equator, imaginary great circle around the earth, everywhere equidistant from the two geographical poles and forming the base line from which latitude is reckoned. The equator, which measures c.24,902 mi (40,076 km), is designated as lat. 0°. It intersects N South America, central Africa, and Indonesia. The celestial equator is the projection of the plane of the earth's equator on the celestial sphere (see equatorial coordinate system).