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The equator is a circle of latitude, about in circumference, that divides Earth into the
Northern Northern may refer to the following: Geography * North, a point in direction * Northern Europe, the northern part or region of Europe * Northern Highland, a region of Wisconsin, United States * Northern Province, Sri Lanka * Northern Range, a ra ...
and
Southern Southern may refer to: Businesses * China Southern Airlines, airline based in Guangzhou, China * Southern Airways, defunct US airline * Southern Air, air cargo transportation company based in Norwalk, Connecticut, US * Southern Airways Express, M ...
hemispheres. It is an
imaginary line In general, an imaginary line is usually any sort of geometric line that has only an abstract definition and does not physically exist. In fact, they are used to properly identify places on a map. Some outside geography do exist, such as th ...
located at 0 degrees latitude, halfway between the North and
South South is one of the cardinal directions or Points of the compass, compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pro ...
poles. The term can also be used for any other celestial body that is roughly spherical. In spatial (3D) geometry, as applied in astronomy, the equator of a rotating spheroid (such as a planet) is the parallel (circle of latitude) at which latitude is defined to be 0°. It is an
imaginary line In general, an imaginary line is usually any sort of geometric line that has only an abstract definition and does not physically exist. In fact, they are used to properly identify places on a map. Some outside geography do exist, such as th ...
on the spheroid, equidistant from its
poles Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, who share a common history, culture, the Polish language and are identified with the country of Poland in Ce ...
, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid with the
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * Planes (gen ...
perpendicular to its axis of
rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
and midway between its
geographical pole A geographical pole or geographic pole is either of the two points on Earth where its axis of rotation intersects its surface. The North Pole lies in the Arctic Ocean while the South Pole is in Antarctica. North and South poles are also define ...
s. On and near the equator (on Earth), noontime
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
appears almost directly overhead (no more than about 23° from the zenith) every day, year-round. Consequently, the equator has a rather stable daytime temperature throughout the year. On the equinoxes (approximately March 20 and September 23) the
subsolar point The subsolar point on a planet is the point at which its sun is perceived to be directly overhead (at the zenith); that is, where the sun's rays strike the planet exactly perpendicular to its surface. It can also mean the point closest to the sun ...
crosses Earth's equator at a shallow angle, sunlight shines perpendicular to Earth's axis of rotation, and all latitudes have nearly a 12-hour day and 12-hour night.


Etymology

The name is derived from medieval Latin word , in the phrase , meaning 'circle equalizing day and night', from the Latin word 'make equal'.


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 A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth (ignoring elevation) at a given latitude coordinate line. Circles of latitude are often called parallels because ...
on Earth; the other four are the two polar circles (the
Arctic Circle The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle. The Arctic Circle marks the southernmost latitude at w ...
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 In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Any arc of a great circle is a geodesic of the sphere, so that great circles in spherical geomet ...
—meaning, one whose
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * Planes (gen ...
passes through the center of the globe. The plane of Earth's equator, when projected outwards to the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of 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
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon due to Earth's rotation. As viewed from everywhere on Earth (except the North and South poles), the equinox Sun sets due west at the moment of both the spring ...
s because the Sun's daily path is nearly perpendicular to the
horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
for most of the year. The length of
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
(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 , but the diameter at the equator is about 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, /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.


Geodesy


Precise location

The precise location of the equator is not truly fixed; the true equatorial plane is perpendicular to the Earth's rotation axis, which drifts about during a year. Geological samples show that the equator significantly changed positions between 48 and 12 million years ago, as sediment deposited by ocean thermal currents at the equator shifted. The deposits by thermal currents are determined by the axis of the Earth, which determines solar coverage of the Earth's surface. Changes in the Earth's axis can also be observed in the geographic layout of volcanic island chains, which are created by shifting hot spots under the Earth's crust as the axis and crust move. This is consistent with the Indian tectonic plate colliding with the Eurasian tectonic plate, which is causing the Himalayan uplift.


Exact length

The International Association of Geodesy (IAG) and the International Astronomical Union (IAU) use an equatorial radius of (codified as the IAU 2009 value). This equatorial radius is also in the 2003 and 2010 IERS Conventions. It is also the equatorial radius used for the IERS 2003 ellipsoid. If it were really circular, the length of the equator would then be exactly 2π times the radius, namely . The GRS 80 (Geodetic Reference System 1980) as approved and adopted by the IUGG at its Canberra, Australia meeting of 1979 has an equatorial radius of . The WGS 84 (World Geodetic System 1984) which is a standard for use in cartography, geodesy, and satellite navigation including
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
, also has an equatorial radius of . For both GRS 80 and WGS 84, this results in a length for the equator of . The
geographical mile The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the international ellipsoid 1924 this equalled 1855.4 metres. ''The American Practical Navigator'' 2017 defines the geographical mile as . Gre ...
is defined as one arc-minute of the equator, so it has different values depending on which radius is assumed. For example, by WSG-84, the distance is , while by IAU-2000, it is . This is a difference of less than over the total distance (approximately ). The earth is commonly modeled as a sphere flattened 0.336% along its axis. This makes the equator 0.16% longer than a
meridian Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon * ...
(a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimetre, , one arc-minute of which is , explaining the SI standardization of the
nautical mile A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude. Today ...
as , more than less than the
geographical mile The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the international ellipsoid 1924 this equalled 1855.4 metres. ''The American Practical Navigator'' 2017 defines the geographical mile as . Gre ...
. The sea-level surface of the Earth (the geoid) is irregular, so the actual length of the equator is not 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° West to 169° East to be greater than its diameter ninety degrees away.


Equatorial countries and territories

The equator passes through the land of 11
countries A country is a distinct part of the world, such as a state (polity), state, nation, or other polity, political entity. It may be a sovereign state or make up one part of a larger state. For example, the country of Japan is an independent, so ...
. Indonesia is the country straddling the greatest length of the equatorial line across both land and sea. Starting at the prime meridian and heading eastwards, the equator passes through: The equator also passes through the territorial seas of three countries: Maldives (south of
Gaafu Dhaalu Atoll Gaafu Dhaalu (also known as Southern Huvadhu Atoll or Huvadhu Atholhu Dhekunuburi, ހުވަދުއަތޮޅު ދެކުނުބުރި) is an administrative district of the Maldives formed by the southwestern section of Huvadhu Atoll. It was created o ...
), Kiribati (south of Buariki Island), and the United States (south of Baker Island). Despite its name, no part of
Equatorial Guinea Equatorial Guinea ( es, Guinea Ecuatorial; french: Guinée équatoriale; pt, Guiné Equatorial), officially the Republic of Equatorial Guinea ( es, link=no, República de Guinea Ecuatorial, french: link=no, République de Guinée équatoria ...
lies on the Equator. However, its island of
Annobón Annobón ( es, Provincia de Annobón; pt, Ano-Bom), and formerly as ''Anno Bom'' and ''Annabona'', is a province (smallest province in both area and population) of Equatorial Guinea consisting of the island of Annobón, formerly also Pigalu a ...
is south of the equator, and the rest of the country lies to the north. France, Norway ( Bouvet Island), and the United Kingdom are the other three
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
-based countries which have territories in the Southern Hemisphere.


Equatorial seasons and climate

Seasons result from the tilt of Earth's axis away from a line perpendicular to the plane of its revolution around the Sun. Throughout the year, the Northern and Southern hemispheres are alternately turned either toward or away from the Sun, depending on Earth's position in its orbit. The hemisphere turned toward the Sun receives more sunlight and is in summer, while the other hemisphere receives less sun and is in winter (see solstice). At the equinoxes, Earth's axis is perpendicular to the Sun rather than tilted toward or away, meaning that day and night are both about 12 hours long across the whole of Earth. Near the equator, this means the variation in the strength of solar radiation is different relative to the time of year than it is at higher latitudes: maximum solar radiation is received during the equinoxes, when a place at the equator is under the
subsolar point The subsolar point on a planet is the point at which its sun is perceived to be directly overhead (at the zenith); that is, where the sun's rays strike the planet exactly perpendicular to its surface. It can also mean the point closest to the sun ...
at high noon, and the intermediate seasons of spring and autumn occur at higher latitudes; and the minimum occurs during ''both'' solstices, when either pole is tilted towards or away from the sun, resulting in either summer or winter in both hemispheres. This also results in a corresponding movement of the equator away from the subsolar point, which is then situated over or near the relevant
tropic The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
circle. Nevertheless, temperatures are high year-round due to the Earth's axial tilt of 23.5° not being enough to create a low minimum midday
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
to sufficiently weaken the Sun's rays even during the solstices. High year-round temperatures extend to about 25° north or south of the equator, although the moderate seasonal temperature difference is defined by the opposing solstices (as it is at higher latitudes) near the poleward limits of this range. Near the equator, there is little temperature change throughout the year, though there may be dramatic differences in rainfall and humidity. The terms summer, autumn, winter and spring do not generally apply. Lowlands around the equator generally have a tropical rainforest climate, also known as an equatorial climate, though cold ocean currents cause some regions to have tropical monsoon climates with a dry season in the middle of the year, and the Somali Current generated by the Asian monsoon due to continental heating via the high Tibetan Plateau causes
Greater Somalia Greater Somalia ( so, Soomaaliweyn, ar, الصومال الكبرى ''As-Sūmal al-Kubra'') is a concept to unite all ethnic Somalis comprising the regions in or near the Horn of Africa in which ethnic Somalis live and have historically inhabited ...
to have an arid climate despite its equatorial location. Average annual temperatures in equatorial lowlands are around during the afternoon and around sunrise. Rainfall is very high away from cold ocean current upwelling zones, from per year. There are about 200 rainy days per year and average annual sunshine hours are around 2,000. Despite high year-round sea level temperatures, some higher altitudes such as the Andes and
Mount Kilimanjaro Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world: above sea level and ab ...
have glaciers. The highest point on the equator is at the elevation of , at , found on the southern slopes of Volcán Cayambe ummit in Ecuador. This is slightly above the
snow line The climatic snow line is the boundary between a snow-covered and snow-free surface. The actual snow line may adjust seasonally, and be either significantly higher in elevation, or lower. The permanent snow line is the level above which snow wil ...
and is the only place on the equator where snow lies on the ground. At the equator, the snow line is around ''lower'' than on Mount Everest and as much as lower than the highest snow line in the world, near the Tropic of Capricorn on Llullaillaco.


Line-crossing ceremonies

There is a widespread maritime tradition of holding ceremonies to mark a sailor's first crossing of the equator. In the past, these ceremonies have been notorious for their brutality, especially in naval practice. Milder line-crossing ceremonies, typically featuring King Neptune, are also held for passengers' entertainment on some civilian ocean liners and cruise ships.


See also

*
1st parallel north The 1st parallel north is a circle of latitude that is 1 degree (69.2 miles/111.36 kilometers) north of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, the Pacific Ocean and South America. ...
*
1st parallel south The 1st parallel south is a circle of latitude that is 1 degree (69.2 miles/111.36 kilometers) south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, Australasia, the Pacific Ocean and S ...
* Bogota Declaration *
Coriolis force In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
*
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
* Planetary equator * Prime meridian * Thermal equator


References


Sources

* (IUGG/WGS-84 data) * (IAU data) {{Authority control Astronomical coordinate systems Circles of latitude Geodesy Tropics