Equator

Equator

The latitude of the Equator is by definition 0x. 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 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 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.
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. Spacecraft must launch to the east, southeast, or northeast to take advantage of this.
Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. During the year the northern and southern hemispheres are inclined toward or away from the sun according to Earth's position in its orbit. The hemisphere inclined toward the sun receives more sunlight and is in summer, while the other hemisphere receives less sun and is in winter. At the equinox the Earth's axis isn't tilted toward the sun and the day is about 12 hours long over most of the Earth.
Near the Equator there is little distinction between summer, winter, autumn, or spring. The temperatures are usually high year-roundâ€"with the exception of high mountains in South America and in Africa. The temperature at the Equator can plummet during rainstorms. In many tropical regions people identify two seasons: the wet season and the dry season, but many places close to the Equator are on the oceans or rainy throughout the year. The seasons can vary depending on elevation and proximity to an ocean.
Certain navies, such as the Royal Navy and the US Navy, have a tradition of holding ceremonies on board ship to mark sailors' 1st 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 sea-level surface of the Earth is irregular, so the actual length of the Equator isn't so easy to determine. Aviation Week and Space Technology on 9 October 1961 reported that measurements using the Transit IV-A satellite had shown the equatorial "diameter" from longitude 11 deg West to 169 East to be 1000 feet greater than its "diameter" ninety degrees away.

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