Among the contemporaries of Newton were several astronomers, who have a particular claim upon the re collection of posterity. Maraldi determined the retro grade motion of Jupiter's nodes, and the progressive motion of his aphelion ; he discovered the variation in the inclination of the orbits of Jupiter's satellites ; he corrected the theory of Mars ; and he sheaved that the motion of Saturn was diminishing. Mr Pound found, that the equatorial was to the polar diameter of Jupiter as 13 to 12. He calculated new tables for computing the eclipses of Jupiter's first satellite, and he rectified the motions of the satellites of Saturn.
The celebrated Dr Halley was one of the most emi nent of Newton's contemporaries, and conferred many important obligations on the science of astronomy. In the 19th year of his age he published a geometrical method of finding the aphelia and eccentricities of the planetary orbits ; and in the following year he went to St Helena to make a catalogue of the southern stars. While he was observing the transit of Mercury during his stay at St Helena, it occurred to him, that the sun's parallax might be ascertained by the transit of Venus over his disc, and he pointed out the method by which this important problem was to be resolved. The sight of the litmous comet of 1680, which he obtained on his road from Calais to Paris, gave occasion to his beau tiful investigation respecting the orbits of these wan dering stars, by which he was enabled to predict the return of the comet in 1759 ; the first and the only pre diction of this kind that has yet been fulfilled. In the year 1691, he published tables of the conjunctions Mercury and Venus with the sun ; and he afterwards directed his chief attention to the lunar theory. Upon the death of Flamstead in 1719, Dr Halley was appoin ted to the office of astronomer-royal ; and in the same year he published his tables of the sun, moon, and pla nets. Though he was now in the 64th year of his age, this indefatigable astronomer continued to observe the moon's transit for 18 years ; and in the space of 9 years he made no fewer than 1500 observations, which enabled him to correct his theory of the moon. The reputation of Halley, however, does not rest merely on his astro nomical labours. His acquaintance with all the sciences, the services which he rendered them, and his fine taste for literature and philosophy, while they prove the uni versality of his genius, will secure him a high rank among the ornaments of the human species.
Upon the death of Dr Halley in 1742, the royal ob servatory received an able successor in the person of Mr James Bradley. To this ingenious astronomer we are indebted for two of the most beautiful discoveries of which the science can boast. While endeavouring to
determine the parallax of the fixed stars, with a large and excellent sector constructed by Grahame, he dis covered the aberration of these bodies, which arises from the motion of light combined with the motion of the earth in its orbit. We have already given a com plete history of this wonderful discovery in the article ABERRATION. In the year 1745, he was led by obser vation to the discovery of the nutation of the earth's axis ; a phenomenon which he justly ascribed to the action of the moon upon the protuberant equatorial parts of the terrestrial spheroid. This libration of the earth's axis amounts to 18 seconds in the space of 19 years, dying which the moon's nodes perform a complete re volution. The precession of the equinoxes, produced by the single action of the moon, is 36"; but on account of the motion of her nodes, this effect is not uniformly produced, and hence arises a variation in the precession, and a nutation in the axis of the earth. Dr Bradley constructed from his 0 observations new tables of Jupiter's satellites. He fixed, with great accuracy, both the quantity and the laws of atmospherical refractions, and gave an elegant method of computing the correc tions which are necessary from a variation in the tem perature and pressure of the air. The numerous and accurate observations which were made by this able ob server, will be soon given to the world. The first vo lume has been already published by Dr Hornsby, and the remainder is at present in the hands of Mr Abra ham Robertson, to whom their publication has been entrusted.
About this time France produced a number of excel lent astronomers, distinguished chiefly by the number and importance of their observations. In 1751, M. de la Caille undertook a voyage to the Cape of Good Hope for the purpose of perfecting the catalogues of the stars in the southern hemisphere. After the most indefatiga ble exertions, he returned to Europe with a catalogue of 9800 stars comprehended between the south pole and the tropic of Capricorn. The places of 1942 stars were calculated from his observations, and published in 1763 in his C.rlunt 4ustrale Stelliferum. The right ascensions and declinations of the remaining 8000, the observations upon which are published, have not been determined. In addition to these labours, La Caille computed new tables of the sun, containing equations for the attraction of Jupiter and Venus ; he made observations on the parallax of Mars and Venus ; on atmospherical refrac tion ; and on the length of pendulums ; and, during his stay at the Cape, he measured a degree of the meri dian.