Celestial sphere is defined as an imaginary sphere that has huge radius and it is concentric to Earth which means Earth lies at the centre of sphere. All the objects in sky are considered to be projected on the inner surface of this sphere such as stars and other objects

Celestial sphere is imaginary plane which is extended to the stars. It is an extension of physical surface of the Earth and Earth’s horizon in the celestial sphere. It is imagined in a way that it blocks the view of the observer so that he cannot see the stars beneath himself and it is drawn in such a way that is seems to appear as the extension of the Earth.

Therefore, the horizon shown is imaginary plane and it extends from the observing location of observer on Earth out to the stars.

Conclusion:

Thus, the horizon shown is imaginary plane that extends from observing location on Earth out to the stars.