158 THE GREEK ASTRONOMY. 



The Horizon (opt^wv) is commonly understood 

 as the boundary of the visible earth and heaven. 

 In the doctrine of the sphere, this boundary is a, 

 great circle., that is, a circle of which the plane 

 passes through the centre of the sphere ; and, there- 

 fore, an entire hemisphere is always above the 

 horizon. The term occurs for the first time in the 

 work of Euclid, called Phenomena (<fran'oVei/a). We 

 possess two treatises written by Autolycus 42 (who 

 lived about 300 B.C.) which trace deductively the 

 results of the doctrine of the sphere. Supposing its 

 diurnal motion to be uniform, in a work entitled 

 Hepl KtvovtJievris ^aipa?, "On the Moving Sphere," 

 he demonstrates various properties of the diurnal 

 risings, settings, and motions of the stars. In another 

 work, Uepl 'Eu-iToXwi/ /ecu Au<jeo>i;, "On Risings and 

 Settings 43 ," tacitly assuming the sun's motion in his 

 circle to be uniform, he proves certain propositions, 

 with regard to the risings and settings of the stars, 

 at the same time when the sun rises and sets 44 , 

 or vice versa** ; and also their apparent risings and 

 settings when they cease to be visible after sun-set, 

 or begin to be visible after sun-rise 46 . Several of 

 the propositions contained in the former of these 

 treatises are still necessary to be understood, as 

 fundamental parts of astronomy. 



The work of Euclid, just mentioned, is of the 



42 Delambre, Astron. Ancienne, p. 19. " Ib. p. 25. 



44 Cosmical setting and rising. 4S Acronical. 



46 Heliacal. 





