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.