Equator - Geodesy of The Equator

Geodesy of The Equator

The latitude of the Equator is 0° (zero degrees). The Equator is one of the five notable circles of latitude on Earth, with the others being the two Polar Circles 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 directly 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 rates of sunrise and sunset in the world since the sun rises and sets nearly vertically (exactly vertically at the equinoxes). They are also the only places in the world where the sun can go directly from the zenith to the nadir and from the nadir to the zenith. Such places also have a theoretical constant 12 hours of day and night throughout the year, though in practice there are variations of a few minutes due to the effects of atmospheric refraction and because sunrise and sunset are measured from the time that the edge of the Sun's disk is on the horizon, rather than the center of the disk.

The Earth bulges slightly at the Equator. It has an average diameter of 12,750 kilometres (7,922 mi), but at the Equator the diameter is approximately 43 kilometres (27 mi) greater than the polar diameter.

Locations near the Equator are theoretically good sites for spaceports, such as the 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. Spacecraft launched in this manner must launch to the east, southeast, or northeast to take advantage of this effect.