One axis of the
altazimuth coordinate system. It is the
circle on
the celestial sphere midway between the observer's
zenith and
nadir. In laymen's terms, if you were standing in the center of a
globe, the
north pole would be your zenith, the
south pole would be your nadir, and the
equator would be your celestial
horizon
An observer on the surface of the earth may be able to see points below his celestial horizon due to atmopsheric refraction.