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celestial horizon,one axis of the altazimuth coordinate systemaltazimuth coordinate system
or horizon coordinate system,
astronomical coordinate system in which the position of a body on the celestial sphere is described relative to an observer's celestial horizon and zenith.
..... Click the link for more information. . It is the great circle on the celestial sphere midway between the observer's zenith and nadir; it divides the celestial sphere into two equal hemispheres. The observer may be unable to see all the stars that lie above his celestial horizon because of obstructions such as buildings, trees, or mountains; he may be able to see some stars that lie below his celestial horizon because of atmospheric refraction.
celestial horizon[sə′les·chəl hə′rīz·ən]
That great circle of the celestial sphere which is formed by the intersection of the celestial sphere and a plane through the center of the earth and is perpendicular to the zenith-nadir line. Also known as rational horizon.
The great circle of the celestial sphere, equidistant from the zenith and the nadir. It is the intersection of the sensible and rational horizons at infinity. Its plane passes through the center of the earth.