From AMS Glossary
Revision as of 18:27, 26 January 2012 by Perlwikibot
The line between the sky and the earth projected on the celestial sphere.
This line is often irregular, due to the earth's topography. On flat surfaces, such as the sea or a level plane, the line approximates a great circle if the observer's elevation above the surface is insignificant and a small circle if the elevation is appreciable.