Tycho Brahe, in the latter part of his life, had, for his disciple and assistant, the celebrated Kepler, who was born in 1751, at Vie], in the duchy of Wirtemberg, and was one of those rare characters that appear in the world only at particular times, to prepare the way for new and important discoveries. Like his master Tycho, he appears to havg attached him self to the science at a very early age ; and if it be the privilege of genius to change received ideas, and to announce truths which had never before been dis covered, he may justly be considered as one of the greatest men that had yet ap peared. Hipparchus, Ptolemy, Tycho Brahe, and even Copernicus himself, were indebted for a great part of their knowledge to the Egyptians, Chaldmans, and Indians, who were their masters in this science ; but Kepler, by his own ta lents and industry, has made discoveries, of which no traces are to be found in the annals of antiquity. See BRARR.
This great man, after seventeen years of mt ditation and calculation, having had the idea of comparing them with the powers of the numbers by which they are expressed, he found that the squares of the times of the revolutions of the planets are to each other as the cubes of their mean distances from the sun; and that the same law applies equally to their satel lites See KEPLER.
At the same time also that Kepler, in Germany, was tracing the orbits of the planets, and settling the laws of their mo tions. Galileo (who was born at Pisa, in Italy, in 1564) was meditating upon the doctrine of motion in general, and inves tigating its principles ; and from the ad mirable discoveries which lie made in this branch of the physico-mechanical sciences, Newton and Huygens were af ter, ands enabled to derive the most brilliant and complete theories of all the planetary motions.
About this period, also, a fortunate ac cident produced the most marvellous in strument that human industry and saga city could have ever hoped to discover ; and which, by giving a far greater exten sion and precision to astronomical obser vations, shewed many irregularities and new phcenomena, which had hitherto re mained unknown. This invention was that of the telescope, which was no soon er known to Galileo, than he set himself about to improve it ; and the discoveries he was by this means enabled to make were as new as they were surprising.
The face of the moon appeared full of cavities and asperities, resembling vallies and mountains. The sun, which had generally been considered as a globe of pure fire, was observed to be sullied by a number of dark spots, which appeared on various parts of his surface. A great number of new stars were discovered in every part of the heavens ; the planet Jupiter was found to be attended with four moons, which moved round him in the same manner that our moon moves round the earth; the phases of Venus ap peared like those of the moon, as had be fore been concluded by Copernicus, from his theory ; and, in short, most of the ob. servations he made furnished new proofs of the truth of the Copernican system. In publishing the discoveries which he had made with this new instrument, Galileo shewed, in the most incontestable man ner, the annual and diurnal motion of the earth ; which doctrine, however, was thought so alarming, that it was immedi ately declared heretical, by a congrega tion of cardinals, who were assembled upon the occasion ; and its venerable au thor, one of the most virtuous and en lightened men of his age, was obliged to abjure, upon his knees, and in the most solemn manner, a truth, which nature and his own understanding had shewn him to be incontrovertible. After this, he was
condemned to perpetual imprisonment ; from which, however, at the end of a year, he was enlarged, by the solicitations of the grand duke ; but that he might not withdraw himself from the power of the inquisition, he was forbid to quit the ter ritory of Florence, where he died in 1642 ; carrying with him the regrets of Europe, enlightened by his labours, and their indignation against the odious tri bunal which had treated him so unwor thily. See GALILEO.
The discoveries of Huygens succeeded those of Kepler and Galileo • and few men have, pei haps, marl ed more of the sciences, by the importance and sublimi ty of his researches. Among other things, his happy application of the pendulum to clocks is one of the most advantageous presents that was ever made to astrono my. He was also the first who found that the singular appearances of Saturn are produced by a ring, by which the planet is surrounded : and his assiduity in ob serving it led him to the discovery of one of its satellites.
About this epoch, astronomy began to be more generally cultivated and improv ed, in consequence of the establishment of several learned societies, which, by ex citing a spirit of emulation and enterprise among their members, greatly contribut ed to the advancement of every branch of the mathematical and physical sciences. The chief of these were, the Royal Soci ety of London, and that of the Academy of Sciences of Paris ; both of which have rendered great services to astronomy, as well by the eminent men they have pro duced, as by the zeal and ardour with which the science has been constantly promoted by them. Towards the latter part of the seventeenth century, and the beginning of the eighteenth, practical as tronomy seems rather to have languished ; but, at the same time, the theoretical part was carried to the highest degree of per fection by the immortal Newton, in his "Principia," and by the astronomy of David Gregory. About this time, also, clock and watch-work was greatly im proved by Mr. Graham, who likewise constructed the old eight feet mural arch at the Royal Observatory at Greenwich, and the zenith sector of 24 feet radius, with which Dr. Bradley discovered the aberration of the fixed stars. The astro nomical improvements in the last century have been chiefly owing to the greater perfection of instruments, and to the esta blishment of regular observatories, in va rious parts of Europe. Romer, a cele brated Danish astronomer, first made use of a meridian telescope ; and, by observ ing the eclipses of Jupiter's satellites, was led to his discovery of the motion of light, which he communicated to the Aca demy of Sciences at Paris, in 1675.