Chapter V: Part 5

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. For example,
time zone, time-zone; Le Verrier, Leverrier; light wave, light-wave;
intrust; wabbling; unexcelled; crape; monumented.

Pg 146, 'James Clark-Maxwell' replaced by 'James Clerk Maxwell'.
Pg 189, 'impossible to measure' replaced by 'possible to measure'.

End of Project Gutenberg's Practical Talks by an Astronomer, by Harold Jacoby