The horizon coordinate system is an astronomical coordinate system that is based on the observer’s local horizon. It is primarily used for observing celestial objects from a specific location on Earth, and the coordinates change as the observer’s position and time change.
In this system, the sky is divided into two main components:
- Altitude (or elevation): This is the angle of an object above the local horizon. It ranges from 0° (the horizon) to 90° (the zenith, which is directly overhead).
- Azimuth: This is the angle measured along the horizon, starting from the north, moving eastward. It ranges from 0° (due north) to 360°, covering the entire horizon in a circular manner.
The horizon coordinate system is altitude-azimuth (alt-az) and is relative to the observer’s local horizon and position. This means that the coordinates of celestial objects will differ based on the time of day and the observer’s location on Earth. For example, an object might have a different altitude and azimuth at different times or from different locations.
Key Points:
- The altitude measures how high an object is above the horizon (0° is at the horizon, 90° is directly overhead).
- The azimuth measures the direction of the object along the horizon, starting at north (0°), moving eastward (90° for east, 180° for south, 270° for west).
- The coordinates are observer-dependent, meaning they are unique to the observer’s location and the time at which the observation is made.
This system is helpful for practical observations, such as when trying to locate stars, planets, or other celestial objects in the sky from a specific location. However, it is not ideal for mapping the entire celestial sphere because the coordinates change depending on the observer’s position and the time of observation. For that reason, other coordinate systems like the equatorial coordinate system are more commonly used for mapping celestial objects universally.