Its center is often taken as the center of the Earth. Although objects are various distances from Earth, each is projected onto the sphere so that each star has an apparent place on the sphere. Relative posistions on the sphere are specified by angles. It is both a vertical circle and an hour circle. The observer's meridian is the most important of all circles of reference.
Vertical circles receive their name from the fact that they are perpendicular to the horizon.
The north and south celestial poles are the centers of rotation of the celestial sphere. 1. the great circle on the celestial sphere midway between the north and south celestial poles. Diurnal circles are parallel to the celestial equator. Also known as the First Point of Aries.