The study of the heavens is the oldest, the study of man the youngest, of the sciences. With regard to himself and the character of his being man only obtained a clear knowledge in the second half of the present century; with regard to the starry heavens, the motions of the planets, and so on, he had acquired astonishing information forty-five hundred years ago. The ancient Chinese, Hindoos, Egyptians, and Chaldæans in the distant East knew more of the science of the spheres than the majority of educated Christians did in the West four thousand years after them. An eclipse of the sun was astronomically observed in China in the year 2697 B.C., and the plane of the ecliptic was determined by means of a gnome eleven hundred years B.C., while Christ himself had no knowledge whatever of astronomy--indeed, he looked out upon heaven and earth, nature and man, from the very narrowest geocentric and anthropocentric point of view. The greatest advance of astronomy is generally, and rightly, said to be the founding of the heliocentric system of Copernicus, whose famous work, _De Revolutionibus Orbium Celestium_, of itself caused a profound revolution in the minds of thoughtful men. In overthrowing the Ptolemaic system, he destroyed the foundation of the Christian theory, which regarded the earth as the centre of the universe and man as the godlike ruler of the earth. It was natural, therefore, that the Christian clergy, with the pope at its head, should enter upon a fierce struggle with the invaluable discovery of Copernicus. Yet it soon cleared a path for itself, when Kepler and Galileo grounded on it their true "mechanics of the heavens," and Newton gave it a solid foundation by his theory of gravitation (1686). A further great advance, comprehending the entire universe, was the application of the idea of evolution to astronomy. It was done by the youthful Kant in 1755; in his famous general natural history and theory of the heavens he undertook the discussion, not only of the "constitution," but also of the "mechanical origin" of the whole world-structure on Newtonian principles. The splendid _Système du Monde_ of Laplace, who had independently come to the same conclusions as Kant on the world-problem, gave so firm a basis to this new _Mécanique Céleste_ in 1796 that it looked as if nothing entirely new of equal importance was left to be discovered in the nineteenth century. Yet here again it had the honor of opening out entirely new paths and infinitely enlarging our outlook on the universe. The invention of photography and photometry, and especially of spectral analysis (in 1860 by Bunsen and Kirchoff), introduced physics and chemistry into astronomy and led to cosmological conclusions of the utmost importance. It was now made perfectly clear that matter is the same throughout the universe, and that its physical and chemical properties in the most distant stars do not differ from those of the earth under our feet.