536 



LECTURE XLV. 



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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 S-i 

 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 e(juator, 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 declination and right ascension are most 

 usually laid down; but with respect to the sun and the planets, performing 

 their revolutions in or near the ecliptic, it is most convenient to calculate 

 their latitude and longitude. 



The plane passing through the earth's axis and the place of a spectator is 



