ABSTRACT
The measurement of the positions on the celestial sphere was among the earliest operations performed on heavenly bodies. The Alt-Azimuth system is based on the horizontal plane and its perpendicular, namely the vertical. Horizon, equator, and ecliptic are three great circles on the celestial sphere, which can be used to define different astronomical reference systems. The plane passing through the observer and perpendicular to the vertical cuts the celestial sphere in the astronomical horizon. The Astronomical Almanac provides a table of reference ellipsoids from the time of Airy to the present. The equatorial system is therefore the fundamental one for all precise descriptions of the celestial sphere, and it is employed by all major star catalogues. Indeed, only the hour angle axis needs an accurate tracking rate, the declination axis being used for fast slewing and occasional corrections to the pointing.
