What is the celestial equator?

The celestial sphere is an imaginary sphere projected out into space, with the observer at its center. It represents the entire sky, and all celestial objects other than Earth are imagined as being located on its inside surface. If Earth’s axis were extended, the points where it would intersect the celestial sphere are called the celestial poles, being directly above Earth’s poles. The circle in the sphere halfway between the celestial poles is called the celestial equator. It can be thought of as Earth’s equator projected onto the celestial sphere. Astronomers use the equatorial coordinate system for indicating the positions of stars and other heavenly objects on the celestial sphere.