ITS EARLIEST STAGES. 131 



The two circles parallel to this which bounded the sun's path among" 

 the stars were called Tropics (rpoTuicai), because the sun turns back 

 again towards the equator when he reaches them. The stars which 

 never set are bounded by a circle called the Arctic Circle (dpKriKog, 

 from dparog, the Bear, the constellation to which some of the prin- 

 cipal stars within that circle belong.) A circle about the opposite 

 pole is called Antarctic, and the stars which are within it can never 

 rise to us. 41 The sun's path or circle of the signs is called the Zodiac, 

 or circle of animals ; the points where this circle meets the equator 

 are the Equinoctial Points, the days and nights being equal when the 

 sun is in them ; the Solstitial Points arc those where the sun's path 

 touches the tropics ; his motion to the south or to the north ceases 

 when he is there, and he appears in that respect to stand still. The 

 Colures {icoXovpoi, mutilated) are circles which pass through the poles 

 and through the equinoctial and solstitial points; they have their name 

 because they are only visible in part, a portion of them being below 

 the horizon. 



The Horizon (6pi%G)v) 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, therefore, an entire hemisphere 

 is always above the horizon. The term occurs for the first time in the 

 work of Euclid, called Phenomena (fyaivoueva). 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 Kivovjxevrjg 

 2(j)aipag, " On the Moving Sphere," he demonstrates various properties 

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

 work, Ilept 'EttitoX&v ml Avoeuv, "On Risings and Settings," 43 

 tacitly assuming the sun's motion in his circle to be uniform, he proves 

 certain propositions, with regard to those risings and settings of the 

 stars, which take place at the same time when the sun rises and sets, 4 ' 

 or vice versa ; 45 and also their apparent risings and settings when they 

 cease to be visible after sunset, or begin to be visible after sunrise. 46 



« The Arctic and Antarctic Circles of modern astronomers are different from 

 these. 

 48 Delambre, Astron,. Ancienne, p. 19. 

 43 Delambre, Astron. Anc. p. 25. 44 Cosmical rising and setting. 



45 Acronycal rising and setting ; (aicpovvKios, happening at the extremity of the 

 night.) 



46 Heliacal rising and setting. 



