equator is the equator of the astronomical
sky. It is the projection of the earth's equator onto the celestial sphere
. If you are standing on earths equator and look directly overhead you are looking at the celestial equator.
The celestial equator is zero degrees of declination
in the sky. As you go to the north the declination
increases to 90 degree's which is directly overhead at the north pole. As you go south the declination
decreases to -90 which is directly overhead at the south pole.
This system of declination starting at the celestail equator is the one which most current day astronomers use to chart stars in the sky.
NOTE: While all stars have fixed relations to the celestial equator, the suns position realitive to the celestial equator changes over the course of a year due to the 23.5 degree tilt of the earths axis realitive to its orbit.