We use coordinate systems to identify the position of a point on a surface. When we talk about coordinates, we usually think of geographical coordinates on the surface of the Earth. However, we can use similar systems to locate objects in the night sky. We call these celestial coordinates. When we think about the night sky in this way, we imagine it as a spherical surface around the Earth.

To locate a point on Earth you need two coordinates: latitude and longitude. Unlike places on Earth, objects in the night sky appear to move over time. This is because the Earth is rotating on its axis and orbiting the Sun. So celestial coordinate systems must take this movement into account.

There are two main coordinate systems used in astronomy: Altitude-Azimuth and Right Ascension and Declination.