# Definition:Celestial Sphere/Observer

## Definition

In the context of astronomy, in particular spherical astronomy, the observer is considered to be a reference point located at the center of the celestial sphere.

In practice, of course, an actual observer is physically located on the surface of Earth.

However, the celestial sphere is considered to be so much larger than the radius of Earth that the observer is considered to be at the center of Earth.

In the above diagram, the position of the observer is indicated by $O$.