An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass. The capitalized term Equator refers to the Earth's equator.
In simpler language, the Equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that 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 0° (zero degrees). The length of Earth's equator is about . To calculate the actual length of the Equator would require taking into consideration that the Equator goes up and down various mountains and hills in South America, in Africa, and on various islands. 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.
The Sun, in its seasonal apparent movement across the sky, passes directly over the Equator twice each year, at 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. 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 , but at the Equator the diameter is approximately 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.
For high precision work, the Equator is not quite as fixed as the above discussion implies. The true equatorial plane must always be perpendicular to the Earth's spin axis. Although this axis is relatively stable, its position drifts about north-south over the course of a year. Thus, the true equator moves slightly, but this effect is only relevant in extremely precise geophysical measurements.
The surface of the Earth at the Equator lies mostly on three of the oceans: the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. The highest point on the Equator is at the elevation of , at , found on the southern slopes of Volcán Cayambe [summit ] in Ecuador. This place is a short distance above the snow line, and this immediate vicinity forms the only section of the Equator where snow lies on the ground year-round.
! Co-ordinates | ! Country, territory or sea | ! Notes |
Ilhéu das Rolas | ||
The Batu Islands, Sumatra and the Lingga Islands | ||
Borneo | ||
Kayoa and Halmahera islands | ||
Gebe Island | ||
Passing 24 km north of central Quito, near Mitad del Mundo | ||
Passing 4.3 km north of the border with Peru | ||
Despite its name, no part of Equatorial Guinea's territory lies on the Equator. However, its island of Annobón is south of the Equator, and the rest of the country lies to the north.
Certain navies, such as the Royal Navy and the US Navy, have a tradition of holding ceremonies on board ship to mark sailors' first crossing of the Equator. These rites of initiation have in the past been notorious for their brutality. Milder line-crossing ceremonies, typically featuring King Neptune, are also held for passengers' entertainment on some civilian ocean liners and cruise ships.
The geographical mile is defined as one arc minute of the Equator, and therefore has different values depending on which standard equator is used, namely or for respectively WGS-84 and IAU-2000, a difference of nearly a millimeter.
The earth is standardly modeled as a sphere flattened 0.336% along its axis. This makes the Equator 0.16% longer than a meridian (as a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimeter, , one arc minute of which is , explaining the SI standardization of the nautical mile as , more than short of the geographical mile.
Category:Lines of latitude Category:Celestial coordinate system Category:Tropics Category:Geodesy Category:Navigation Category:GPS
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