# celestial sphere

The celestial sphere is the imaginary sphere upon whose inner surface celestial objects can be considered to lie for the purpose of describing their position. The celestial sphere is centered on the origin of whatever system of celestial coordinates is being used.

The **celestial axis** is the projection of Earth’s rotation
axis, north and south, onto the celestial sphere. The celestial
poles are the two points where an extension of Earth’s axis intersects
the celestial sphere and about which the celestial sphere appears to rotate
daily. As a result of precession, the
celestial poles complete a circle around the ecliptic poles every 25,800
years.

The **celestial meridian** is the great circle on the
celestial sphere that passes through the celestial poles and the zenith
of the observer. The celestial equator is the great circle on the celestial sphere that divides the northern and
southern hemispheres and serves as the zero-mark for declination;
it is the projection into space of Earth’s equatorial plane.

The **celestial
latitude** is the angular distance on the celestial sphere measured
north or south of the ecliptic along the great circle passing through the
poles of the ecliptic and the celestial object. The **celestial longitude** is the angular distance along the ecliptic from the vernal equinox eastward.

An **hour circle** is a great circle on the celestial sphere that passes through a celestial object and the north and south celestial poles. Everywhere on an hour circle the right ascension is the same.