The celestial sphere is an imaginary sphere, an indeterminate distance from Earth, which intersects the straight lines from the celestial bodies to the observer.
It is then convenient to identify the directions of the celestial bodies with their position on the celestial sphere.
In the context of astronomy, in particular spherical astronomy, the observer is considered to be a reference point located at the center of the celestial sphere.
In practice, of course, an actual observer is physically located on the surface of Earth.
However, the celestial sphere is considered to be so much larger than the radius of Earth that the observer is considered to be at the center of Earth.
- Results about the celestial sphere can be found here.
The adjective celestial, in the context of astronomy, is derived from the Latin caelestis, from caelum (meaning sky or heaven).
The term has religious connotations, dating from the time where the prevailing belief was that supernatural beings lived somewhere out there in the sky, taking an interest in what happens on Earth.
- 1976: W.M. Smart: Textbook on Spherical Astronomy (6th ed.) ... (next): Chapter $\text I$. Spherical Trigonometry: $1$. Introduction.
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): celestial sphere
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): celestial sphere
- 2008: Ian Stewart: Taming the Infinite ... (previous) ... (next): Chapter $5$: Eternal Triangles: Astronomy