Popular Science Monthly/Volume 56/January 1900/Advance of Astronomy During the Nineteenth Century
APPLETONS’
POPULAR SCIENCE
MONTHLY.
JANUARY, 1900.
| ADVANCE OF ASTRONOMY DURING THE NINETEENTH CENTURY. |
By Sir ROBERT BALL,
LOWNDEAN PROFESSOR OF ASTRONOMY AT THE UNIVERSITY OF CAMBRIDGE, ENGLAND.
ONE of the most remarkable chapters in the astronomy of the past century was commenced on the very first night with which that century began. It was, indeed, on the 1st of January, 1801, that the discovery of a new planet was announced. The five great orbs—Jupiter, Saturn, Mercury, Mars, and Venus—had been known from the earliest times of which we have records, and the planet Uranus had been discovered nearly twenty years before the previous century closed. The solar system was thus thought to consist of these six planets and, of course, the earth. On the memorable night to which I have referred, Piazzi, the astronomer, made a remarkable advance. He discovered yet another planet—the seventh, or eighth, if the earth be included. The new body was a small object in comparison with those which were previously known. It was invisible to the unaided eye, and seemed no more than a starlike point even when viewed through a telescope. It revolved around the sun in the wide region between the orbits of Mars and Jupiter. This discovery was speedily followed by others of the same kind, and, as the century has advanced to its close, the numbers of these planets—asteroids, as they are generally called—has been gradually increasing, so much so that now, of these little bodies known to astronomers, the number amounts to about four hundred and fifty.
But just as the beginning of the century was heralded by the discovery of the first of these asteroids, so the close of the century will be signalized in the history of astronomy by the detection among these little objects of one which has entirely cast into the shade all other discoveries of the same nature. On the night of the 13th of August, 1898, a German astronomer, Herr Witt, exposed a photographic plate to the heavens in his telescope in the Observatory of Urania, at Berlin. On that plate a picture of the heavens was obtained, and in that picture a new planet was revealed. At first the discovery of one more asteroid does not imply very much. Hundreds of such planets might be found, and indeed have been found, and yet no particular comment has been called forth. But this planet found by Witt is a unique object; it is more interesting than the whole of the four hundred and thirty-two other minor planets which have preceded it—not, indeed, on account of its size, for Witt's planet is a wholly insignificant object from this point of view. The special interest which this new planet has for us dwellers on the earth lies in the fact that it seems to be the nearest to the earth of all the other worlds in space—the moon, of course, excepted. This is the reason why the attention of all who are interested in the science of astronomy has been concentrated on Witt's discovery. It is certainly the most interesting telescopic revelation which has been made for many years.
It may illustrate a characteristic feature in the progress of modern astronomy if I describe how Witt succeeded in obtaining this picture. He had selected one of the most rapid plates that the skilled manufacturer can supply to the photographer. He put this plate into his telescope, and he directed it to the heavens. If that plate had been used in broad daylight for the more ordinary purpose of obtaining a photographic portrait, an exposure of half a second would have been quite long enough. But the very faint stars can not work their charm on the plate with equal rapidity; a second is not long enough, nor is ten seconds, nor even ten minutes. If we desire to secure an imprint of the faintest stars we must expose the plate for an hour, and sometimes for even much longer than an hour. Of course, an exposure of such duration would utterly ruin the picture if a gleam of any other light obtained access. But in the darkness of night the plate is secure from this danger. Each star is thus given time enough to impress its little image at leisure.
The photographer has often occasion to deplore the poorness of his light. It is, of course, in the endeavor to counteract the poorness of the light that so long an exposure is frequently given. But it will not be any longer supposed that, from the astronomer's point of view, a tedious exposure must necessarily be a disadvantage. Let it be henceforth recollected that it was the very requirement of a long exposure which led to the present important discovery. If the stars had been bright enough to be photographed by an exposure not longer than a few seconds or even than a few minutes, then this new and wonderful planet Eros would not have been revealed.
Many points of light which were undoubtedly stars, and merely stars, were shown on this picture taken by the German astronomer at Urania. Among these points of light was, however, one object which, though in appearance hardly distinguishable from a faint star, was in truth a body of a very different character. No telescope, however powerful, would show by mere inspection any appreciable difference between the dot of light indicating a star and the dot of light indicating the asteroid Eros. The fundamental difference between the star and Eros was, however, revealed by the long exposure. The stars in such a picture are, of course, at rest. They have occupied for years and for centuries the places where we now find them. If they are moving at all, their movements are so slow that they need not now be considered. But this starlike point, or, as we may at once call it, this asteroid, Eros, is moving. Not that its movements seem very rapid from the distance at which alone we are compelled to view it. No casual glance would indicate that Eros was flying along. The ordinary observer would see no change in its place in a second—no change in its place even in a minute. But when the exposure has lasted for an hour this asteroid, in the course of the hour, has moved quite appreciably. Hence arose a great difference between the representation which the photograph has given of the stars, properly so called, and of the asteroid. Each star is depicted as a sharp, well-defined point. This little body which is not a star, this unsteady sitter in the picture, could not be so represented; it merely appeared as a streak. The completed photograph accordingly shows a large number of well-marked dots for the stars, and among them one faint line for the asteroid.
Such a feature on a picture, though very unusual, does sometimes present itself. To detect such a streak on a photograph of the stars is a moment of transcendent joy to the astronomer. It is often for him the exciting occasion on which a discovery is made. This little moving point is in actual fact as different from a star as a pebble is different from a brilliant electric light. The resemblance of the asteroid to a star is merely casual; the resemblance would wholly disappear if we were able to make a closer inspection. The star is a brilliant blazing orb like a sun, but so far away that its luster is diminished to that of a point; the planet is comparatively near us; it is a dark body like our earth, and is like our earth also in this further respect that all the light it enjoys has been derived from the sun.
Though there is this immense difference between a star and a planet, yet the observer must not expect to notice any such difference by merely taking a peep through the telescope. It was only the long exposure in the photograph that revealed the little body.
Such is the manner in which an asteroid is generally discovered in these latter days. A discovery like this comes as the well-earned reward of the skill and patience of the astronomical photographer. There are, indeed, a large number of known asteroids; our catalogues contained four hundred and thirty-two of them up to the time when Witt exposed his now famous plate. Had the asteroid Witt then found been merely as other asteroids, it would never have received the prominent position that has now to be assigned to it in any account of the astronomy of the century. That object found by Witt on this night which is to be henceforth memorable in astronomy is of a wholly exceptional kind. Had Eros been merely an ordinary asteroid, Witt might no doubt have received the credit to which his labors and success would have entitled him. Another asteroid would have been added to the long list of such objects already known, but the newspapers would never have troubled their readers about the matter, and the only persons who would have been affected would have been the astronomers, and perhaps even among them no particular sympathy would have been felt in certain quarters. Those particular astronomers to whom has been intrusted the special work of looking after the asteroids and of calculating the tables of their movements might even have received with no very great enthusiasm the announcement of this further addition to the burden on their heavily laden shoulders.
I have said that Eros is quite a small globe; it may be well for us fully to realize how small that asteroid actually is. If the moon were to be crushed into two million equal fragments, each of those parts would be as big as Eros. If the whole of Eros were to be covered with houses, the city thus formed would not be so large as greater London. So far as mere size is concerned, Eros is quite unimportant. We can further illustrate this if we compare Eros with some of the other planets. The well-known evening star, Venus, the goddess of love, is a hundred million times as big as that tiny orb we now call Eros, the god of love. After all this it may seem strange to have to maintain what is, however, undoubtedly the fact, that the discovery of Eros is one of the most remarkable discoveries of this century.
Until Eros was discovered, our nearest neighbors among the planets were considered to be Venus on one side and Mars on the other. The other great planets are much more distant, while, of course, the stars properly so called are millions of times as far.
Great, then, was the astonishment of the astronomers when, by the discovery of Eros, Mars and Venus were suddenly dethroned from their position of being the earth's nearest neighbors among the planetary host. This little Eros will, under favorable circumstances, approach the earth to within about one third the distance of Mars when nearest, or about one half the distance of Venus when nearest. We thus concentrate on Eros all the interest which arises from the fact that, the moon of course excepted, Eros is the nearest globe to the earth in the wide expanse of heaven. To the astronomer this statement is of the utmost significance; when Eros comes so close it will be possible to determine its distance with a precision hitherto unattainable in such measurements. Once the distance of Eros is known, the distance of the sun and of all the other planets can be determined. The importance of the new discovery arises, then, from the fact that by the help of Eros all our measurements in the celestial spaces will gain that for which every astronomer strives—namely, increased accuracy.
Seeing that the existence of intelligence is a characteristic feature of this earth, we feel naturally very much interested in the question as to whether there can be intelligent beings dwelling on other worlds around us. It is only regrettable that our means of solving this problem are so inadequate. Indeed, until quite lately it would have been almost futile to discuss this question at all. All that could then have been said on the subject amounted to little more than the statement that it would be intolerable presumption for man to suppose that he alone, of all beings in the universe, was endowed with intelligence, and that his insignificant little earth, alone amid the myriad globes of space, enjoyed the distinction of being the abode of life. Recent discovery has, however, given a new aspect to this question. At the end of this century certain observations have been made disclosing features in the neighboring planet. Mars, which have riveted the attention of the world. On this question, above most others, extreme caution is necessary. It is especially the duty of the man of science to weigh carefully the evidence offered to him on a subject so important. He will test that evidence by every means in his power, and if he finds the evidence establishes certain conclusions, then he is bound to accept such conclusions irrespective of all other circumstances.
Mr. Percival Lowell has an observatory in an eminently favorable position at Flagstaff, in Arizona. He has a superb telescope, and enjoys a perfect climate for astronomical work. Aided by skillful assistants, he has observed Mars under the most favorable circumstances with great care for some years. I must be permitted to say that, having carefully studied what Mr. Lowell has set forth, and having tested his facts and figures in every way in my power, most astronomers have come to the conclusion that, however astonishing his observations may seem to be, we can not refuse to accept them.
No one has ever seen inhabitants on Mars, but Mr. Percival Lowell and one or two other equally favored observers have seen features on that planet which, so far as our experience goes, can be explained in no other way than by supposing that they were made by an intelligent designer for an intelligent purpose. Mr. Lowell has discovered that there are certain operations in progress on the surface of Mars which, if we met with on this earth, we should certainly conclude, without the slightest hesitation, were the result of operations conducted under what we consider rational guidance.
A river, as Nature has made it, wends its way to and fro; it never takes the shortest route from one point to another; the width of the river is incessantly changing; sometimes it expands into a lake, sometimes it divides so as to inclose an island. If we could discern through our telescopes a winding line such as I have described on Mars it might perhaps represent a river.
But suppose, instead of a winding line, there was a perfectly straight line, or rather a great circle on the globe drawn as straight as a surveyor could lay it out—if we beheld an object like that on Mars I think we should certainly infer that it was not a river made in the ordinary course of natural operations; no natural river ever runs in that regular fashion. If such a straight line were indeed a river, then it must have been designedly straightened by human agency or by some other intelligent agency for some particular purpose. In its larger features Nature does not work by straight lines. A long and perfectly straight object, if found on our earth, might be a canal or it might be a road; it might be a railway or a terrace of some kind; but assuredly no one would expect it to be a natural object.
We have the testimony of Schiaparelli, now strengthened by that of Mr. Lowell and his assistants, that there are many straight lines of this kind on Mars. They appear to be just as straight as a railway would have to be if laid across the flat and boundless prairie, where the engineer encountered no obstacle whatever to make him swerve from the direct path. These lines on Mars run for hundreds of miles, sometimes, indeed, I should say for thousands of miles. They are far wider than any terrestrial river, except perhaps the Amazon for a short part of its course. The lines on Mars are about forty miles wide. Indeed, the planet is so distant that if these lines were much narrower than forty miles they would be invisible. Each of them is marvelous in its uniformity throughout its entire length.
The existence of these straight lines on the planet contains perhaps the first suggestion of the presence of some intelligent beings on Mars. The mere occurrence of a number of perfectly straight, uniform lines on such a globe would in itself be a sufficiently remarkable circumstance. But there are other features exhibited by these objects which also suggest the astonishing surmise that they have been constructed by some intelligent beings for some intelligent purposes.
Sometimes two of these lines will start from a certain junction, sometimes there will be a third or a fourth from the same junction; in one case there are as many as seven radiating from the same point. Such an arrangement of these straight lines is certainly unlike anything that we find in Nature. We are led to seek for some other explanation of the phenomenon, and here is the explanation which Mr. Lowell offers:
It has recently been found that there are no oceans of water on the planet Mars. In earlier days it used no doubt to be believed that the dark marks easily seen in thetelescope could represent nothing but oceans, but I think we must now give up the notion that these are watery expanses. Indeed, there is not much water on that globe anywhere in comparison with the abundance of water on our earth. It is the scarcity of water which seems to give a clew to some of the mysteries discovered on Mars by Schiaparelli and Lowell.
As our earth moves round the sun we have, of course, the changing seasons of the year. In a somewhat similar manner Mars revolves around the sun, and accordingly this planet has also its due succession of seasons. There is a summer on Mars, and there is a winter; during the winter on that globe the poles of the planet are much colder than at other seasons, and the water there accumulates in the form of ice or snow to make those ice-caps that telescopic observers have so long noticed. In this respect Mars, of course, is like our earth. The ice-cap at each pole of our globe is so vast that even the hottest summer does not suffice to melt the accumulation; much of the ice and snow there remains to form the eternal snow which every arctic explorer so well knows. It would seem, however, that the contrast between winter and summer on Mars must be much more deeply marked than the contrast between winter and summer on our earth. During the summer of Mars ice and snow vanish altogether from the poles of that planet.
Mr. Lowell supposes that water is so scarce on Mars that the inhabitants have found it necessary to economize to the utmost whatever stock there may be of this most necessary element. The observations at Flagstaff tend to show that the dark lines on Mars mark the course of the canals by which the water melted in summer in the arctic regions is conducted over the globe to the tracts where the water is wanted. Not that the line as we see it represents actually the water itself; the straight line so characteristic of Mars's globe seems rather to correspond to the zones of vegetation which are brought into culture by means of water that flows along a canal in its center. In much the same way would the course of the Nile be exhibited to an inhabitant on Mars who was directing a telescope toward this earth: the river itself would not be visible, but the cultivated tracts which owe their fertility to the irrigation from the river would be broad enough to be distinguishable. The appearance of these irrigated zones would vary, of course, with the seasons; and we observe, as might have been expected, changes in the lines on Mars corresponding to the changes in the seasons of the planet.
A noteworthy development of astronomy in the last century has been the erection of mighty telescopes for the study of the heavens. It must here suffice to mention, as the latest and most remarkable of these, the famous instrument at the Yerkes Observatory, which belongs to the University of Chicago. Just as the century is drawing to its close, the Yerkes telescope has begun to enter on its sublime task of exhibiting the heavens under greater advantages than have ever been previously afforded to any astronomers since the world began.
The University of Chicago having been recently founded, it was desired to associate with the university an astronomical observatory which should be worthy of the astonishingplace that this wonderful city has assumed in the world's history. Mr. Yerkes, an American millionaire, generously undertook to provide the cost of this observatory. Two noble disks of glass, forty inches in diameter, were produced at the furnaces of Messrs. Mantois, in Paris; these disks were worked by Mr. Alvan Clark, of Boston, into the famous object glass which, weighing nearly half a ton, has now been mounted in what we may describe as a temple or a palace such as had never been dreamed of before in the whole annals of astronomy.
Perhaps if we could now place the science of the nineteenth century in its proper perspective the most remarkable discovery which it contains would be that of the planet Neptune. Indeed, the whole annals of science present no incident of a more dramatic character.
It will be remembered that at the latter part of the eighteenth century William Herschel had immortalized himself by the discovery of a great planet, to which was presently assigned the name of Uranus. After the movements of Uranus had been carefully studied, it was found that on many previous occasions Uranus had been unwittingly observed by astronomers, who regarded it as a star. When these observations were all brought together, and when the track which Uranus followed through the heavens was thus opened to investigation, it was found that the movements of the planet presented considerable anomalies. The planet did not move precisely as it would have moved had it been subjected solely to the supreme attractive power of the sun. Astronomers are, of course, accustomed to irregularities of this description in the movements of the planets. These irregularities have as their origin the attractions of the various other members of the solar system. It is possible to submit these attractions to calculation and thus to estimate their amount. The effect, for instance, of Saturn in disturbing Jupiter can be allowed for, and the nature of Jupiter's motion as thus modified can be precisely estimated. In like manner, the influence of the earth on Venus can be determined, and so for the other planets; and thus, generally speaking, it was found that when the proper allowances had been made for the action of known causes of disturbance, then the calculated movement of each planet could be reconciled with observation.
The circumstances of Uranus were, however, in this respect wholly exceptional. Due allowance was first made for the attraction of Uranus by Saturn, and for the attraction of Uranus by Jupiter, as well as by the other planets. It was thus found that the irregularities of Uranus could be to some extent explained, but that it was not possible in this manner to account for those irregularities completely. It was therefore evident that some influence must be at work affecting the movement of Uranus, in addition to those arising from any planet of which astronomers hitherto had cognizance. The only available supposition would be that some other planet, at present unrecognized, must be in our system, and that the attraction of this unknown body must give rise to those irregularities of Uranus which remained still outstanding.
A great problem was thus proposed for mathematicians. It was nothing less than to affect the determination of the orbit and the position of this unknown planet, the sole guide to the solution of the problem being afforded by the discrepancies between the places of Uranus as actually observed and the places which were indicated by the calculations, when every allowance had been made for known causes. The problem was indeed a difficult one, but, fortunately, two mathematicians proved to be equal to the task of solving it—Adams, in England, and Le Verrier, in France. Each of these astronomers, in independence of the other, succeeded in determining the place of the planet in the sky. The dramatic incident of this discovery was afforded when the mathematicians had done their work. When the place of the planet had been ascertained, then the telescopic search was undertaken to verify if it were indeed the case that a planet hitherto unknown did actually lurk in the spot to which the calculations pointed. Every one who has ever read a book on astronomy is well acquainted with the wonderful manner in which this verification was made. Just where the mathematicians indicated, there was the great planet discovered! To this object the name of "Neptune" has been assigned, and its discovery may be said to mark an epoch in the history of gravitation. It provided a most striking illustration of the truth of those great laws which Newton had discovered.
The latter half of the century will be also remarkable in the history of science from the fact that within that period mankind has been enabled to make some acquaintance with the chemistry of the celestial bodies. It was in 1859 that Kirchhoff and Bunsen first expounded to the world the true meaning of the dark lines in the solar spectrum. In this they were following out a line of reasoning that had been previously suggested by Prof. Sir G. Stokes, of Cambridge, England. Those who are at all conversant with that wonderful branch of knowledge known as spectrum analysis are aware how these discoveries have rendered it possible for us to determine in many cases the actual material elements found in the most distant bodies.
One of the striking results to which this investigation has led is the demonstration of the substantial unity of the materials from which the earth and the various heavenly bodies have been constructed. Those elements which enter most abundantly into the composition of the earth are also the elements which appear to enter most abundantly into the composition of the sun and of the stars. The iron and the hydrogen, the sodium and the many other materials of which our globe is so largely formed, are also the selfsame materials which, in widely different proportions and in very different associations, go to form the heavenly bodies. This conclusion is as interesting as it was unexpected. It might naturally have been thought that, seeing the sun is separated from us by nearly a hundred million miles, and seeing that the stars are separated from us by millions of millions of miles, all these celestial bodies must be constructed in quite a different manner and of substances quite distinct from the substances which we know on this earth. But this is not the case. Indeed, at the present moment it seems doubtful if there be any element which spectrum analysis has hitherto disclosed in the celestial bodies which is not also a recognized terrestrial body. The well-known case of helium gives a striking illustration. In the year 1868 Sir Norman Lockyer detected the presence of rays in the solar spectrum which were unknown at that time in terrestrial chemistry. These rays appeared to emanate from some substance which, though present in the sun, did not then appear to belong to the earth. This element was accordingly named "helium," to indicate its solar origin. Twenty-five years later Professor Ramsay discovered a substance on the earth which had been hitherto unrecognized, and which, on examination, yielded in the spectrum precisely those same rays which had been found in the so-called helium from the sun. In consequence of this discovery this element is now recognized as a terrestrial body. It is indeed a remarkable illustration of the extraordinary character of modern methods of research that a substance should have first been discovered at a distance of nearly one hundred million miles, that same substance being all the time, though no doubt in very small quantities, a constituent of our earth as well as of the sun.
Much has been done within the past century in many other branches of astronomy. I must especially mention the important subject of meteoric showers. For the development of our knowledge of this attractive part of astronomy we are largely indebted to the labors of the late Prof. H. Newton, of Yale. By his investigations, in conjunction with those of the late Professor Adams, it was demonstrated that the shower of shooting stars which usually appears in the middle of November is derived from a shoal of small bodies which revolve around the sun in an elliptic track, and accomplish that circuit in about thirty-three years and a quarter. The earth crosses the track of these meteors in the middle of November. If it should happen that the great shoal is passing through the junction at the time the earth also arrives there, then the earth rushes through the shoal of little bodies. These plunge into our atmosphere, they are ignited by the friction, and a great shower is observed. It is thus that we account for the recurrence of specially superb displays at intervals of about thirty-three years.
But one more great astronomical discovery of this century must be mentioned, and here again, as in so many other instances, we are indebted to American astronomers. It was in 1877 that Prof. Asaph Hall discovered that the planet Mars was attended by two satellites. This was indeed a great achievement, and excited the liveliest interest and attention. Since the days when telescopes were first invented all the astronomers have been looking at Mars, and yet they never noticed (their telescopes were not good enough) those interesting satellites which the acute observation of Professor Hall detected with the help of the great telescope of the Naval Observatory at Washington. This discovery was followed by another of a still more delicate nature, when that consummate observer, Professor Barnard, using the great Lick telescope, detected the fifth satellite of Jupiter. This is indeed a most difficult object to observe, requiring, as it does, the highest optical power, the most perfect atmospheric conditions, and the most skillful of astronomical observers. We may take this observation to represent the high-water mark of telescopic astronomy in the nineteenth century. This being so, it may fitly conclude this brief account of some of the most remarkable astronomical discoveries which that century has produced.
