Definition:Celestial Horizon

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.

However, the word horizon has multiple uses, so on $\mathsf{Pr} \infty \mathsf{fWiki}$ the full name is preferred.

Also see

• Definition:Horizontal

• Definition:Visible Hemisphere

• Results about the celestial horizon can be found here.

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 $(3)$
• 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 $(3)$
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): horizon