Definition:Celestial Horizon/Pole

Definition

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

Zenith

Let an observer $O$ on the surface of Earth be assumed to be the center of the celestial sphere $C$.

The zenith $Z$ is the point on $C$ which is vertically overhead.

Nadir

Consider the celestial sphere $C$ with an observer $O$ at the center.

Let $Z$ be the zenith of $C$

Let the straight line $ZO$ be produced from $O$ to meet $C$ at $R$.

The point $R$ is known as the nadir of $C$.

Sources

• 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
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): nadir
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): zenith
• 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
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): nadir
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): zenith