An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and midway between the poles. The Equator usually refers to the Earth's equator: 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. Other planets and astronomical bodies have equators similarly defined. Earth's equator is about long; 78.7% is across water and 21.3% is over land.
== Geodesy of the Earth's Equator ==
The latitude of the Earth's Equator is by definition 0° (zero degrees). The Equator is one of the five notable circles of latitude on Earth, with the others being 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. The imaginary circle obtained when the Earth's equator is projected onto the sky is called the celestial equator.
In its seasonal apparent movement across the sky the sun passes over the Equator twice each year, at the March and September equinoxes. At the moment of the equinox, light rays from the center of the Sun are perpendicular to the surface of the Earth at the point on the Equator experiencing solar noon.
Places on the Equator experience the quickest sunrises and sunsets since the sun rises and sets almost vertically throughout the year. The length of a day (sunrise to sunset) at the Equator is almost constant during the year; each day is about 14 minutes longer than night because of atmospheric refraction and the fact that sunrise/sunset is the moment when the ''edge'' of the Sun's disk passes the horizon, rather than its centre.
The Earth bulges slightly at the Equator. Its "average" diameter is , but at the Equator the diameter is about greater than at the poles.
Locations near the Equator are good sites for spaceports, such as the Guiana Space Centre in Kourou, French Guiana, as they are moving faster than other latitudes due to the Earth's rotation, and the added velocity reduces the fuel needed to launch spacecraft. As the Earth spins toward the east, spacecraft must launch to the east, southeast or northeast to take advantage of this.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』