Celestial coordinates are a reference system used to define the positions of objects on the celestial sphere. There are two main coordinate systems in use:- The equatorial coordinate system is by far the most common for astronomical observations, and is basically an extension of the latitude and longitude coordinate system used on Earth. By defining a right ascension, a declination and an epoch, every astronomical object is identified with a unique position on the sky.
- The horizontal coordinate system uses the observer's horizon as a reference, and measures an object's altitude (height above the horizon) and azimuth (angular distance from due north measured eastwards). Unlike the equatorial coordinate system, the position of each object depends on the location of the observer and the time of the observation. This coordinate system is commonly used in telescope design and construction.