behavior became much worse as time went on. -"Has Saturn devoured his children, according to the old legend?" -he inquired soon afterward; 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 -edgewise, and was, therefore, perfectly invisible to Galileo's -"optic tube." The puzzle remained unsolved by Galileo; it was left -for another great man to find the true answer. Huygens, in 1656, -first announced that the ring _is_ a ring. - -The manner in which this announcement was made is characteristic -of the time; to-day it seems almost ludicrous. Huygens published a -little pamphlet in 1656 called "_De Saturni Luna Observatio Nova_" -or, "A New Observation of Saturn's Moon." He gave the explanation -of what had been observed by himself and preceding astronomers in -the form of a puzzle, or "logogriph." Here is what he had to say of -the phenomenon in question: - -"aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s -ttttt uuuuu." - -It was not until 1659, three years later, in a book entitled -"_Systema Saturnium_," that Huygens rearranged the above letters in -their proper order, giving the Latin sentence: - -"_Annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam -inclinato._" Translated into English, this sentence informs us that -the planet "is girdled with a thin, flat ring, nowhere touching -Saturn, and inclined to the ecliptic"! - -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 himself. It -was an explanation that _explained_. The reason for its preliminary -announcement in the above manner must have been the following: -Huygens was probably not quite sure of his ground in 1656, -while three years afterward he had become quite certain. By the -publication of the logogriph of 1656 he secured for himself the -credit of what he had done. If any other astronomer had published -the true explanation after 1656, Huygens could have proved his -claim to priority by rearranging the letters of his puzzle. On the -other hand, if further researches showed him that he was wrong, -he would never have made known the true meaning of his logogriph, -and would thus have escaped the ignominy of making an erroneous -explanation. Thus, the method of announcement was comparable in -ingenuity with the Huygenian explanation itself. - -We are compelled to pass over briefly the entertaining history of -subsequent observations of the ring, in order to explain the new -work of Keeler and others. Cassini, about 1675, been able to show -that the ring was double; that there are really two independent -rings, with a distinct dark space between them. It was a case of -wheels within wheels. To our own eminent countryman, W. C. Bond, -of Cambridge, Mass., we owe the further discovery (Harvard College -Observatory, November, 1850) of the third ring. This is also -concentric with the other two, and interior to them, but difficult -to observe, because of its much smaller luminosity. - -It is almost transparent, and the brilliant light of the planet's -central ball is capable of shining directly through it. For this -reason the inner ring is called the "gauze" or "crape" ring. If we -add to the above details the fact that our modern large telescopes -show slight irregularities in the surface of the rings, especially -when seen edgewise, we have a brief statement of all that the -telescope has been able to reveal to us since Galileo's time. - -But of far greater interest than the mere fact of their existence -is the important cosmic question as to the constitution, structure, -and, above all, durability of the ring system. Astronomers often -use the term "stability" with regard to celestial systems like the -ring system of Saturn. By this they mean permanent durability. A -system is stable if its various parts can continue in their present -relationship to one another, without violating any of the known -laws of astronomy. Whenever we study any collection of celestial -objects, and endeavor to explain their motions and peculiarities, -we always seek some explanation not inconsistent with the continued -existence of the phenomena in question. For this there is, perhaps, -no sufficient philosophical basis. Probably much of the great -celestial procession is but a passing show, to be but for a moment -in the endless vista of cosmic time. - -However this may be, we are bound to assume as a working theory -that Saturn has always had these rings, and will always have them; -and it is for us to find out how this is possible. The problem has -been attacked mathematically by various astronomers, including -Laplace; but no conclusive mathematical treatment was obtained -until 1857, when James Clerk Maxwell proved in a masterly manner -that the rings could be neither solid nor liquid. He showed, -indeed, that they would not last if they were continuous bodies -like the planets. A big solid wheel would inevitably be torn -asunder by any slight disturbance, and then precipitated upon -the planet's surface. Therefore, the rings must be composed of -an immense number of small detached particles, revolving around -Saturn in separate orbits, like so many tiny satellites. - -This mathematical theory of the ring system being once established, -astronomers were more eager than ever to obtain a visual -confirmation of it. We had, indeed, a sort of analogy in the -assemblage of so-called "minor planets" (p. 64), which are known -to be revolving around our sun in orbits situated between Mars and -Jupiter. Some hundreds of these are known to exist, and probably -there are countless others too small for us to see. Such a swarm -of tiny particles of luminous matter would certainly give the -impression of a continuous solid body, if seen from a distance -comparable to that separating us from Saturn. But arguments founded -on analogy are of comparatively little value. - -Astronomers need direct and conclusive telescopic evidence, and -this was lacking until Keeler made his remarkable spectroscopic -observation in 1895. The spectroscope is a peculiar instrument, -different in principle from any other used in astronomy; we study -distant objects with it by analyzing the light they send us, rather -than by examining and measuring the details of their visible -surfaces. The reader will recall that according to the modern -undulatory theory, light consists simply of a series of waves. -Now, the nature of waves is very far from being understood in the -popular mind. Most people, for instance, think that the waves of -ocean consist of great masses of water rolling along the surface. - -This notion doubtless arises from the behavior of waves when -they break upon the shore, forming what we call surf. When a -wave meets with an immovable body like a sand beach, the wave is -broken, and the water really does roll upon the beach. But this -is an exceptional case. Farther away from the shore, where the -waves are unimpeded, they consist simply of particles of water -moving straight up and down. None of the water is carried by mere -wave-action away from the point over which it was situated at first. - -Tides or other causes may move the water, but not simple -wave-motion alone. That this is so can be proved easily. 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 waves, but it will not move. -Similarly, wind-waves are often quite conspicuous on a field of -grain; but they are caused by the individual grain particles moving -up and down. The grain certainly cannot travel over the ground, -since each particle is fast to its own stalk. - -But while the particles do not travel, the wave-disturbance -does. At times it is transmitted to a considerable distance from -the point where it was first set in motion. Thus, when a stone -is dropped into still water, the disturbance (though not the -water) travels in ever-widening circles, until at last it becomes -too feeble for us to perceive. Light is just such a travelling -wave-disturbance. Beginning, perhaps, in some distant star, it -travels through space, and finally the wave impinges on our eyes -like the ocean-wave breaking on a sand beach. Such a light-wave -affects the eye in some mysterious way. We call it "seeing." - -The spectroscope (p. 21) enables us to measure and count the waves -reaching us each second from any source of light. No matter how far -away the origin of stellar light may be, the spectroscope examines -the character of that light, and tells us the number of waves set -up every second. It is this characteristic of the instrument that -has enabled us to make some of the most remarkable observations of -modern times. If the distant star is approaching us in space, more -light-waves per second will reach us than we should receive from -the same star at rest. Thus if we find from the spectroscope that -there are too many waves, we know that the star is coming nearer; -and if there are too few, we can conclude with equal certainty that -the star is receding. - -Keeler was able to apply the spectroscope in this way to the planet -Saturn and to the ring system. The observations required dexterity -and observational manipulative skill in a superlative degree. These -Keeler had; and this work of his will always rank as a classic -observation. He found by examining the light-waves from opposite -sides of the planet that the luminous ball rotated; for one side -was approaching us and the other receding. This observation was, -of course, in accord with the known fact of Saturn's rotation -on his axis. With regard to the rings, Keeler showed in the same -way the existence of an axial rotation, which appears not to have -been satisfactorily proved before, strange as it may seem. But the -crucial point established by his spectroscope was that the interior -part of the rings rotates _faster_ than the exterior. - -The velocity of rotation diminishes gradually from the inside to -the outside. This fact is absolutely inconsistent with the motion -of a solid ring; but it fits in admirably with the theory of a -ring comprised of a vast assemblage of small separate particles. -Thus, for the first time, astronomy comes into possession of an -observational determination of the nature of Saturn's rings, and -Galileo's puzzle is forever solved. - - - - -THE HELIOMETER - - -Astronomical discoveries are always received by the public with -keen interest. Every new fact read in the great open book of nature -is written eagerly into the books of men. For there exists a strong -curiosity to ascertain just how the greater world is built and -governed; and it must be admitted that astronomers have been able -to satisfy that curiosity with no small measure of success. But it -is seldom that we hear of the means by which the latest and most -refined astronomical observations are effected. Popular imagination -pictures the astronomer, as he doubtless once was, an aged -gentleman, usually having a long white beard, and spending entire -nights staring at the sky through a telescope. - -But the facts to-day are very different. The working astronomer -is an active man in the prime of life, often a young man. He -wastes no time in star-gazing. His observations consist of -exact measurements made in a precise, systematic, and almost -business-like manner. A night's "watch" at the telescope is -seldom allowed to exceed about three hours, since it is found -that more continued exertions fatigue the eye and lead to less -accurate results. To this, of course, there have been many notable -exceptions, for endurance of sight, like any form of physical -strength, differs greatly in different individuals. Astronomical -research does not include "picking out" the constellations, and -learning the Arabic names of individual stars. These things are not -without interest; but they belong to astronomy's ancient history, -and are of little value except to afford amusement and instruction -to successive generations of amateurs. - -Among the instruments for carefully planned measurements of -precision the heliometer probably takes first rank. It is at -once the most exquisitely accurate in its results, and the most -fatiguing to the observer, of all the varied apparatus employed by -the astronomer. The principle upon which its construction depends -is very peculiar, and applies to all telescopes, even ordinary -ones for terrestrial purposes. If part of a telescope lens be -covered up with the hand, it will still be possible to see through -the instrument. 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 point, or focus, a greater -amount of their light than could be brought together by the far -smaller lens in the unaided eye. - -The telescope might very properly be likened to an enlarged eye, -which can see more than we can, simply because it is bigger. If -a telescope lens has a surface one hundred times as large as -that of the lens in our eye, it will gather and bring to a focus -one hundred times as much light from a distant object. Now, if -any part of this telescope be covered, the remaining part will, -nevertheless, gather and focus light just as though the whole lens -were in action; only, there will be less light collected at the -focus within the tube. 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 instrument. For of this simple character is the operation of -any telescope: the large glass lens at one end collects a distant -planet's light, and brings it to a focus near the other end of the -tube, where it forms a tiny picture of the planet, which, in turn, -is examined with the little magnifier at the eye-end. - -Having arrived at the fundamental principle that part of a lens -will act in a manner similar to a whole one, it is easy to explain -the construction of a heliometer. An ordinary telescope lens is -sawed in half by means of a thin round metal disk revolved rapidly -by machinery, and fed continually with emery and water at its -edge. The cutting effect of emery is sufficient to make such a -disk enter glass much as an ordinary saw penetrates wood. The two -"semi-lenses," as they are called, are then mounted separately in -metal holders. These are attached to one end of the heliometer, -called the "head," in such a way that the two semi-lenses can slide -side by side upon metal guides. This head is then fastened to one -end of a telescope tube mounted in the usual way. The "head" end -of the instrument is turned toward the sky in observing, and at -the eye-end is placed the usual little magnifier we have already -described. - -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 tube. Now, if he moves the semi-lenses so -as to bring them side by side exactly, the whole arrangement will -act like an ordinary telescope. For the semi-lenses will then fit -together just as if the original glass had never been cut. But -if the half-lenses are separated a little on their slides, each -will act by itself. Being slightly separated, each will cover a -different part of the sky. The whole affair acts as if the observer -at the eye-end were looking through two telescopes at once. For -each semi-lens acts independently, just as if it were a complete -glass of only half the size. - -Now, suppose there were a couple of stars in the sky, one in the -part covered by the first semi-lens, and one in the part covered -by the second. The observer would, of course, see both stars at -once upon looking into the little magnifier at the eye-end of the -heliometer. - -We must remember that these stars will appear in the field of view -simply as two tiny points of light. The astronomer, as we have -said, is provided with a simple system of long rods, by means of -which he can manipulate the semi-lenses while the observation is -being made. If he slides them very slowly one way or the other, the -two star-points in the field of view will be seen to approach each -other. In this way they can at last be brought so near together -that they will form but a single dot of light. Observation with -the heliometer consists in thus bringing two star-images together, -until at last they are superimposed one upon the other, and we see -one image only. Means are provided by which it is then possible to -measure the amount of separation of the two half-lenses. Evidently -the farther asunder on the sky are the two stars under observation, -the greater will be the separation of the semi-lenses necessary -to make a single image of their light. Thus, measurement of the -lenses' separation becomes a means of determining the separation -of the stars themselves upon the sky. - -The two slides in the heliometer head are supplied with a pair of -very delicate measures or "scales" usually made of silver. These -can be examined from the eye-end of the instrument by looking -through a long microscope provided for this special purpose. 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 -examination. Measures made in this way with the heliometer are -counted the most precise of astronomical observations. - -Having thus described briefly the kind of observations obtained -with the heliometer, we shall now touch upon their further -utilization. We shall take as an example but one of their many -uses--that one which astronomers consider the most important--the -measurement of stellar distances. (See also p. 94.) - -Even the nearest fixed star is almost inconceivably remote from -us. And astronomers are imprisoned on this little earth; we cannot -bridge the profound distance separating us from the stars, so as -to use direct measurement with tape-line or surveyor's chain. We -are forced to have recourse to some indirect method. Suppose a -certain star is suspected, on account of its brightness, or for -some other reason, of being near us in space, and so giving a -favorable opportunity for a determination of distance. A couple of -very faint stars are selected close by. These, on account of their -faintness, the astronomer may regard as quite immeasurably far -away. He then determines with his heliometer the exact position on -the sky of the bright star with respect to the pair of faint ones. -Half a year is then allowed to pass. During that time the earth has -been swinging along in its annual path or orbit around the sun. -Half a year will have sufficed to carry the observer on the earth -to the other side of that path, and he is then 185,000,000 miles -away from his position at the first observation. - -Another determination is made of the bright star's position as -referred to the two faint ones. Now, if all these stars were -equally distant, their relative positions at the second observation -would be just the same as at the former one. But if, as is very -probable, the bright star is very much nearer us than are the two -faint ones, we shall obtain a different position from our second -observation. For the change of 185,000,000 miles in the observer's -location will, of course, affect the direction in which we see -the near star, while it will leave the distant ones practically -unchanged. Without entering into technical details, we may say that -from a large number of observations of this kind, we can obtain -the distance of the bright star by a process of calculation. The -only essential is to have an instrument that can make the actual -observations of position accurately enough; and in this respect the -heliometer is still unexcelled. - - - - -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 see. Of course, it has this in common with the sun and stars -and planets, that it rises in the eastern horizon, slowly climbs -the dome of the sky, and again goes down and sets in the west. -This motion of the heavenly bodies is known to be an apparent one -merely, and caused by the turning of our own earth upon its axis. A -man standing upon the earth's surface can look up and see above him -one-half the great celestial vault, gemmed with twinkling stars, -and bearing, perhaps, within its ample curve one or two serenely -shining planets and the lustrous moon. But at any given moment the -observer can see nothing of the other half of the heavenly sphere. -It is beneath his feet, and concealed by the solid bulk of the -earth. - -The earth, however, is turning on an axis, carrying the observer -with it. And so it is continually presenting him to a new part of -the sky. At any moment he sees but a single half-sphere; yet the -very next instant it is no longer the same; a small portion has -passed out of sight on one side by going down behind the turning -earth, while a corresponding new section has come into view on -the opposite side. It is this coming into view that we call the -rising of a star; and the corresponding disappearance on the other -side is called setting. Thus rising and setting are, of course, -due entirely to a turning of the earth, and not at all to actual -motions of the stars; and for this reason, all objects in the sky, -without exception, must rise and set again. But the moon really has -a motion of its own in addition to this apparent one caused by the -earth's rotation. - -Somewhere in the dawn of time early watchers of the stars thought -out those fancied constellations that survive even down to our own -day. They placed the mighty lion, king of beasts, upon the face -of night, and the great hunter, too, armed with club and dagger, -to pursue him. Among these constellations the moon threads her -destined way, night after night, so rapidly that the unaided eye -can see that she is moving. 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 Moon. "To-night she is -near the bright star in the eye of the Bull." And again: "To-night -she rides full, and near to the heart of the Virgin." - -And why does the moon ride thus through the stars of night? Modern -science has succeeded in disentangling the intricacies of her -motion, until to-day only one or two of the very minutest details -of that motion remain unexplained. But it has been a hard problem. -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 stature, but whose height no one of them may -ever hope to scale. - -But for our present purpose it is unnecessary to pursue the subject -of lunar motion into its abstruser details. To understand why the -moon moves rapidly among the stars, it is sufficient to remember -that she is whirling quickly round the earth, so as to complete -her circuit in a little less than a month. We see her at all times -projected upon the distant background of the sky on which are set -the stellar points of light, as though intended for beacons to -mark the course pursued by moon and planets. The stars themselves -have no such motions as the moon; situated at a distance almost -inconceivably great, they may, indeed, be travellers through empty -space, yet their journeys shrink into insignificance as seen from -distant earth. 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 eyes. - -Let us again recur to our supposed observer watching the moon night -after night, so as to record the stars closely approached by her. -Why should he not some time or other be surprised by an approach so -close as to amount apparently to actual contact? The moon covers -quite a large surface on the sky, and when we remember the nearly -countless numbers of the stars, it would, indeed, be strange if -there were not some behind the moon as well as all around her. - -A moment's consideration shows that this must often be the case; -and in fact, if the moon's advancing edge be scrutinized carefully -through a telescope, small stars can be seen frequently to -disappear behind it. This is a most interesting observation. At -first we see the moon and star near each other in the telescope's -field of view. But the distance between them lessens perceptibly, -even quickly, until at last, with a startling suddenness, the star -goes out of sight behind the moon. After a time, ranging from a few -moments to, perhaps, more than an hour, the moon will pass, and we -can see the star suddenly reappear from behind the other edge. - -These interesting observations, while not at all uncommon, can -be made only very rarely by naked-eye astronomers. The reason is -simple. The moon's light is so brilliant that it fairly overcomes -the stars whenever they are at all near, except in the case of -very bright ones. Small stars that can be followed quite easily -up to the moon's edge in a good telescope, disappear from a -naked-eye view while the moon is still a long distance away. But -the number of very bright stars is comparatively small, so that -it is quite unusual to find anyone not a professional astronomer -who has actually seen one of these so-called "occultations." -Moreover, most people are not informed in advance of the occurrence -of an opportunity to make such observations, although they can be -predicted quite easily by the aid of astronomical calculations. -Sometimes we have occultations of planets, and these are the most -interesting of all. When the moon passes between us and one of the -larger planets, it is worth while to observe the phenomenon even -without a telescope. - -Up to this point we have considered occultations chiefly as being -of interest to the naked-eye astronomer. Yet occultations have a -real scientific value. It is by their means that we can, perhaps, -best measure the moon's size. By noting with a telescope the time -of disappearance and reappearance of known stars, astronomers can -bring the direct measurement of the moon's diameter within the -range of their numerical calculations. Sometimes the moon passes -over a condensed cluster of stars like the Pleiades. The results -obtainable on these occasions are valuable in a very high degree, -and contribute largely to making precise our knowledge of the lunar -diameter. - -There is another thing of scientific interest about occultations, -though it has lost some of its importance in recent years. When -such an event has been observed, 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 motion. We -have already called attention to the great inherent difficulty of -this theory. 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 known, we can both check our calculations -and gather material for improving our theory. The same principle -can be used also in the converse direction. Within the limits of -precision imposed by the state of our knowledge of lunar theory, -we can utilize occultations to help determine the position on -the earth of places whose longitude is unknown. It is a very -interesting bit of history that the first determination of the -longitude of Washington was made by means of occultations, and that -this early determination led to the founding of the United States -Naval Observatory. - -On March 28, 1810, Mr. Pitkin, of Connecticut, reported to -the House of Representatives on "laying a foundation for the -establishment of a first meridian for the United States, by which a -further dependence on Great Britain or any other foreign nation for -such meridian may be entirely removed." This report was the result -of a memorial presented by one William Lambert, who had deduced the -longitude of the Capitol from an occultation observed October 20, -1804. Various proceedings were had in Congress and in committee, -until at last, in 1821, Lambert was appointed "to make astronomical -observations by lunar occultations of fixed stars, solar eclipses, -or any approved method adapted to ascertain the longitude of the -Capitol from Greenwich." Lambert's reports were made in 1822 -and 1823, but ten years passed before the establishment of a -formal Naval Observatory under Goldsborough, Wilkes, and Gilliss. -But to Lambert belongs the honor of having marked out the first -fundamental official meridian of longitude in the United States. - - - - -MOUNTING GREAT TELESCOPES - - -There are many interesting practical things about an astronomical -observatory with which the public seldom has an opportunity to -become acquainted. Among these, perhaps, the details connected -with setting up a great telescope take first rank. The writer -happened to be present at the Cape of Good Hope Observatory when -the photographic equatorial telescope was being mounted, and the -operation of putting it in position may be taken as typical of -similar processes elsewhere. (See also p. 86.) - -[Illustration: Forty-Inch Telescope, Yerkes Observatory, University -of Chicago.] - -In the first place, it is necessary to explain what is meant by an -"equatorial" telescope. One of the chief difficulties in making -ordinary observations arises from the rising and setting of the -stars. They are all apparently moving across the face of the sky, -usually climbing up from the eastern horizon, only to go down again -and set in the west. If, therefore, we wish to scrutinize any -given object for a considerable time, we must move the telescope -continuously so as to keep pace with the motion of the heavens. For -this purpose, the tube must be attached to axles, so that it can be -turned easily in any direction. The equatorial mounting is a device -that permits the telescope to be thus aimed at any part of the sky, -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 view. - -To understand the equatorial mounting it is necessary to remember -that the rising and setting motions of the heavenly bodies are -apparent ones only, and due in reality to the turning of the earth -on its own axis. As the earth goes around, it carries observer, -telescope, and observatory past the stars fixed upon the distant -sky. Consequently, to keep a telescope pointed continuously at a -given star, it is merely necessary to rotate it steadily backward -upon a suitable axis just fast enough to neutralize exactly the -turning of our earth. - -By a suitable axis for this purpose we mean one so mounted as -to be exactly parallel to the earth's own axis of rotation. A -little reflection shows how simply such an arrangement will work. -All the heavenly bodies may be regarded, for practical purposes, -as excessively remote in comparison with the dimensions of our -earth. The entire planet shrinks into absolute insignificance -when compared with the distances of the nearest objects brought -under observation by astronomers. It follows that if we have our -telescope attached to such a rotation-axis as we have described, -it will be just the same for purposes of observation as though the -telescope's axis were not only parallel to the earth's axis, but -actually coincident with it. The two axes may be separated by a -distance equal to that between the earth's surface and its centre; -but, as we have said, this distance is insignificant so far as our -present object is concerned. - -There is another way to arrive at the same result. We know that -the stars in rising and setting all seem to revolve about the pole -star, which itself seems to remain immovable. Consequently, if we -mount our telescope so that it can turn about an axis pointing at -the pole, 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 direction. Astronomical considerations teach us -that an axis thus pointing at the pole will be parallel to the -earth's own axis. Thus we arrive at the same fundamental principle -for mounting an astronomical telescope from whichever point of view -we consider the subject. - -Every large telescope is provided with such an axis of rotation; -and for the reason stated it is called the "polar axis." The -telescope itself is then called an "equatorial." The advantage -of this method of mounting is very evident. Since we can follow -the stars' motions by turning the telescope about one axis only, -it becomes a very simple matter to accomplish this turning -automatically by means of clock-work. - -The "following" of a star being thus provided for by the device -of a polar axis, it is, of course, also necessary to supply some -other motion so as to enable us to aim the tube at any point in the -heavens. For it is obvious that if it were rigidly attached to the -polar axis, we could, indeed, follow any star that happened to be -in the field of view, but we could not change this field of view at -will so as to observe other stars or planets. To accomplish this, -the telescope is attached to the polar axis by means of a pivot. -By turning the telescope around its polar axis, and also on this -pivot, we can find any object in the heavens; and once found, we -can leave to the polar axis and its automatic clock-work the task -of keeping that object before the observer's eye. - -In setting up the Cape of Good Hope instrument the astronomers were -obliged to do a large part of the work of adjustment personally. -Far away from European instrument-makers, the parts of the -mounting and telescope had to be "assembled," or put together, by -the astronomers of the Cape Observatory. A heavy pier of brick -and masonry had been prepared in advance. Upon this was placed a -massive iron base, intended to support the superstructure of polar -axis and telescope. This base rested on three points, one of which -could be screwed in and out, so as to tilt the whole affair a -little forward or backward. By means of this screw we effected the -final adjustment of the polar axis to exact parallelism with that -of the earth. Other screws were provided with which the base could -be twisted a little horizontally either to the right or left. Once -set up in a position almost correct, it was easy to perfect the -adjustment by the aid of these screws. - -Afterward the tube and lenses were put in place, and the clock -properly attached inside the big cast-iron base. This clock-work -looked more like a piece of heavy machinery than a delicate clock -mechanism. But it had heavy work to do, carrying the massive -telescope with its weighty lenses, and needed to be correspondingly -strong. It had a driving-weight of about 2,000 pounds, 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 -case. - -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 clock. This object -having been attained successfully, the instrument stood ready to be -used in the routine business of the observatory. - -Before leaving the subject of telescope-mountings, we must mention -the giant instrument set up at the Paris Exposition of 1900. The -project of having a _Grande Lunette_ had been hailed by newspapers -throughout the world and by the general public in their customary -excitable way. It was tremendously over-advertised; exaggerated -notions of the instrument's powers were spread abroad and eagerly -credited; the moon was to be dragged down, as it were, from its -customary place in the sky, so near that we should be able almost -to touch its surface. As to the planets--free license was given to -the journalistic imagination, and there was no effective limitation -to the magnificence of astronomical discovery practically within -our grasp, beyond the necessity for printed space demanded by -sundry wars, pestilences, and other mundane trifles. - -[Illustration: Yerkes Observatory, University of Chicago.] - -Now, the present writer is very far from advocating a lessening of -the attention devoted to astronomy. Rather would he magnify his -office than diminish it. But let journalistic astronomy be as good -an imitation of sober scientific truth as can be procured at space -rates; let editors encourage the public to study those things -in the science that are ennobling and cultivating to the mind; -let there be an end to the frenzied effort to fabricate a highly -colored account of alleged discoveries of yesterday, capable of -masquerading to-day under heavy head-lines as News. - -The manner in which the big telescope came to be built is not -without interest, and shows that enterprise is far from dead, even -in the old countries. A stock company was organized--we should -call it a corporation--under the name _Société de l'Optique_. It -would appear that shares were regularly put on the market, and -that a prospectus, more or less alluring, was widely distributed. -We may say at once that the investing public did not respond -with obtrusive alacrity; but at all events, the promoters' -efforts received sufficient encouragement to enable them to begin -active work. From the very first a vigorous attempt was made to -utilize both the resources of genuine science and the devices of -quasi-charlatanry. It was announced that the public were to be -admitted to look through the big glass (apparently at so much an -eye), and many, doubtless, expected that the man in the street -would be able to make personal acquaintance with the man in the -moon. A telescopic image of the sun was to be projected on a big -screen, and exhibited to a concourse of spectators assembled in -rising tiers of seats within a great amphitheatre. And when clouds -or other circumstances should prevent observing the planets or -scrutinizing the sun, a powerful stereopticon was to be used. -Artificial pictures of the wonders of heaven were to be projected -on the screen, and the public would never be disappointed. It -was arranged that skilled talkers should be present to explain -all marvels: and, in short, financial profit was to be combined -with machinery for advancing scientific discovery. Astronomers -the world over were "circularized," asked to become shareholders, -and, in default of that, to send lantern-slides or photographs of -remarkable celestial objects for exhibition in the magic-lantern -part of the show. - -The project thus brought to the attention of scientific men three -years ago did not have an attractive air. It savored too much of -charlatanism. But it soon appeared that effective government -sanction had been given to the enterprise; and, above all, -that men of reputation were allowing the use of their names in -connection with the affair. More important still, we learned that -the actual construction had been undertaken by Gautier, of Paris, -that finances were favorable, and that real work on parts of the -instrument was to commence without delay. - -Gautier is a first-class instrument-builder; he has established -his reputation by constructing successfully several telescopes of -very large size, including the _equatorial coudé_ of the Paris -Observatory, a unique instrument of especial complexity. The -present writer believes that, if sufficient time and money were -available, the _Grande Lunette_ would stand a reasonable chance of -success in the hands of such a man. And by a reasonable chance, we -mean that there is a large enough probability of genuine scientific -discovery to justify the necessary financial outlay. But the -project should be divorced from its "popular" features, and every -kind of advertising and charlatanism excluded with rigor. - -As planned originally, and actually constructed, the _Grande -Lunette_ presents interesting peculiarities, distinguishing it -from other telescopes. Previous instruments have been built on the -principle of universal mobility. It is possible to move them in -all directions, and thus bring any desired star under observation, -irrespective of its position in the sky. But this general mobility -offers great difficulties in the case of large and ponderous -telescopes. Delicacy of adjustment is almost destroyed when the -object to be adjusted weighs several tons. And the excessive -weight of telescopes is not due to unavoidably heavy lenses alone. -It is essential that the tube be long; and great length involves -appreciable thickness of material, if stiffness and solidity are to -remain unsacrificed. Length in the tube is necessitated by certain -peculiar optical defects of all lenses, into the nature of which we -shall not enter at present. 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 tube. The length -of a good telescope should be at least twelve times the diameter -of its large lens. If the relative length can be still further -increased, so much the better; for then the optical defects can be -further reduced. - -In the case of the Paris instrument a radical departure consists -in making the tube of unprecedented length, 197 feet, with a -lens diameter of 49¼ inches. This great length, while favorable -optically, precludes the possibility of making the instrument -movable in the usual sense. In fact, the entire tube is attached -to a fixed horizontal base, and no attempt is made to change its -position. Outside the big lens, and disconnected altogether from -the telescope proper, is mounted a smooth mirror, so arranged that -it can be turned in any direction, and thus various parts of the -sky examined by reflection in the telescope. - -While this method unquestionably has the advantage of leaving -the optician quite free as to how long he will make his tube, it -suffers from the compensating objection that a new optical surface -is introduced into the combination, viz., the mirror. 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 eye. - -But it is not yet possible to pronounce definitely upon the merit -of this form of instrument, since, as we have said, the maker -has not been given time enough to try the idea to the complete -satisfaction of scientific men. In the early part of August, 1900, -when the informant of the present writer left Paris, after serving -as a member of the international jury for judging instruments of -precision at the Exposition, the condition of the _Grande Lunette_ -was as follows: Two sets of lenses had been contemplated, one -intended for celestial photography, and the other to be used for -ordinary visual observation. Only the photographic lenses had been -completed, however, and for this reason the public could not be -permitted to look through the instrument. The photographic lenses -were in place in the tube, but at that time their condition was -such that, though some photographs had been obtained, it was -not thought advisable to submit them to the jury. Consequently, -the _Lunette_ did not receive a prize. Since that time various -newspapers have reported wonderful results from the telescope; -but, disregarding the fusillade from the sensational press, we may -sum up the present state of affairs very briefly. Gautier is still -experimenting; and, given sufficient time and money, he may succeed -in producing what astronomers hope for--an instrument capable of -advancing our knowledge, even if that advance be only a small one. - - - - -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 men. 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 journeying. The celestial -pole is, indeed, a fundamentally important thing in astronomical -science, and the determination of its exact position upon the -sky has always engaged the closest attention of astronomers. -Quite recently new methods of research have been brought to -bear, promising a degree of success not hitherto attained in the -astronomers' pursuit of their pole. - -In the first place, we must explain what is meant by the celestial -pole. We have already mentioned the poles of the earth (p. 136). -Our planet turns once daily upon an axis passing through its -centre, and it is this rotation that causes all the so-called -diurnal phenomena of the heavens. Rising and setting of sun, moon, -and stars are simply results of this turning of the earth. Heavenly -bodies do not really rise; 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 them. The terrestrial poles are those two points -on the earth's surface where it is pierced by the rotation axis of -the planet. Now we can, if we choose, imagine this axis lengthened -out indefinitely, further and further, until at last it reaches the -great round vault of the sky. Here it will again pierce out two -polar points; and these are the celestial poles. - -The whole thing is thus quite easy to understand. On the sky the -poles are marked by the prolongation of the earth's axis, just as -on the earth the poles are marked by the axis itself. And this -explains at once why the stars seem nightly to revolve about the -pole. If the observer is being turned round the earth's axis, of -course it will appear to him as if the stars were rotating around -the same axis in the opposite direction, just as houses and fields -seem to fly past a person sitting in a railway train, unless he -stops to remember that it is really himself who is in motion, and -not the trees and houses. - -The existence of such a centre of daily motions among the stars -once recognized, it becomes of interest to ascertain whether the -centre itself always retains precisely the same position in the -sky. It was discovered as early as the time of Hipparchus (p. 39) -that such is not the case, and that the celestial pole is subject -to a slow motion among the stars on the sky. If a given star were -to-day situated exactly at the pole, it would no longer be there -after the lapse of a year's time; for the pole would have moved -away from it. - -This motion of the pole is called precession. It means that certain -forces are continually at work, compelling the earth's axis to -change its position, so that the prolongation of that axis must -pierce the sky at a point which moves as time goes on. These forces -are produced by the gravitational attractions of the sun, moon, -and planets upon the matter composing our earth. If the earth -were perfectly spherical in shape, the attractions of the other -heavenly bodies would not affect the direction of the earth's -rotation-axis in the least. But the earth is not quite globular in -form; it is flattened a little at the poles and bulges out somewhat -at the equator. (See p. 135.) - -This protuberant matter near the equator gives the other bodies -in the solar system an opportunity to disturb the earth's -rotation. 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 23½ degrees; and it requires 25,800 years to complete -a circuit of this precessional cycle. One of the most striking -consequences of this motion will be the change of the polar star. -Just at present the bright star Polaris in the constellation of -the Little Bear is very close to the pole. But after the lapse of -sufficient ages the first-magnitude star Vega of the constellation -Lyra will in its turn become Guardian of the Pole. - -It must not be supposed, however, that the motion of the pole -proceeds quite uniformly, and in an exact circle; the varying -positions of the heavenly bodies whose attractions cause -the phenomena in question are such as to produce appreciable -divergencies from exact circular motion. Sometimes the pole -deviates a little to one side of the precessional circle, and -sometimes it deviates on the other side. The final result is a -sort of wavy line, half on one side and half on the other of an -average circular curve. It takes only nineteen years to complete -one of these little waves of polar motion, so that in the -whole precessional cycle of 25,800 years there are about 1,400 -indentations. This disturbance of the polar motion is called by -astronomers nutation. - -The first step in a study of polar motion is to devise a method of -finding just where the pole is on any given date. If the astronomer -can ascertain by observational processes just where the pole is -among the stars at any moment, and can repeat his observations year -after year and generation after generation, he will possess in -time a complete chart of a small portion at least of the celestial -pole's vast orbit. From this he can obtain necessary data for a -study of the mathematical theory of attractions, and thus, perhaps, -arrive at an explanation of the fundamental laws governing the -universe in which we live. - -The instrument which has been used most extensively for the -study of these problems is the transit (p. 118) or the "meridian -circle." This latter consists of a telescope firmly attached to a -metallic axis about which it can turn. The axis itself rests on -massive stone supports, and is so placed that it points as nearly -as possible in an east-and-west direction. Consequently, when the -telescope is turned about its axis, it will trace out on the sky -a great circle (the meridian) which passes through the north and -south points of the horizon and the point directly overhead. The -instrument has also a metallic circle very firmly fastened to -the telescope and its axis. 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 angles. - -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 telescope. This centre is marked with a cross made -by fastening into the focus some pieces of ordinary spider's web, -which give a well-marked, delicate set of lines, even under the -magnifying power of the telescope's eye-piece. In addition to thus -noting the time when the star crosses the field of the telescope, -the astronomer can measure by means of the circle, how high up it -was in the sky at the instant when it was thus observed. - -If the telescope of the meridian circle be turned toward the north, -and we observe stars close to the pole, it is possible to make two -different observations of the same star. 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 it. Double observations of this class enable us to -obtain the elevation of the pole above the horizon, and to fix its -position with respect to the stars. - -Now, there is one very serious objection to this method. In order -to secure the two necessary observations of the same star, it is -essential to be stationed at the instrument at two moments of time -separated by exactly twelve hours; and if one of the observations -occurs in the night, the other corresponding observation will occur -in daylight. - -It is a fact not generally known that the brighter stars can be -seen with a telescope, even when the sun is quite high above -the horizon. Unfortunately, however, 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 Polaris. -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 problem. - -The new method of observation to which we have referred above -consists in an application of photography to the polar problem. If -we aim at the pole a powerful photographic telescope, and expose a -photographic plate throughout the entire night, we shall find that -all stars coming within the range of the plate will mark out little -circles or "trails" upon the developed negative. It is evident that -as the stars revolve about the pole on the sky, tracing out their -daily circular orbits, these same little circles must be reproduced -faithfully upon the photographic plate. The only condition is that -the stars shall be bright enough to make their light affect the -sensitive gelatine surface. - -But even if observations of this kind are continued throughout -all the hours of darkness, we do not obtain complete circles, -but only those portions of circles traced out on the sky between -sunset and sunrise. If the night is twelve hours in length, we -get half-circles on the plate; if it is eighteen hours long, we -get circles that lack only one-quarter of being complete. In -other words, we get a series of circular arcs, one corresponding -to each close polar star. There are no fewer than sixteen stars -near enough to the pole to come within the range of a photographic -plate, and bright enough to cause measurable impressions upon -the sensitive surface. 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 centre; and that centre -is the pole. Moreover, as the arcs are distributed at all sorts -of distances from the pole and in all directions, corresponding to -the accidental positions of the stars on the sky, 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 plate. - -It will be perceived that this method is extremely simple, and, -therefore, likely to be successful; though its simplicity is -slightly impaired by the phenomenon known to astronomers as -"atmospheric refraction." The rays of light coming down to our -telescopes from a distant star must pass through the earth's -atmosphere before they reach us; and in passing thus from the -nothingness of outer space into the denser material of the air, -they are bent out of their straight course. The phenomenon is -analogous to what we see when we push a stick down through the -surface of still water; we notice that the stick appears to be bent -at the point where it pierces the surface of the water; and in -just the same way the rays of light are bent when they pierce into -the air. Fortunately, the mathematical theory of this atmospheric -bending of light is well understood, so that it is possible to -remove the effects of refraction from our results by a process of -calculation. In other words, we can transform our photographic -measures into what they would have been if no such thing as -atmospheric refraction existed. This having been done, all the arcs -on the plate should be exactly circular, and their common centre -should be the position of the pole among the stars on the night -when the photograph was made. - -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 latitude. The elevation of the -pole above the horizon is greatest in high latitudes. Indeed, if -Arctic voyagers could ever reach the pole of the earth they would -see the pole of the heavens directly overhead. Now, the higher up -the pole is in the sky, the less will be the effects of atmospheric -refraction; for the rays of light will then strike the atmosphere -in a direction nearly perpendicular to its surface, which is -favorable to diminishing the amount of bending. - -There is also another very important advantage in placing the -telescope in a high latitude; in the middle of winter the nights -are very long there; if we could get within the Arctic. Circle -itself, there would be nights when the hours of darkness would -number twenty-four, and we could substitute complete circles -for our broken arcs. This would, indeed, be most favorable from -the astronomical point of view; but the essential condition of -convenience for the observer renders an expedition to the frozen -Arctic regions unadvisable. - -But it is at least possible to place the telescope as far north -as is consistent with retaining it within the sphere of civilized -influences. We can put it in that one of existing observatories -on the earth which has the highest latitude; and this is the -observatory of Helsingfors, in Finland, which belongs to a great -university, is manned by competent astronomers, and has a latitude -greater than 60 degrees. - -Dr. Anders Donner, Director of the Helsingfors Observatory, has -at its disposal a fine photographic telescope, and with this some -preliminary experimental "trail" photographs were made in 1895. -These photographs were sent to Columbia University, New York, and -were there measured under the writer's direction. Calculations -based on these measures indicate that the method is promising in -a very high degree; and it was, therefore, decided to construct a -special photographic telescope better adapted to the particular -needs of the problem in hand. - -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 exposure. It is clear that -if the telescope moves while the stars are tracing out their -little trails on the plate, the circularity of the curves will -be disturbed. Now, ordinary astronomical telescopes are always -mounted upon very stable foundations, well adapted to making the -telescope stand still; 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 purposes. - -It is a remarkable peculiarity of the instrument needed for the -new trail photographs that it is never moved at all. Once pointed -at the pole, it is ready for all the observations of successive -generations of astronomers. It should have no machinery, no -pivots, axes, circles, clocks, or other paraphernalia of the usual -equatorial telescope. All we want is a very heavy stone pier, -with a telescope tube firmly fastened to it throughout its entire -length. The top of the pier having been cut to the proper angle of -the pole's elevation, and the telescope cemented down, everything -is complete from the instrumental side; and just such an instrument -as this is now ready for use at Helsingfors. - -The late Miss Catharine Wolfe Bruce, of New York, was much -interested in the writer's proposed polar investigations, and -in October, 1898, she contributed funds for the construction of -the new telescope, and the Russian authorities have generously -undertaken the expense of a building to hold the instrument and the -granite foundation upon which it rests. Photographs are now being -secured with the new instrument, and they will be sent to Columbia -University, New York, for measurement and discussion. It is hoped -that they will carry out the promise of the preliminary photographs -made in 1895 with a less suitable telescope of the ordinary form. - - - - -THE MOON HOAX - - -The public attitude toward matters scientific is one of the -mysteries of our time. It can be described best by the single -word, Credulity; simple, absolute credulity. Perfect confidence -is the most remarkable characteristic of this unbelieving age. -No charlatan, necromancer, or astrologer of three centuries ago -commanded more respectful attention than does his successor of -to-day. - -Any person can be a scientific authority; he has but to call -himself by that title, and everyone will give him respectful -attention. Numerous instances can be adduced from the experience -of very recent years to show how true are these remarks. We have -had the Keeley motor and the liquid-air power schemes for making -something out of nothing. Extracting gold from sea-water has been -duly heralded on scientific authority as an easy source of fabulous -wealth for the million. Hard-headed business men not only believe -in such things, but actually invest in them their most valued -possession, capital. Venders of nostrums and proprietary medicines -acquire wealth as if by magic, though it needs but a moment's -reflection to realize that these persons cannot possibly be in -possession of any drugs, or secret methods of compounding drugs, -that are unknown to scientific chemists. - -If the world, then, will persistently intrust its health and wealth -into the safe-keeping of charlatans, what can we expect when things -supposedly of far less value are at stake? The famous Moon Hoax, as -we now call it, is truly a classic piece of lying. Though it dates -from as long ago as 1835, it has never had an equal as a piece of -"modern" journalism. Nothing could be more useful than to recall it -to public attention at least once every decade; for it teaches an -important lesson that needs to be iterated again and again. - -On November 13, 1833, Sir John Herschel embarked on the Mountstuart -Elphinstone, bound for the Cape of Good Hope. He took with him a -collection of astronomical instruments, with which he intended to -study the heavens of the southern hemisphere, and thus extend his -father's great work to the south polar stars. An earnest student -of astronomy, he asked no better than to be left in peace to -seek the truth in his own fashion. Little did he think that his -expedition would be made the basis for a fabrication of alleged -astronomical discoveries destined to startle a hemisphere. Yet that -is precisely what happened. Some time about the middle of the year -1835 the New York _Sun_ began the publication of certain articles, -purporting to give an account of "Great Astronomical Discoveries, -lately made by Sir John Herschel at the Cape of Good Hope." It was -alleged that these articles were taken from a supplement to the -Edinburgh _Journal of Science_; yet there is no doubt that they -were manufactured entirely in the United States, and probably in -New York. - -The hoax begins at once in a grandiloquent style, calculated to -attract popular attention, and well fitted to the marvels about -to be related. Here is an introductory remark, as a specimen: -"It has been poetically said that the stars of heaven are the -hereditary regalia of man as the intellectual sovereign of the -animal creation. He may now fold the zodiac around him with a -loftier consciousness of his mental supremacy." Then follows a -circumstantial and highly plausible account of the manner in -which early and exclusive information was obtained from the Cape. -This was, of course, important in order to make people believe -in the genuineness of the whole; but we pass at once to the more -interesting account of Herschel's supposed instrument. - -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 instrument. Sir John is supposed to have had an -interesting conversation in England "with Sir David Brewster, -upon the merits of some ingenious suggestion by the latter, in -his article on optics in the Edinburgh Encyclopædia (p. 644), for -improvements in the Newtonian reflectors." The exact reference to -a particular page is here quite delightful. After some further -talk, "the conversation became directed to that all-invincible -enemy, the paucity of light in powerful magnifiers. After a few -moments' silent thought, Sir John diffidently inquired whether -it would not be possible to effect a _transfusion of artificial -light through the focal object of vision_! Sir David, somewhat -startled at the originality of the idea, paused awhile, and then -hesitatingly referred to the refrangibility of rays, and the angle -of incidence.... Sir John continued, 'Why cannot the illuminated -microscope, say the hydro-oxygen, be applied to render distinct, -and, if necessary, even to magnify the focal object?' Sir David -sprang from his chair in an ecstasy of conviction, and leaping -half-way to the ceiling, exclaimed, 'Thou art the man.' "This -absurd imaginary conversation contains nothing but an assemblage -of optical jargon, put together without the slightest intention of -conveying any intelligible meaning to scientific people. Yet it was -well adapted to deceive the public; and we should not be surprised -if it would be credited by many newspaper readers to-day. - -The authors go on to explain how money was raised to build the -new instrument, and then describe Herschers embarkation and the -difficulties connected with transporting his gigantic machines -to the place selected for the observing station. "Sir John -accomplished the ascent to the plains by means of two relief -teams of oxen, of eighteen each, in about four days, and, aided -by several companies of Dutch boors [_sic_], proceeded at once to -the erecting of his gigantic fabric." The place really selected -by Herschel cannot be described better than in his own words, -contained in a genuine letter dated January 21, 1835: "A perfect -paradise in rich and magnificent mountain scenery, sheltered from -all winds.... I must reserve for my next all description of the -gorgeous display of flowers which adorn this splendid country, as -well as the astonishing brilliancy of the constellations." The -author of the hoax could have had no knowledge of Herschers real -location, as described in this letter. - -The present writer can bear witness to the correctness of -Herschel's words. Feldhausen is truly an ideal secluded spot -for astronomical study. A small obelisk under the sheer cliff -of far-famed Table Mountain now marks the site of the great -reflecting telescope. Here Herschel carried on his scrutiny of the -Southern skies. He observed 1,202 double stars and 1,708 nebulæ -and clusters, of which only 439 were already known. He studied the -famous Magellanic clouds, and made the first careful drawings of -the "keyhole" nebula in the constellation Argo. - -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 time, when a sudden blazing up of the -wonderful star Eta Argus was seen within the nebula. This object -has, perhaps, undergone more remarkable changes of light than -any other star in the heavens. It is as though there were some -vast conflagration at work, now blazing into incandescence, and -again sinking almost into invisibility. In 1843 Maclear estimated -the brilliancy of Eta to be about equal to that of Sirius, the -brightest star in the whole sky. Later it diminished in light, -and cannot be seen to-day with the naked eye, though the latest -telescopic observations indicate that it is again beginning to -brighten. - -Such was Herschel's quiet study of his beloved science, in glaring -contrast to the supposed discoveries of the "Hoax." Here are a few -things alleged to have been seen on the moon. 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 rock." There were forests, -too, and water, "fairer shores never angels coasted on a tour -of pleasure. A beach of brilliant white sand, girt with wild -castellated rocks, apparently of green marble." - -There was animal life as well; "we beheld continuous herds of -brown quadrupeds, having all the external characteristics of the -bison, but more diminutive than any species of the bos genus in our -natural history." There was a kind of beaver, that "carries its -young in its arms like a human being," and lives in huts. "From the -appearance of smoke in nearly all of them, there is no doubt of its -(the beaver's) being acquainted with the use of fire." Finally, -as was, of course, unavoidable, human creatures were discovered. -"Whilst gazing in a perspective of about half a mile, we were -thrilled with astonishment to perceive four successive flocks of -large-winged creatures, wholly unlike any kind of birds, descend -with a slow, even motion from the cliffs on the western side, and -alight upon the plain.... Certainly they were like human beings, -and their attitude in walking was both erect and dignified." - -We have not space to give more extended extracts from the hoax, -but we think the above specimens will show how deceptive the whole -thing was. The rare reprint from which we have extracted our -quotations contains also some interesting "Opinions of the American -Press Respecting the Foregoing Discovery." The _Daily Advertiser_ -said: "No article, we believe, has appeared for years, that will -command so general a perusal and publication. 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 science." The _Mercantile -Advertiser_ said: "Discoveries in the Moon.--We commence to-day -the publication of an interesting article which is stated to have -been copied from the Edinburgh _Journal of Science_, and which -made its first appearance here in a contemporary journal of this -city. It appears to carry intrinsic evidence of being an authentic -document." Many other similar extracts are given. The New York -_Evening Post_ did not fall into the trap. The _Evening Post's_ -remarks were as follows: "It is quite proper that the _Sun_ should -be the means of shedding so much light on the _Moon_. 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 -earth; and that there does or did exist such a race rests on the -evidence of that most veracious of voyagers and circumstantial of -chroniclers, Peter Wilkins, whose celebrated work not only gives an -account of the general appearance and habits of a most interesting -tribe of flying Indians, 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 tribe." - -We shall limit our extracts from the contemporary press to the few -quotations here given, hoping that enough has been said to direct -attention once more to that important subject, the Possibility of -Being Deceived. - - - - -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 system. There -was one significant fault in Newton's superbly simple conception of -an eternal law governing the world in which we live. 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 gravitation. But -they proved just as conclusively that these planetary orbits, as -they are called, could not maintain indefinitely the same shapes or -positions. Slow indeed might be the changes they were destined to -undergo; slow, but sure, with that sureness belonging to celestial -science alone. And so men asked: Has this magnificent solar system -been built upon a scale so grand, been put in operation subject -to a law sublime in its very simplicity, only to change and change -until at length it shall lose every semblance of its former self, -and end, perhaps, in chaos or extinction? - -Laplace was able to answer confidently, "No." Nor was his answer -couched in the enthusiastic language of unbalanced theorists who -work by the aid of imagination alone. Based upon the irrefragable -logic of correct mathematical reasoning, and clad in the sober garb -of mathematical formulæ, his results carried conviction to men of -science the world over. So was it demonstrated that changes in our -solar system are surely at work, and shall continue for nearly -countless ages; yet just as surely will they be reversed at last, -and the system will tend to return again to its original form and -condition. The objection that the Newtonian law meant ultimate -dissolution of the world was thus destroyed by Laplace. From that -day forward the law of gravitation has been accepted as holding -sway over all phenomena visible within our planetary world. - -The intricacies of our own solar system being thus illumined, the -restless activity of the human intellect was stimulated to search -beyond for new problems and new mysteries. 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 night. Does the same law of gravitation -cast its magic spell over that hazy cloud of Pleiades, binding -them, like ourselves, with bonds indissoluble? Who shall answer, -yes or no? We can only say that astronomers have as yet but stepped -upon the threshold of the universe, and fixed the telescope's great -eye upon that which is within. - -Let us then begin by reminding the reader what is meant by the -Newtonian law of gravitation. It appears all things possess the -remarkable property of attracting or pulling each other. Newton -declared that all substances, solid, liquid, 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 existing. His law further -formulates certain conditions governing the manner in which this -gravitational attraction is exerted; but these are mere matters of -detail; interest centres about the mysterious fact of attraction -itself. How can one thing pull another with no connecting link -through which the pull can act? Just here we touch the point -that has never yet been explained. Nature withholds from science -her ultimate secrets. They that have pondered longest, that have -descended farthest of all men into the clear well of knowledge, -have done so but to sound the depths beyond, never touching bottom. - -This inability of ours, to give a good physical explanation of -gravitation, has led certain makers of paradoxes to doubt or even -deny that there is any such thing. But, fortunately, we have a -simple laboratory experiment that helps us. Unexplained it may -ever remain, but that there can be attraction between physical -objects connected by no visible link is proved by the behavior -of an ordinary magnet. Place a small piece of steel or iron near -a magnetized bar, and it will at once be so strongly attracted -that it will actually fly to the magnet. Anyone who has seen this -simple experiment can never again deny the possibility, at least, -of the law of attraction as stated by Newton. Its possibility -once admitted, the fact that it can predict the motions of all -the planets, even down to their minutest details, transforms the -possibility of its truth into a certainty as strong as any human -certainty can ever be. - -But this demonstration of Newton's law is limited strictly to -the solar system itself. We may, indeed, reason by analogy, and -take for granted that a law which holds within our immediate -neighborhood is extremely likely to be true also of the entire -visible universe. But men of science are loath to reason thus; and -hence the fascination of researches in cosmic astronomy. Analogy -points out the path. The astronomer is not slow to follow; but he -seeks ever to establish upon incontrovertible evidence those truths -which at first only his daring imagination had led him to half -suspect. - -If we are to extend the law of gravitation to the utmost, we must -be careful to consider the law itself in its most complete form. -A heavenly body like the sun is often said to govern the motions -of its family of planets; but such a statement is not strictly -accurate. The governing body is no despot; 'tis an abject slave of -law and order, as much as the tiniest of attendant planets. The -action of gravitation is mutual, and no cosmic body can attract -another without being itself in turn subject to that other's -gravitational action. - -If there were in our solar system but two bodies, sun and planet, -we should find each one pursuing a path in space under the -influence of the other's attraction. These two paths or orbits -would be oval, and if the sun and planet were equally massive, the -orbits would be exactly alike, both in shape and size. But if the -sun were far larger than the planet, the orbits would still be -similar in form, but the one traversed by the larger body would be -small. 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 orb. - -Whenever the preponderance of the larger body is extremely great, -its orbit will be correspondingly insignificant in size. This is -in fact the case with our own sun. 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 instruments. The path -traced out by the sun's centre would not fill a space as large as -the sun's own bulk. Nevertheless, true orbital motion is there. - -So we may conclude that as a necessary consequence of the law of -gravitation every object within the solar system is in motion. To -say that planets revolve about the sun is to neglect as unimportant -the small orbit of the sun itself. This may be sufficiently -accurate for ordinary purposes; but it is unquestionably necessary -to neglect no factor, however small, if we propose to extend our -reasoning to a consideration of the stellar universe. For we shall -then have to deal with systems in which the planets are of a size -comparable with the sun; and in such systems all the orbits will -also be of comparatively equal importance. - -Mathematical analysis has derived another fact from discussion -of the law of gravitation which, perhaps, transcends in simple -grandeur everything we have as yet mentioned. It matters not how -great may be the number of massive orbs threading their countless -interlacing curved paths in space, there yet must be in every -cosmic system one single point immovable. This point is called the -Centre of Gravity. If it should so happen that in the beginning of -things, some particle of matter were situated at this centre, then -would that atom ever remain unmoved and imperturbable throughout -all the successive vicissitudes of cosmic evolution. 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 motion. - -But in general, we cannot suppose that the centres of gravity in -the various stellar systems are really occupied by actual physical -bodies. The centre may be a mere mathematical point in space, -situated among the several bodies composing the system, but, -nevertheless, endowed, in a certain sense, with the same remarkable -property of relative immobility. - -Having thus defined the centre of gravity in its relation to the -constituent parts of any cosmic system, we can pass easily to its -characteristic properties in connection with the inter-relation of -stellar systems with one another. 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 whole. It is this -property of the centre of gravity which makes it pre-eminently -important in cosmic researches. For, while we know that centre to -be at rest relatively to all the planets in the system, it may, -nevertheless, in its quality as a sort of concentrated essence of -them all, be moving swiftly through space under the pull of distant -stars. In that case, the attendant bodies will go with it--but they -will pursue their evolutions within the system, all unconscious -that the centre of gravity is carrying them on a far wider circuit. - -What is the nature of that circuit? This question has been for -many years the subject of earnest study by the clearest minds -among astronomers. The greatest difficulty in the way is the -comparatively brief period during which men have been able to -make astronomical observations of precision. Space and time are -two conceptions that transcend the powers of definition possessed -by any man. But we can at least form a notion of how vast is the -extent of time, 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 dome. 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 Greenwich, in England. Yet so slowly do the -movements of the stars unroll themselves upon the firmament, 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 space. - -Travellers upon a railroad cannot tell at any given moment whether -they are moving in a straight line, or whether the train is -turning upon some curve of huge size. The St. Gothard railway -has several so-called "corkscrew" tunnels, within which the rails -make a complete turn in a spiral, the train finally emerging from -the tunnel at a point almost vertically over the entrance. In this -way the train is lifted to a higher level. Passengers are wont to -amuse themselves while in these tunnels by watching the needle of -an ordinary pocket-compass. This needle, of course, always points -to the north; and as the train turns upon its curve, the needle -will make a complete revolution. But the passenger could not know -without the compass that the train was not moving in a perfectly -straight line. Just so we passengers on the earth are unaware of -the kind of path we are traversing, until, like the compass, the -astronomer's instruments shall reveal to us the truth. - -But as we have seen, astronomical observations of precision have -not as yet extended through a period of time corresponding to the -few minutes during which the St. Gothard traveller watches the -compass. We are still in the dark, and do not know as yet whether -mankind shall last long enough upon the earth to see the compass -needle make its revolution. We are compelled to believe that the -motion in space of our sun is progressing upon a curved path; but -so far as precise observations allow us to speak, we can but say -that we have as yet moved through an infinitesimal element only of -that mighty curve. However, we know the point upon the sky toward -which this tiny element of our path is directed, and we have an -approximate knowledge of the speed at which we move. - -More than a century ago Sir William Herschel was able to fix -roughly what we call the apex of the sun's way in space, or the -point among the stars toward which that way is for the moment -directed. We say for the moment, but we mean that moment of which -Bradley saw the beginning in 1750, and upon whose end no man of -those now living shall ever look. Herschel found that a comparison -of old stellar observations seemed to indicate that the stars in a -certain part of the sky were opening out, as it were, and that the -constellations in the opposite part of the heavens seemed to be -drawing in, or becoming smaller. There can be but one reasonable -explanation of this. We must be moving toward that part of the sky -where the stars are separating. Just so a man watching a regiment -of soldiers approaching, will see at first only a confused body of -men; but as they come nearer, the individual soldiers will seem to -separate, until at length each one is seen distinct from all the -others. - -Herschel fixed the position of the apex at a point in the -constellation Hercules. The most recent investigations of Newcomb -and others have, on the whole, verified Herschel's conclusions. -With the intuitive power of rare genius, Herschel had been able to -sift truth out of error. The observational data at his disposal -would now be called rude, but they disclosed to the scrutiny of -his acute understanding the germ of truth that was in them. Later -investigators have increased the precision of our knowledge, until -we can now say that the present direction of the solar motion is -known within very narrow limits. A tiny circle might be drawn on -the sky, to which an astronomer might point his hand and say: -"Yonder little circle contains the goal toward which the sun and -planets are hastening to-day." Even the speed of this motion has -been subjected to measurement, and found to be about ten miles per -second. - -The objective point and the rate of motion thus stated, exact -science holds her peace. Here genuine knowledge stops; and we can -proceed further only by the aid of that imagination which men of -science need to curb at every moment. But let no one think that the -sun will ever reach the so-called apex. To do so would mean cosmic -motion upon a straight line, while every consideration of celestial -mechanics points to motion upon a curve. When shall we turn -sufficiently upon that curve to detect its bending? 'Tis a problem -we must leave as a rich heritage to later generations that are to -follow us. The visionary theorist's notion of a great central sun, -controlling our own sun's way in space, must be dismissed as far -too daring. 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 member. Then we reach a conception that -has lost nothing in the grandeur of its simplicity, and is yet -in accord with the probabilities of sober mechanical science. We -cease to be a lonely world, and stretch out the bonds of a common -relationship to yonder stars within the firmament. - - - - -INDEX - - PAGE - - Airy, Astronomer Royal, 1 - - Allis, photographs comet, 101 - - Andromeda nebula, 28 - temporary star, 28, 29, 45 - - Apex, of solar motion, explained, 221 - - Aquila, constellation, temporary star in, 40 - - Arctic regions, position of pole in, 194 - - Argo, constellation, variable star in, 205 - - Association, international geodetic, 139 - - Asteroids, first discovery by Piazzi, 59, 106 - discovery by photography, 64 - group of, 63 - photography of, invented by Wolf, 104 - - Astronomer, royal, 1 - working, description of, 152 - - 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 - - - Campbell, observes Pole-star, 18 - - Cape of Good Hope, observatory, photography at, 101 - telescope, 170, 174 - - _Capriccio_, Galileo's, 55 - - Cassini, shows Saturn's rings to be double, 144 - - Cassiopeia, temporary star in, 40 - - Celestial pole, 184 - - Central sun theory, 223 - - Centre of gravity, 217 - - Chart-room, on ship-board, 5 - - Chronometer, invention of, 8 - - 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, 116 - astronomical, its dial, 116 - error of, determined with transit, 118 - 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 - - 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, 158 - of Sun, 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, 118 - - 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 - - Geography, maps, astronomical side of, 112 - - Geology, polar motion in, 131 - - Gill, photographs comet, 100 - - Gilliss, at Naval Observatory, Washington, 169 - - Goldsborough, at Naval Observatory, Washington, 169 - - _Grande Lunette_, Paris, 1900, 176, 180 - - Gravitation, 13 - in Pleiades, 14, 212 - law of, Newton's, 212 - - Gravity, centre of, 217 - - Greenwich, origin of longitudes, 7, 124 - time, 7 - - Groombridge, English astronomer, 1 - - - 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, 11, 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 - - 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, 121 - - Journalistic astronomy, 176 - - Jupiter's satellites, discovered by Galileo, 50 - discovered by Barnard, 51 - - - Keeler, observes Saturn's rings, 140, 147, 150 - photographs nebulæ, 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, 112 - - Meridian circle, explained, 189 - - Milky-way, poor in nebulæ, 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 nebulæ, 28 - - Naples, Royal Observatory, latitude observations, 139 - - Naval Observatory, Washington, noon signal, 120 - - NAVIGATION, 1 - before chronometers, 3 - use of astronomy in, 113 - - NEBULÆ, 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, 120 - - Number, of nebulæ, 31, 33 - of temporary stars, 38 - - Nutation, explained, 188 - - - Occultations, 161 - explained, 165 - - 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, 176 - - 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, 81 - 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, 110 - wholesale methods in, 103 - - Piazzi, discovers first asteroid, 59, 106 - - Pitkin, report to House of Representatives, 168 - - Planetary nebulæ, 31 - - 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, 11 - - Polar axis, of telescope, 173 - - 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-nebulæ, 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 - Greenwich, Bradley's observations, 219 - Naples, latitude observations, 139 - - 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 - - _Société de l'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 nebulæ, 31 - - Stability, of Saturn's rings, 145 - of Solar System, 210 - - Standards, time, of the world, 111 - table of, 130 - - "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, 176, 180 - double, for photography, 86 - equatorial, explained, 170 - first used by Galileo, 49 - motion of, 87 - mounting great, 170 - 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 - - TIME STANDARDS OF THE WORLD, 111 - standards of the World, table of, 130 - system, in New York, 120 - zones, explained, 128 - - Trails, photographic, 191 - - Transit, for determining clock error, 118 - - 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, 112 - - - 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, 11 - - World's time standards, table of, 130 - - - Yale College, Pleiades measured at, 15 - - - Zones, time, explained, 128 - - - - - TRANSCRIBER'S NOTE - - Italic text is denoted by _underscores_. - - Fractions in the two tables on pg 74 and pg 78 are displayed in the form - "a-b/c" as 4-1/2 or 2-7/16 for example. The original text in the - tables used the form "a b-c". A few other basic fractions in the text - such as ½ and ⅖ are displayed in this same form in the etext. - - There is only one Footnote in this book, with its anchor on pg 69. - It has been placed at the end of the chapter containing the anchor. - - Obvious typographical errors and punctuation errors have been - corrected after careful comparison with other occurrences within - the text and consultation of external sources. - - Except for those changes noted below, all misspellings in the text, - and inconsistent or archaic usage, have been retained. 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