Equator

Line across the Earth
An equator of a rotating spheroid (such as a planet) is its zeroth circle of latitude (parallel). It is the imaginary line on the spheroid's surface, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid's surface with the plane perpendicular to its axis of rotation and midway between its poles.

On Earth, the Equator is about 40,075 kilometres (24,901 mi) long, of which 78.7% lies across water and 21.3% over land. Indonesia is the country with the longest equatorial line lay across the water and land.

The name is derived from medieval Latin aequator, in the phrase circulus aequator diei et noctis ‘circle equalizing day and night’, from Latin aequare ‘make equal’.

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 — that is, 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 plane of the equator passes through the Sun twice per year: at the March and September equinoxes. To a person on the Earth, the Sun appears to travel North or South over the Equator (or the celestial equator) at these times. Light rays from the center of the Sun are perpendicular to the surface of the Earth at the point of solar noon on the Equator.

Locations on the Equator experience the quickest sunrises and sunsets, because the sun moves 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 fourteen minutes longer than nighttime due to atmospheric refraction and the fact that sunrise is begun (or sunset is ended) as the upper edge, not the center, of the Sun's disk crosses the horizon.

This page was last edited on 6 June 2018, at 17:57 (UTC).
Reference: https://en.wikipedia.org/wiki/Equator under CC BY-SA license.

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