This section provides some basic knowledge of the celestial sphere.
The celestial sphere is an imaginary sphere of infinite radius centered on the Earth,
on which all celestial bodies are assumed to be projected. Note that:
The celestial sphere is an Earth-centered model of our universe.
The celestial sphere is a 2-dimensional model where the 3-dimensional universe is projected to the surface of an imaginary
Two coordinate systems are used for the surface of the celestial sphere: a fixed coordinate system and a relative coordinate
The following picture provides a good illustration of the celestial sphere, http://www.pfm.howard.edu:
The celestial sphere can be viewed as the sphere ceiling of the Earth where stars are located. Of course,
this is not true, our view of stars from the Earth is a projection of stars located in a 3-dimensional space: