Definitions

If you want to complete some additional reading beyond what is in our Lesson 1 pages, I recommend:

• Astronomy Without a Telescope (www.astronomynotes.com)
  • Reference Markers

In this lesson, we will be using some vocabulary that may be either unfamiliar or may be used differently, in an astronomical context, from your usual usage. Normally, I advocate for discussing vocabulary in the context of a lesson, instead of presenting it like this to begin the lesson. In our case, though, I'm going to break my own rule so that we can start discussing the sky immediately and make sure that we agree on the meaning of these terms. The vocabulary you will encounter in this lesson includes:

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

All of these terms are used to describe the location or behavior of objects in the sky. For example, you can refer to the altitude of the Sun. Or, when the Sun passes from one side of the meridian to the other, you can talk about the Sun "transiting the meridian." These terms and their meanings are illustrated using the following two diagrams. Read over the list of terms and definitions in the list and/or mouse over the terms in the image to view their definitions.

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.