The
equator is the intersection of the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's surface with the
planeIn mathematics, a plane is a flat surface. Planes can arise as subspaces of some higher dimensional space, as with the walls of a room, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry....
perpendicular to the Earth's
axis of rotationA rotation is a movement of an object in a circular motion. A two-dimensional object rotates around a center of rotation. A three-dimensional object rotates around a line called an axis. If the axis of rotation is within the body, the body is said to rotate upon itself, or spin—which implies...
and containing the Earth's
center of massThe center of mass of a system of particles is a specific point where, for many purposes, the system behaves as if its mass were concentrated there. The center of mass is a function only of the positions and masses of the particles that compose the system...
. In simpler language, it is an imaginary line on the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's surface equidistant from the
North PoleThe North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets the Earth's surface...
and
South PoleThe South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects the surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
that divides the Earth into a
Northern HemisphereThe Northern Hemisphere is the half of a planet that is north of the equator—the word hemisphere literally means 'half sphere'. It is also that half of the celestial sphere north of the celestial equator...
and a
Southern HemisphereThe Southern Hemisphere is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'...
. The equators of other
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s and astronomical bodies are defined analogously.
Geodesy of the equator
The
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
of the equator is 0°. The length of
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's equator is about .
The equator is one of the five main
circles of latitudeA circle of latitude, on the Earth, is an imaginary east-west circle connecting all locations that share a given latitude. A location's position along a circle of latitude is given by its longitude....
that are based on the relationship between the Earth's axis of
rotationA rotation is a movement of an object in a circular motion. A two-dimensional object rotates around a center of rotation. A three-dimensional object rotates around a line called an axis. If the axis of rotation is within the body, the body is said to rotate upon itself, or spin—which implies...
and the plane of the Earth's
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
around the sun. It is the only line of latitude which is also a
great circleA great circle of a sphere is a circle that runs along the surface of that sphere so as to cut it into two equal halves, as distinct from a small circle. The great circle therefore has both the same circumference and the same center as the sphere...
. The imaginary circle obtained when the Earth's equator is projected onto the sky is called the
celestial equatorThe celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...
.
The
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
in its
seasonA season is a division of the year, marked by changes in weather.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...
al movement through the sky, passes directly over the equator twice each
yearA year is the amount of time it takes the Earth to make one revolution around the Sun...
, on the March and September
equinoxAn equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator...
es. At the equator, the rays of the sun are
perpendicularIn geometry, two lines or planes , are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective. Thus, referring to Figure 1, the line AB is the perpendicular to CD through the point B...
to the surface of the earth on these dates.
Places on the equator experience the quickest rates of
sunriseSunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...
and
sunsetSunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".In astronomy the time of sunset is...
in the world. They are also the only places in the world where the sun can go directly from the
zenithIn general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there...
to the
nadirThe nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms...
and from the
nadirThe nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms...
to the
zenithIn general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there...
. Such places also have a theoretical constant 12 hours of day and night throughout the year (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 the edge of the Sun's disc is on the
horizonThe horizon is the apparent line that separates earth from sky.It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not...
, rather than its centre). North or south of the equator day length increasingly varies with latitude and the seasons.
The Earth
bulges slightly at the equatorAn equatorial bulge is a bulge which a planet may have around its equator, distorting it into an oblate spheroid. The Earth has an equatorial bulge of 42.72 km due to its rotation: its diameter measured across the equatorial plane is 42.72 km more than that measured between the poles...
. It has an average diameter of , but at the equator the diameter is approximately greater than the polar diameter.
Locations near the equator are good sites for
spaceportA spaceport, astrodrome or cosmodrome is a site for launching spacecraft, by analogy with seaport for ships or airport for aircraft. The word spaceport, and even more so cosmodrome, has traditionally been used for sites capable of launching spacecraft into orbit around Earth or on interplanetary...
s, such as the Guiana Space Centre in
KourouKourou is a town and commune in French Guiana, an overseas région and département of France located in South America.Kourou is the location of the Guiana Space Centre, France and ESA's main spaceport.-Geography:...
,
French GuianaFrench Guiana is an overseas department of France, located on the northern coast of South America. It has borders with two nations, Brazil and Suriname. Like the other DOMs, French Guiana is also an overseas region of France, one of the 26 regions of France. Its currency is the euro...
, 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 to use 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 wanders in approximately a radius circular motion each year. Thus, the true equator moves slightly. This, however, is only important for detailed scientific studies. The effect is quite small, and the width of a line marking the equator on almost any map will be much wider than the error.
Equatorial seasons and climate
Near the equator there is little distinction between summer, winter, autumn or spring. Temperatures are high year round (permanent "summer"), with the exception of periods during the wet season and at higher altitudes. In many tropical regions people identify two seasons: wet and dry. However, most places close to the equator are wet throughout the year, and seasons can vary depending on a variety of factors including elevation and proximity to an ocean. The rainy and humid conditions mean that the equatorial climate is not the hottest in the world.
The surface of the Earth at the equator is mostly ocean. The highest point on the equator is , at 00°00′00″S, 77°59′31″W, on the south slopes of Volcán Cayambe (summit ) in Ecuador. This is a short distance above the snow line, and this point and its immediate vicinity form the only section of the equator where snow lies on the ground.
Equatorial countries and territories
The equator traverses the land and/or territorial waters of 14
countriesIn geography, a country is a geographical region. The term is often applied to a political division or the territory of a state, or to a smaller, or former, political division of a geographical region...
. Starting at the
Prime MeridianThe Prime Meridian is the meridian at which longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.Unlike the parallels of...
and heading eastwards, the equator passes through:
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Atlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
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Gulf of GuineaThe Gulf of Guinea is the part of the Atlantic Ocean southwest of Africa. The intersection of the Equator and Prime Meridian is in the gulf...
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Ilhéu das Rolas Ilhéu das Rolas is a small island in São Tomé and Príncipe, in Africa. The island lies on the equator, off the southern tip of São Tomé Island. It is in the Caué District of São Tomé Province. The island is home to a small resort.-External links:****...
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Atlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
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Gulf of GuineaThe Gulf of Guinea is the part of the Atlantic Ocean southwest of Africa. The intersection of the Equator and Prime Meridian is in the gulf...
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Lake Victoria Lake Victoria or Victoria Nyanza is one of the African Great Lakes. The lake was named after the United Kingdom's Queen Victoria, by John Hanning Speke, the first European to see the lake....
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Indian OceanThe Indian Ocean is the third largest of the world's oceanic divisions, covering about 20% of the water on the Earth's surface. It is bounded on the north by South Asia ; on the west by Africa; on the east by Indochina, the Sunda Islands, and Australia; and on the south by the Southern Ocean...
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Passing between Gaafu Dhaalu Southern Huvadhu Atoll or Gaafu Dhaalu is an administrative district of the Maldives formed by the southwestern section of Huvadhu Atoll. It was created on February 8, 1962 when Huvadhu Atoll was divided into two districts...
and GnaviyaniGnaviyani Atoll is one of the administrative divisions of the Maldives. It corresponds to the island known as Fuvammulah in the local language and Fua Mulaku in the International Navigational Charts and by H. C. P. Bell, the first person who made serious research on the Maldives...
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The Batu Islands The Batu Islands are an archipelago of Indonesia located in the Indian Ocean off the west coast of Sumatra, between Nias and Siberut. The three primary islands, of approximately equal size, are Pini, Tanahmasa, and Tanahbala. There are forty-eight smaller islands, of which less than half are...
, SumatraSumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world .-Etymology:Sumatra was known in ancient times by the Sanskrit...
and the Lingga IslandsNot to be confused with "Linga", a common Scottish island name, see Linga The Lingga Islands or Lingga Archipelago are a group of islands in Indonesia, located south of Singapore, along both sides of the equator, off the eastern coast of Riau Islands province on Sumatra island...
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Karimata Strait The Karimata Strait is the wide strait that connects the South China Sea to the Java Sea, between the islands of Sumatra and Borneo in Indonesia....
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BorneoBorneo is the third largest island in the world and is located at the centre of Maritime Southeast Asia. Administratively, this island is divided among Indonesia , Malaysia and Brunei . Indonesians refer to the island as Kalimantan...
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Makassar StraitMakassar Strait is a strait between the islands of Borneo and Sulawesi in Indonesia. To the north it joins the Celebes Sea, while to the south it meets the Java Sea.The Mahakam River of Borneo empties into the strait....
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SulawesiSulawesi is one of the four larger Sunda Islands of Indonesia and is situated between Borneo and the Maluku Islands.- Etymology :...
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Gulf of Tomini The Gulf of Tomini is a gulf near the island of Sulawesi, Indonesia.It is bounded on its north and west sides by the Minahassa Peninsula and on the south side by the East Peninsula, Sulawesi....
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Molucca Sea The Molucca Sea is located in the western Pacific Ocean, within the country of Indonesia. The region is rich in coral and has many diving sites.-Location:...
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Kayoa and HalmaheraHalmahera is the largest island in the Maluku Islands. It is part of the North Maluku province of Indonesia.Halmahera has a land area of 17,780 km² and a population in 1995 of 162,728...
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Halmahera Sea Halmahera Sea is a regional sea located in the central eastern part of the Australasian Mediterranean Sea. It is centered at about 1°S and 129°E and is bordered by the Pacific Ocean to the north, Halmahera to the west, Waigeo and Irian Jaya to the east, and the Seram Sea to the south...
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Pacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. Its name is derived from the Latin name Tepre Pacificum, "peaceful sea", bestowed upon it by the Portuguese explorer Ferdinand Magellan. It extends from the Arctic in the north to Antarctica in the south, bounded by Asia and...
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Passing just north of WaigeoWaigeo is an island in West Papua province of eastern Indonesia. The island is also known as Amberi, or Waigiu.It is the largest of the four main islands in the Raja Ampat Islands archipelago, between Halmahera and about 65 km to the northwest coast of New Guinea...
island,
> Passing just south of AranukaAranuka is an atoll of Kiribati, located just north of the equator, in the Gilbert Islands...
atoll,
> Passing just south of Baker IslandBaker Island is an uninhabited atoll located just north of the equator in the central Pacific Ocean about ) southwest of Honolulu. The island lies almost halfway between Hawaii and Australia, and is a possession of the United States. Its nearest neighbor is Howland Island, to the north.Baker is...
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Isabela IslandIsabela Island is the largest island of the Galápagos with an area of 4,640 km², and length of 62 miles nearly 4 times larger than Santa Cruz, the next largest of the islands. This island was named in honor of Queen Isabella of Spain, who sponsored the voyage of Columbus....
in the Galápagos IslandsThe Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, 972 km west of continental Ecuador...
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Pacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. Its name is derived from the Latin name Tepre Pacificum, "peaceful sea", bestowed upon it by the Portuguese explorer Ferdinand Magellan. It extends from the Arctic in the north to Antarctica in the south, bounded by Asia and...
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Including some islands in the mouth of the Amazon RiverThe Amazon River of South America is the largest river in the world by volume, with a total river flow greater than the next eight largest rivers combined. The Amazon, which has the largest drainage basin in the world, accounts for approximately one-fifth of the world's total river flow. During...
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Atlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
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Despite its name, no part of
Equatorial GuineaEquatorial Guinea, officially the Republic of Equatorial Guinea is a country located in Central Africa. With an area of 28,000 km
2 it is one of the smallest countries in continental Africa. It has a population estimated at half a million...
's territory lies on the equator. However, its island of
AnnobónAnnobón , also known as Pagalu or Pigalu, is an island of Equatorial Guinea. It is located in the South Atlantic Ocean at , about 220 miles off the west coast of Africa and south west of São Tomé Island...
is about south of the equator, and the rest of the country lies to the north. The country that comes closest to the equator without actually touching it is
PeruPeru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean.Peruvian territory was home to the Norte Chico...
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"Crossing the Line"
The English-speaking seafaring tradition maintains that all sailors who cross the equator during a nautical voyage must undergo rites of passage and elaborate rituals initiating them into The Solemn Mysteries of the Ancient Order of the Deep. Those who have never "crossed the line" are derisively referred to as "pollywogs" or simply "slimy wogs". Upon entering the domain of His Royal Majesty, Neptunus Rex, all wogs are subject to various initiation rituals performed by those members of the crew who have made the journey before. Upon completion of the initiation ceremony, the wogs are then known as "trusty Shellbacks". If the crossing of the equator is done at the
180th meridianThe 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian passing through the Royal Observatory, Greenwich. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes...
, the title of "Golden Shellback" is conferred, recognizing the simultaneous entry into the realm of the Golden Dragon. If the crossing occurs at the Greenwich or
Prime MeridianThe Prime Meridian is the meridian at which longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.Unlike the parallels of...
, the sailor is considered to be an "Emerald Shellback".
Exact length of the equator
The equator is modeled exactly in two widely used standards as a circle of radius an integer number of meters. In 1976 the
IAUThe International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
standardized this radius as , subsequently refined by the
IUGGThe International Union of Geodesy and Geophysics, or IUGG, is a non-governmental organisation dedicated to the scientific study of the Earth using geophysical and geodesic techniques. The IUGG was established in 1919. Some areas within its scope are environmental preservation, reduction of the...
to and adopted in
WGS-84The World Geodetic System is a standard for use in cartography, geodesy, and navigation. It comprises a standard coordinate frame for the Earth, a standard spheroidal reference surface for raw altitude data, and a gravitational equipotential surface that defines the nominal sea level.The latest...
, though the yet more recent IAU-2000 has retained the old IAU-1976 value. In either case, the length of the equator is by definition exactly 2π times the given standard, which to the nearest millimeter is in WGS-84 and in IAU-1976 and IAU-2000.
The
geographical mileThe geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheriod this equalled 1855.4 metres...
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 about 0.336% along its axis. This results in the equator being about 0.16% longer than a
meridianA meridian is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations running along it with a given longitude. The position of a point on the meridian is given by the latitude. Each meridian is perpendicular to all circles of latitude at the...
(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
SIThe International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science.The older metric system included several groups...
standardization of the
nautical mileThe nautical mile is a unit of length corresponding approximately to one minute of arc of latitude along any meridian....
as , more than short of the
geographical mileThe geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheriod this equalled 1855.4 metres...
.
See also
- 1st parallel north
The 1st parallel north is a circle of latitude that is 1 degree 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 south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, Australasia, the Pacific Ocean and South America....
- Antarctic Circle
The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. As of , it lies at latitude 66° 33′ 39″ south of the equator. The area south of the Antarctic Circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone...
- Arctic Circle
The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. In , it is the parallel of latitude that runs approximately 66° 33′ 39″ north of the Equator. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern...
- Intertropical Convergence Zone
The Intertropical Convergence Zone is a term that is used to describe the North-East and South-East trade wind convergence. Convectively active portions of the ITCZ are known as the monsoon trough...
- Thermal equator
The thermal equator is a belt encircling the Earth, defined by the set of locations having the highest mean annual temperature at each longitude around the globe...
- Tropic of Cancer
The Tropic of Cancer, or the Northern tropic, is one of five major degree measures or major circles of latitude that mark maps of the Earth. It is the northmost latitude at which the sun can appear directly overhead at noon...
- Tropic of Capricorn
The Tropic of Capricorn, or Southern tropic, is one of the five major circles of latitude that mark maps of the Earth. It lies 23° 26′ 22″ south of the equator, and marks the most southerly latitude at which the sun can appear directly overhead at noon...
External links