celestial horizon

The celestial horizon is a plane perpendicular to a point's gravity vector.

The great circle separating hemispheres is called celestial horizon or rational horizon.

Some examples of great circles on the celestial sphere include the celestial horizon, the celestial equator, and the ecliptic.

There is a vertical circle for any given azimuth, where azimuth is the angle measured east from the north on the celestial horizon.

Altazimuth, used to measure the position of celestial bodies relative to the celestial horizon and the zenith - the altitude azimuth.

Consequently, it also contains the horizon's north and south points and it is perpendicular to the celestial equator and the celestial horizon.

By 84 CE the elliptical ring was added to the armillary sphere, while Zhang Heng's model of 125 CE added the celestial horizon ring and meridian ring.