Astronomical Coordinate Systems
Mapping locations on Earth is easy because, except for an occasional earthquake and the slow glide of the tectonic plates, things stay put on Earth’s surface. But in the sky, things are moving, and of course we are constantly moving with respect to the sky. Therefore, coordinate systems for locating objects in the sky have to take all this movement into account. Several systems have been devised to describe positions of celestial objects relative to Earth. The choice of which one to use depends on what you are observing and how.