Definition:Celestial Sphere/Observer

Definition

In the context of astronomy, in particular spherical astronomy, the (celestial) 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 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 $O$.

Sources

• 1976: W.M. Smart: Textbook on Spherical Astronomy (6th ed.) ... (previous) ... (next): Chapter $\text {II}$. The Celestial Sphere: $18$. Altitude and azimuth.
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): celestial sphere
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): celestial sphere