MODERN HISTORY.] 



ASTRONOMY. 



915 



revolution round the sun, in common with the other five 

 known planets, whose distances from the great central 

 body he proportioned. As regards the " precession of 

 the equinoxes," he conceived it to arise from a small 

 motion of the earth's axis. In consequence, however, of 

 his ignorance of the elliptical motions of the planets, he 

 was unable totally to abolish the system of epicycles. 

 The work which contained his explanations was dedicated 

 to Pope Paul III. ; and it was owing only to the urgent 

 representations of his scientific friends, that he was in- 

 duced to publish it in the form of an hypothesis. He 

 had only revised the last proof sheet when death put an 

 end to his labours in his seventieth year. 



One of the most important inventions ever introduced 

 by the genius of man, was afterwards applied to the 

 observations of the heavenly bodies by Galileo. This 

 was the telescope, which has enabled the astronomer to 

 overcome the feebleness of his natural vision, and to 

 examine leisurely, and attentively, the physical consti- 

 tution of the celestial bodies. Galileo, an ardent follower 

 of Copernicus, was immediately convinced of the truth 

 of the system, when, having observed the phases of 

 Venus, he traced their connection to their elongation 

 from the sun, and found that the orbit of this planet was 

 included within the earth's orbit. Galileo also discovered 

 the four satellites of Jupiter, and was acquainted with 

 the existence of spots on the sun, from which he con- 

 cluded the rotation of that body on its axis. He appears 

 lo to have first applied the pendulum to the clock. 



The overthrow of the Ptolemaic system followed 

 quickly on the footsteps of Copernicus and Galileo, but 

 not till the new system had encountered much opposition. 

 Tycho Brahe, who was born in 1546, is celebrated in 

 having supplied observations of greater excellence than 

 any of his predecessors. The cultivation of practical 

 astronomy had made but little progress since the time of 

 the Arabs, and the new theory required accurate obser- 

 vations to test its correctness. Tycho thus paved the 

 way for Kepler to make his splendid discoveries. Per- 

 sonally, however, he resisted the Copernican theory, and 

 invented an hypothesis of his own, retaining the earth 

 as the centre of the universe, but making the sun the 

 centre of the planetary motions. This system had few 

 admirers ; but the accuracy and number of his planetary 

 observations, his star catalogue, his discovery of the 

 " variation" of the moon, and his table of refractions, 

 give him a position among the astronomers of his period. 

 From the planetary observations of Mars, the immortal 

 discovery of the elliptic motions of the planets has re- 

 sulted. Kepler placed such confidence in the obser- 

 vations of Tycho, that he affirmed his belief that the 

 true system of the universe might be founded on them. 

 Taking at first the Copernican doctrine of a circular 

 hypothesis, he found that a difference of 8' existed be- 

 tween the theory and observation of Mars, which 

 difference, from the accuracy of the observations, he had 

 no doubt was attributive to an erroneous hypothesis. 

 After labour infinitely great, extending over a period of 

 twenty years, he discovered his three famous Jaws which 

 serve as the foundation of modern astronomy. Kepler 

 appears also to have had some notions on gravity ; and 

 he left also some tracts on optics. The laws which he 

 had determined for the sun and planets, were found also 

 to extend to the satellites of Jupiter, they having been 

 just discovered by Galileo, Huygens, and Cassini, 



Among the distinguished men who followed Kepler, 

 mention must be made of Huygens, Hevelius, Cassini, 

 and Roemer. The first, in addition to his optical and 

 mechanical improvements, described the ring surround- 

 ing Saturn as well as one of its satellites, and is said to 

 have applied the pendulum to clocks a discovery also 

 claimed for Galileo. Hevelius is known as a persevering 

 observer. But all his observations were made with the 

 naked eye ; and although he was able to estimate an- 

 gular spaces by these means to a wonderful degree of 

 accuracy, which Dr. Halley confirmed by direct com- 

 parison, his observations cannot be made use of in any 

 delicate inquiry. His researches on comets, and his 

 tables of the sun, rank him in a high position among 



practical astronomers. Cassini formed tables of Jupiter's 

 satellites from observation, and determined the rotation 

 of Jupiter and Mars. He also occupied himself with 

 refractions, and made a theory of the libration of the 

 moon, which had been left in an imperfect state by 

 Hevelius. Roemer is known as the inventor of the 

 transit instmment and meridian circle, with which in- 

 struments he carried on a series of observations of the 

 heavenly bodies for several years ; but only those of 

 three days have been preserved, the others having been 

 destroyed in the great fire which occurred at Hafnia. 

 Roemer also discovered, by comparison of Cassini's 

 tables of Jupiter's first satellite, the successive trans- 

 mission of light. The discovery of universal gravitation 

 by Newton, at the latter end of the seventeenth century, 

 produced a total revolution in astronomy, and enabled 

 its illustrious author to account for the previous empirical 

 deductions of Kepler. 



Since the discovery of gravitation by Sir Isaac Newton, 

 astronomy has been much advanced by the establish- 

 ment of observatories. And, in this respect, England 

 has great reason to be proud of her position, her astro- 

 nomers having followed up this immortal discovery by 

 a series of observations unexampled for their extent an 

 object which is essentially necessary, in order to continue 

 the comparison of theory and observation, for the cor- 

 rection of the elements of the system. The accuracy of 

 the science has been considerably increased by the in. 

 stitution of the Royal Observatory at Greenwich, both 

 with regard to the eminent men who have directed this 

 establishment, and to the continuous mass of obser- 

 vations which it has furnished. Founded by King 

 Charles II., in 1675, it has had for its directors, succes- 

 sively, Flamsteed, Halley, Bradley, Bliss, Maskelyne, 

 Pond, and Airy. 



Flamsteed is known as the greatest observer of the 

 age in which he lived. Patronised by Sir Jonas Moore, 

 to whom the foundation of the Greenwich Observatory 

 is due, he carried on for many years continuous obser- 

 vations of the planets and fixed stars, and contributed 

 materially to furnish Sir Isaac Newton with the obser- 

 vations on which his system of the world is based. He 

 was succeeded by Dr. Halley, an astronomer to whom 

 we are indebted for many useful investigations, but 

 whose province did not so much lie in practical astro- 

 nomy as that of his predecessor. This distinguished 

 man has the merit of adding considerably to our know- 

 ledge of cometary astronomy, and the lunar theory. But 

 one of his greatest triumphs, which will always render 

 liis name illustrioi;s, is his proposition of determining 

 the sun's distance, by the transits oi Venus over its 

 disc. 



Dr. Bradley, the next in succession, enriched this 

 science by two important discoveries viz., the aberra- 

 tion of light, and a more exact knowledge of the nuta- 

 tion of the earth's axis. Distinguished as the most 

 accurate astronomer of his age, he reobserved, with 

 better instruments, all the stars of Flamsteed, as well 

 as the sun, moon, and planets, the former observations 

 of which, have been incorporated in the Fuiidamenta 

 Astronomies of Bessel. His observations of the moon 

 and planets, are included in the Greenwich planetary and 

 uiuir reductions. 



The same important class of observations was con- 

 ;inued bv Dr. Maskelyne, from 1765 to 1811, extending 

 over an interval of forty-six years. In addition to other 

 important works, this astronomer has the credit of 

 establishing the Nautical Almanac for the use of mari- 

 ners, and of introducing in that work, tabular elements 

 for the improved method of finding the longitude at 

 sea. 



On the same regular system have been carried on, at 

 the Royal Observatory, observations of the sun, moon, 

 and planets, under the superintendence, successively, of 

 Mr. Pond and Professor Airy. If it were necessary to 

 discuss the importance of the immediate comparison of 

 theory and observation, it would be merely requisite to 

 mention, that the discovery of the planet Neptune has 

 thus resulted a discovery which, in the opinion of all 



