The Celestial Sphere is an imaginary sphere that contains a smaller sphere called Earth. When we look from Earth towards the Celestial Sphere we see all the stars, planets and different space objects projected in the sphere (regardless of the actual distance).
This Celestial Sphere has its Equator, and a system of coordinates called Right Ascension (longitude) and Declination (latitude). Every space object has a RA and Dec. The stars are usually fixed in the Celestial Sphere (with slow variations with precession). The planets and the Sun coordinates are variable
Galaxy image obtained from Pixabay under CC0. No Copyright. The person who associated a work with this deed has dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law
The Celestial Sphere picture has been released into the public domain by its author, Offnfopt.