quality:correction
Overview

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 —meaning, 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 equatorial plane runs through the Sun twice a year: on the equinoxes in March and September. To a person on Earth, the Sun appears to travel along the equator (or along the celestial equator) at these times.

Locations on the equator experience the shortest sunrises and sunsets because the Sun's daily path is nearly perpendicular to the horizon for most of the year. The length of daylight (sunrise to sunset) is almost constant throughout the year; it is about 14 minutes longer than nighttime due to atmospheric refraction and the fact that sunrise begins (or sunset ends) as the upper limb, not the center, of the Sun's disk contacts the horizon.

Earth bulges slightly at the Equator; its average diameter is 12,742 km (7,918 mi), but the diameter at the equator is about 43 km (27 mi) greater than at the poles.

Sites near the Equator, such as the Guiana Space Centre in Kourou, French Guiana, are good locations for spaceports as they have the fastest rotational speed of any latitude, 460 m (1,509 ft) /sec. The added velocity reduces the fuel needed to launch spacecraft eastward (in the direction of Earth's rotation) to orbit, while simultaneously avoiding costly maneuvers to flatten inclination during missions such as the Apollo Moon landings.