# Definition:Celestial Horizon

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## Definition

The **celestial horizon** is the great circle on the celestial sphere whose plane is perpendicular to the line from the zenith to the observer.

Thus the **celestial horizon** is the edge of the visible part of the celestial sphere.

The rest of it is hidden by Earth itself.

### Poles of Horizon

The **poles** of the **celestial horizon** are known as the **zenith** and the **nadir**.

## Also known as

The **celestial horizon** is also known just as the **horizon**, and as such is used in common parlance to mean **as far as can be seen**, figuratively as well as literally.

## Also see

## 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** - 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**horizon** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**celestial sphere** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**horizon**