Equator
The Equator usually refers to an imaginary line on the Earth's surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere. The Equator is about 40,075 kilometres (24,901 mi) long; some 78.7% lies across water and 21.3% over land.
Other planets and astronomical bodies have equators similarly defined. Generally, an equator is the intersection of the surface of a rotating sphere with the plane that is perpendicular to the sphere's axis of rotation and midway between its poles.
The latitude of the Earth's equator is by definition 0° (zero degrees) of arc. The Equator is one of the five notable circles of latitude on Earth; the other four are the two Polar Circles: the Arctic Circle and the Antarctic Circle; and the two Tropical Circles: the Tropic of Cancer and the Tropic of Capricorn. The equator is the only line of latitude which is also a great circle — that is, one whose plane passes through the center of the globe. The plane of Earth's equator when projected outwards to the celestial sphere defines the celestial equator.
In the cycle of Earth's seasons, the plane of the equator passes through the Sun twice per year: at the March and September equinoxes. To an observer on the Earth, the Sun appears to travel North or South over the equator (or the celestial equator) at these times. Light rays from the center of the Sun are perpendicular to the surface of the Earth at the point of solar noon on the Equator.
...
Wikipedia