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.