The equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
The latitude of the equator is, by definition, 0°. The length of Earth's equator is about 40,075.0 km, or 24,901.5 miles. The exact length is 40,075,016.6856 m in WGS-84, and 40,075,035.5351 m in IAU-2000.
The equator is one of the five main circles of latitude that are based on the relationship between the Earth's axis of rotation and the plane of the Earth's orbit around the sun. It is the only line of latitude which is also a great circle. The imaginary circle obtained when the Earth's equator is projected onto the heavens is called the celestial equator.
The Sun, in its seasonal movement through the sky, passes directly over the equator twice each year, on the March and September equinoxes. At the equator, the rays of the sun are perpendicular to the surface of the earth on these dates.
Places on the equator experience the quickest rates of sunrise and sunset in the world. Such places also have a constant 12 hours of day and night throughout the year, while north or south of the equator day. Locations near the equator are good sites for spaceports (e.g., Guiana Space Centre in Kourou, French Guiana), as they are already moving faster than any other point on the Earth due to the Earth's rotation, and the added velocity reduces the amount of fuel needed to launch spacecraft.