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 Earths rotation axis, north and south, onto the celestial sphere. The celestial poles are the two points where an extension of Earths 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 Earths 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.