Used to describe the position of objects in the sky, the celestial sphere is a fictitious sphere centred on the Earth upon which all celestial bodies can be projected.
At any one time, an observer on the Earth’s surface can only see half of the celestial sphere since the other half lies below the horizon. Although the rotation of the Earth is constantly bringing new regions of the celestial sphere into view, unless the observer is located at the equator, there will always be part of the celestial sphere that remains hidden.
The projection of the Earth’s equator on the celestial sphere is called the celestial equator. Similarly the Earth’s coordinate system of longitude (meridians) and latitude (parallels) can be projected on the celestial sphere, giving rise to the celestial coordinates: right ascension and declination.