ON PRACTICAL ASTRONOMY. 425 



or even vegetables could exist in any of them ; their minerals may, per- 

 haps, resemble ours, and if the stones which Mr. Howard has analyzed are 

 really lunar productions, we have proofs that the moon at least contains 

 some substances resembling those which compose the earth ; but the seas 

 and rivers of the other planets must consist of some fluid unknown to us, 

 since almost all our liquids would either be frozen, or converted into 

 vapour, in any of them. 



LECT. XLIV. ADDITIONAL AUTHORITIES. 



Librations of the Moon. Cassini, Hist, et Mem. 1721, p. 168, H. 53. Lalande, 

 ibid. 1764, p. 555, H. 112. Sejour, ibid. 1776. 



Eclipses. Hevelius, Ph. Tr. i. 369 ; v. 2023. Louville's Geometrical Mode of 

 calculating Eclipses, Hist, et Mem. 1724, p. 63, H. 74. Gersten's Meth. Ph. Tr. 

 1744, p. 22. Lalande on the Effect of Ellipticity, Hist, et Mem. 1756, p. 364, 

 H. 96 ; 1763, p. 413. Lambert, Table Ecliptique, 12mo, Berlin, 1765. Boscovich 

 de Solis et Lunae Defectibus, 4to, Lond. 1760. Jeaurat on the Projection of Eclipses, 

 Mem. des Sav. Etr. iv. p. 818. Goudin, Mem. sur les Eclipses du Soleil, 4to, 

 1803. Lubbock, Elementary Treatise on the Computation of Eclipses, 1835. 



LECTURE XLV. 



ON PRACTICAL ASTRONOMY. 



IT is generally most convenient in practical astronomy to neglect the real, 

 and to consider only the apparent motions of the sun, the stars, and planets, 

 for the visible effects must be precisely the same, whether the sun or the 

 earth perform a revolution in the plane of the ecliptic, and whether the 

 earth actually move on its axis, or the whole of the celestial bodies move 

 round it in a day. We may, therefore, suppose the sun to move, as he 

 appears to do, from west to east in the ecliptic, so as to advance almost a 

 degree in 24 hours, and from east to west, together with all the stars and 

 planets, so as to perform a whole revolution in a day. Speaking more 

 correctly, the sun appears to describe, in every sidereal day, a spiral, which 

 differs a little from a circle, and is also about a degree shorter, so that about 

 four minutes more are required for the return of the sun to the same part 

 of the heavens, and the completion of a solar day. 



In order to determine the place of any point in the heavens, it is usual to 

 compare its situation either with the plane of the earth's equator, or with the 

 ecliptic ; its angular distance from the equator being called its declination, 

 and from the ecliptic, its latitude ; these distances must be measured in 

 planes perpendicular to those of the equator or ecliptic, and the distances of 

 .these planes from their intersection, or from the equinoctial point in Aries, 

 are called respectively the right ascension and the longitude of the point to 

 be described. For the stars, the decimation and right ascension are most 



