The Celestial Equator
The Celestial Equator is an imaginary great circle on the celestial sphere. The celestial equator is the fundamental plane of the Equatorial Coordinate System, so it is defined as the locus of points with Declination of zero degrees. It is also the projection of the Earth's equator onto the sky.
The Celestial Equator and the Ecliptic are set at an angle of 23.5 degrees in the sky. The points where they intersect are the Vernal and Autumnal Equinoxes.