# Definition:Celestial Sphere/Observer

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## 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 $O$.

## 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**