CHAPTER III.

HISTORY OF PHYSICAL SCIENCE CONTINUED.

I. ASTRONOMY.

SECT. I.-ON THE PRINCIPAL ASTRONOMERS OF THE

SIXTEENTH CENTURY.

268. THE period which is now to pass in review, forms a most splendid æra in the history of astronomical science. The progress made in this important branch of philosophy was so rapid and great-the new discoveries were so various and astonishingthat, compared with them, all the attainments of preceding ages, from the earliest records of authentic history, vanish into utter insignificance. It will only be possible to glance at some of these discoveries in the order in which they were announced to the world, and to enumerate a few of the distinguished individuals to whose inventive genius they are attributed.

269. COPERNICUS must be first mentioned, as confessedly the great founder of Modern Astronomy. He was born at Thorn, in Prussia, A. D. 1472, but did not commence that astronomical career, which has given celebrity to his name, till the beginning of the following century. Originally intended for a physician, he applied himself, in early youth, to the

study of medicine, but afterwards took orders in the Church of Rome, that he might pursue, with greater advantage, his philosophical researches. His literary ardour first manifested itself in the department of mathematics, by which he was naturally led to astronomy, as a science which opens the widest field to geometrical investigation. He soon perceived the deficiency and fallacy of all the theories of the ancients respecting the planetary system, except that which Pythagoras is said to have darkly conjectured many ages before, though he was unable to demonstrate its truth. (§ 87.)

It appears, from the statement of Copernicus himself, that he first began to entertain doubts of the truth of the Ptolemaic system, about A. D. 1507; and that these doubts were occasioned by the difficulty he found in reconciling that system with the simplicity and uniformity of the laws of nature. It soon appeared, that by transferring the sun to the centre of the planetary system, instead of the earth ; by giving to the earth a twofold motion, one on its own axis, and another in a prescribed path or orbit round the sun; and by applying the same theory to the rest of the planets, many of his difficulties vanished, and the celestial phænomena, the solution of which before seemed impracticable, were explained with a facility at which he was perfectly astonished. The simple fact that led the mind of Copernicus to this discovery, was one of those familiar and everyday scenes, which must have been observed a thousand times both by the peasant and the philosopher; namely, the optical illusion which takes

place when a spectator imagines the objects on the banks of a river to be moving in an opposite direction to that in which he is himself proceeding. The conjecture was natural, that the same illusion may happen with respect to the sun's apparent motion, and those of the heavenly bodies. In his reasonings on the subject, there are some sound and solid arguments indicative of true science; but others were errors into which he was betrayed by his remaining attachment to ancient systems. Well knowing the dread of innovations, whether in philosophy or religion, which had ever characterized the Church to which he belonged, he did not venture to announce publicly either his rejection of the ancient, or his discovery of a new theory, till nearly the close of life. It is more than probable, that if he had earlier developed a scheme so completely at variance with the orthodox faith, he would have fallen a sacrifice to the ignorance and bigotry of the times, and, like one of his successors, languished away the remnant of his existence in the prisons of the Inquisition. He was, however, induced in advanced life, by the persuasions of Cardinal Schoenberg, to publish a work, written with great caution, and containing many apologies for the novelty of some of its statements, in which the true doctrine of the solar system was developed. This book, which originally was entitled " De Revolutionibus Coelestibus," but afterwards more appropriately styled " Astronomia Instaurata," was published A. D. 1543, and dedicated to the Roman Pontiff; nor is it a little remarkable, that this

great astronomer died on the very day in which the first complete printed copy of this work was put into his hands.

270. The new theory of Copernicus attracted little attention, when first promulgated. "It lay fermenting in secret," says Playfair, "with other new discoveries, for more than fifty years, till by the exertions of Galileo, it was kindled into so bright a flame, as to consume the philosophy of Aristotle, to alarm the hierarchy of Rome, and to threaten the existence of every opinion not founded on experience and observation." During this interval, which may be regarded as a kind of intellectual twilight, TYCHO BRAHE, the celebrated Danish astronomer, arose and flourished; a man of genius, of science, of unwearied industry; but who, unhappily, suffered himself to be so fettered by early prejudices, or so intimidated by the fear of man, as not to dare to follow up his own convictions, or pursue the course of his own discoveries. He was the inventor of a system, which bears his name, and which takes a middle course between the Ptolemaic and the Copernican; for while it agrees with the former, in supposing the earth to be the immoveable centre of the whole planetary sphere; it represented first the moon as revolving round it; and next the sun, carrying with him in his revolution, the five primary planets as his satellites. Had this theory been developed before the publication of the "Astronomia Instaurata" of Copernicus, it might have been hailed as one, and that not an unimportant step in advance; but appearing when it did, it must

rather be accounted a retrograde movement. There is, however, reason to believe, that Tycho Brahe was far from being satisfied with this theory, though he attempted to reconcile with it the actual phanomena of the celestial motions. Nor can we wonder that, rapidly progressive as mathematical science then was, this system should have obtained few converts and been soon forgotten, except as a matter of history.

271. The services rendered by Tycho Brahe to the sublime science of astronomy, were however great and highly important, for (1.) he prevailed upon the King of Denmark to establish an observatory at Uranïberg, and to furnish it with the most accurate instruments that could be procured from any part of Europe. Aided by these advantages, he not only made an immense number of astronomical observations himself, but collected around him a band of scientific men ardently devoted to the same pursuits, and founded a school of astronomy, from which not a few eminent astronomers proceeded to illumine the world.

(2.) Many years had now elapsed, since the attempt of Hipparchus and Ptolemy to number the stars, and far superior instruments for the purpose were now constructed (§. 86. 89.) The Danish astronomer determined, therefore, to renew the attempt, and, after a long series of minute observations and accurate measurements, produced a catalogue of the fixed stars, amounting to 777, whose situation in the heavens was determined with as much precision as the state of science would then permit, though far