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LATITUDE AND LONGITUDE. 



diameter into the account : for let C be the earth's 

 centre, T some point on its surface, L any star whatever 

 ZQM the concave surface of the heavens. If we ob- 

 serve the star L, from the point T, it is clear that the 

 point of the heavens in which we should see it is B. 



But if we could observe from the centre C, the point of 

 the heavens in which we should perceive it is D, so that 

 in one case it would only appear above the horizon by the 

 arc OB, or angle OTB, whilst from the centre C it 

 would appear. to have an elevation equal to QCD or 

 OTA, (drawing TA parallel to CD) by which we per- 

 ceive that the difference is measured by the angle 

 BTA = TLC (by the properties of parallels). 



The situation of a celestial body when viewed from 

 the surface of the earth is therefore called its apparent 

 place, and that part of the heavens in which it would 

 appear if observed at the same time from the centre of 

 the earth its true place, and the difference of those is 

 consequently the parallax or parallactic angle ; all of 

 which is further illustrated by the accompanying figure, 

 where AV is the earth, A the situation of an observer 

 at some given place on the surface, consequently GN 

 is the sensible horizon, and HO the rational horizon, 

 Z the zenith ; also let FG be part of a vertical circle, 

 whose radius is the moon's distance from the earth'* 

 centre C ; WX part of a vertical circle, whose radius 

 is the distance of a planet from the earth's centre, and 



