Equator is a geographic and cartographic term for an imaginary east-west circle on Earth’s surface, everywhere equidistant from the NORTH POLE and the SOUTH POLE, and dividing Earth into regions defined as the Northern Hemisphere and the Southern Hemisphere. The largest of all such imaginary east-west lines, it is that from which terrestrial latitudes are measured and is thus considered the prime paralel or zero latitude. The equator, as it appears on a map or a GLOBE, intersects northern South America, central Africa, Indonesia, and stretches of the Pacific, Atlantic, and Indian Oceans.
Celestial equator (also equinoctial circle or equinoctial line) is an astronomical term for the circle on the celestial sphere, lying midway between the celestial poles in the same plane as the geographic equator. (The celestial sphere is the imaginary sphere of infinite radius used for defining the positions of celestial bodies, with Earth considered the center of the sphere.) When the Sun’s path intersects the celestial equator, twice a year, on March 21, the vernal equinox, and September 23, the autumnal equinox, day and night are of equal length. The equatorial coordinate system is the most commonly used astronomical coordinate system for indicating the positions of celestial bodies, thus relating to navigation.
The equator, the TROPIC OF CANCER, the northern limit of the Sun’s apparent annual path across the sky, and the TROPIC OF CAPRICORN, the southern limit of the Sun’s apparent path, were the first three reference lines on maps. Ancient cartographers had conceived of this system, based on the Sun’s motion, some time before the third century B.C. Greek astronomer and geographer ERATOSTHENES used it to help determine Earth’s circumference and tilt with a high degree of accuracy.