Definitions – Terms used in Astronomy

Astronomy uses some words and terms that may be unfamiliar to those new to the subject, or perhaps not as experienced in observational astronomy as others. Some of these terms are;

• Altitude
• Azimuth
• Meridian (and transit of the meridian)
• Horizon
• Zenith

All of these terms are used to describe the location or behaviour of objects in the sky. An example, you speak about the Sun or Moon and it’s altitude in relation to the horizon. Alternatively, when the Sun or the Moon passes from one side of the meridian to the other, you might say the Sun or the Moon has “transited or passed the meridian.”

These terms and their meanings are illustrated using the following two diagrams.

Horizon Coordinate System, Part A

• Zenith – Point on celestial sphere directly overhead.
• Nadir – Point on celestial sphere directly beneath observer.
• North Point – Point on horizon in direction of geographical north.
• South Point – Point on horizon in direction of geographical south.
• Vertical Circle – Any great circle which passes through the zenith.
• Meridian – The vertical circle which passes through the north and south points.

Horizon Coordinate System, Part B

• Altitude – The angular distance from horizon to object, measured along a vertical circle.
• Azimuth – The angular distance along horizon from N (S) eastwards to vertical circle through object (for Northern (Southern) hemisphere.