Introduction
All celestial bodies, except the Sun and the Moon, are so far away that simple observation of the sky cannot actually discern differences in the distance between the Earth and them. In fact, they all look like small, bright dots of light. However, we still need a way to map the sky and describe the positions of celestial bodies.
The Celestial Sphere
To achieve this, we introduce the concept of the celestial sphere.
Definition: The Celestial Sphere
The celestial sphere is an imaginary sphere of arbitrary radius whose center is at the center of the Earth.
We imagine that all celestial bodies are positioned on the surface of this sphere and then use coordinates to describe their location, ignoring the distance between us and them. Obviously, the same celestial body will have a different location, depending on where on Earth you are. In fact, two observers might not even be able to see the same object.