An equator is the intersection of a sphere’s surface with the plane that is perpendicular to the sphere’s axis of rotation and midway between the poles. The Equator is normally referred to the Earth’s equator: an imaginary line on the Earth’s surface halfway between the North Pole and the South Pole, separating the Earth into the Northern Hemisphere and Southern Hemisphere. Other planets and astronomical bodies have equators that are similarly defined. The Equator is approximately 24,901 miles long; 78.7 percent of it is across water and 21.3 percent of it is over land.

The latitude of the Equator is, by definition, zero degrees. It is one of the five notable circles of latitude on the Earth, with the other being the two Polar Circles and the two Tropical Circles: the Tropic of Cancer and the Tropic of Capricorn. It’s the only line of latitude which is also a great circle. The imaginary circle acquired when the Earth’s equator is projected onto the sky is called the celestial equator.

Places on the equator have the quickest rate of sunrise and sunset because the sun rises and set almost exactly vertically throughout the year. These places on the equator have, in theory, almost exactly 12 hours of day and 12 hours of night throughout the year, varying by only seconds because of the effects of the Earth’s tilt and orbit.

Season change results from the yearly revolution of the Earth around the Sun and the tilt of the Earth’s axis relative to the plane of revolution. During the year, the northern and southern hemispheres are inclined towards or away from the sun according to the Earth’s location in its orbit. Near the equator, there is barely any distinction between summer, spring, autumn, or winter. The temperatures usually stay high all year-round, with the exception of high mountains in South America and in Africa.

In 1976, the IAU standardized the radius as 3,963 miles, later refined by the IUGG to 3,963 miles and adopted in WGS-84, though the more recent IAU-2000 has preserved the old IAU-1976 value. In either case, the length of the Equator is, by definition, 3.14 times 2 times the given standard, which to the nearest millimeter is 24,901.460897 miles in WGS-84 and 24,901.472609 miles in IAU-1976 and IAU-2000.

Lowlands around the Equator usually have a tropical rainforest climate, known also as an equatorial climate, though cold currents cause some regions to have tropical monsoon climates with a dry season in the middle of the year.

Image Caption: The nations of the equator and the prime meridian. Credit: NuclearVacuum/Wikipedia (CC BY-SA 3.0)