A Short History of Astronomy 



[Cn. IL 



positions on the sky ; they may thus be regarded as the 

 authors of the first real star catalogue, earlier astronomers 

 having only attempted to fix the position of the stars by 

 more or less vague verbal descriptions. They also made a 

 number of valuable observations of the planets, the sun, 

 etc., of which succeeding astronomers, notably Hipparchus 

 and Ptolemy, were able to make good use. 



33. Among the important contributions of the Greeks 

 to astronomy must be placed the development, chiefly from 

 the mathematical point of view, of the consequences of the 

 rotation of the celestial sphere and of some of the simpler 

 motions of the celestial bodies, a development the indi- 

 vidual steps of which it is difficult to trace. We have, 



FIG. 14. The equator and the ecliptic. 



however, a series of minor treatises or textbooks, written 

 for the most part during the Alexandrine period, dealing 

 with this branch of the subject (known generally as 

 Spherics, or the Doctrine of the Sphere), of which the 

 Phenomena of the famous geometer Euclid (about 300 B.C.) 

 is a good example. In addition to the points and circles 

 of the sphere already mentioned (chapter i., 8-n), we 

 now find explicitly recognised the horizon, or the great 

 circle in which a horizontal plane through the observer 

 meets the celestial sphere, and its pole,* the zenith,f or 



* The poles of a great circle on a sphere are the ends of a diameter 

 perpendicular to the plane of the great circle. Every point on the 

 great circle is at the same distance, 90, from each pole. 



f The word "zenith " is Arabic, not Greek : cf. chapter in., 64. 



