AskDefine | Define equator

Dictionary Definition



1 an imaginary line around the Earth forming the great circle that is equidistant from the north and south poles; "the equator is the boundary between the northern and southern hemispheres"
2 a circle dividing a sphere or other surface into two usually equal and symmetrical parts

User Contributed Dictionary



Late Latin (circulus) aequator (diei et noctis), aequatoris



  1. An imaginary great circle around the earth, equidistant from the two poles, and dividing earth's surface into the northern and southern hemisphere.
  2. A similar great circle on any sphere, especially on a celestial body, or on other reasonably symmetrical three-dimensional body.
  3. A short form of the celestial equator.


circle around the earth
circle around any sphere
celestial equator

Extensive Definition

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.

Geodesy of the equator

The latitude of the equator is, by definition, 0°. The length of Earth's equator is about 40,075.0 km, or 24,901.5 miles.
The equator is one of the five main circles of latitude that are based on the relationship between the Earth's axis of rotation and the plane of the Earth's orbit around the sun. It 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 movement through the sky, passes directly over the equator twice each year, on 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. 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 horizon, rather than its centre). North or south of the equator day length increasingly varies with the seasons.
The Earth bulges slightly at the equator. It has an average diameter of 12,750 km, but at the equator the diameter is approximately 43 km greater.
Locations near the equator are good sites for spaceports (e.g., 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. (They must be launched to the East to use this effect.)
For high precision work, we find that 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. And although this axis is pretty stable, its position wanders in roughly a 9 meter 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 climate

Temperatures near the equator are high all year round with the exception for 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 mainly ocean. The highest point on the equator is 4,690 m (15,387'), at on the south slopes of Volcán Cayambe (summit 5,790 m, 18,996') 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 countries. Starting at west Africa and moving east, these are:
Despite its name, no part of flagicon Equatorial Guinea Equatorial Guinea's territory lies on the equator. However, its island of Annobón is about 100 miles (155 km) south of the equator, and the rest of the country lies to the north

"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. These rituals date back to the Middle Ages, though the current ceremonies are most likely derived from Viking traditions. 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 meridian, 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 Meridian, 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 IAU standardized this radius as 6,378,140 m, subsequently refined by the IUGG to 6,378,137 m and adopted in WGS-84, 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 40,075,016.686 m in WGS-84 and 40,075,035.535 m in IAU-1976 and IAU-2000.
(Although millimeter precision can be important up to the scale of a mile, it has negligible physical significance at the scale of a geographic feature such as the equator. From a computational standpoint however millimeter precision or better can be valuable for maintaining consistent results when used in programs for surveying etc. As an overly simple example, if a program were to convert back and forth between the radius and the circumference of the earth sufficiently often while maintaining precision only to a meter each time, errors might accumulate until they became noticeable.)
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 1855.3248 m or 1855.3257 m 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 meridian (as a great circle passing through the two poles). The IUGG standard meridian is to the nearest millimeter 40,007,862.917 m, one arc minute of which is 1852.216 m, explaining the SI standardization of the nautical mile as 1852 m, more than 3 meters short of the geographical mile.

Notes and references

  • (IUGG/WGS-84 data)
  • (IAU data)

See also

equator in Afrikaans: Ewenaar
equator in Tosk Albanian: Äquator
equator in Arabic: خط الاستواء
equator in Asturian: Llinia ecuatorial
equator in Azerbaijani: Ekvator
equator in Bengali: নিরক্ষরেখা
equator in Min Nan: Chhiah-tō
equator in Belarusian (Tarashkevitsa): Экватар
equator in Bosnian: Ekvator
equator in Breton: Keheder
equator in Bulgarian: Екватор
equator in Catalan: Línia equatorial
equator in Czech: Rovník
equator in Welsh: Cyhydedd
equator in Danish: Ækvator
equator in German: Äquator
equator in Estonian: Ekvaator
equator in Modern Greek (1453-): Ισημερινός
equator in Spanish: Ecuador terrestre
equator in Esperanto: Ekvatoro
equator in Basque: Ekuatore
equator in Persian: استوا
equator in French: Équateur (ligne équinoxiale)
equator in Western Frisian: Evener
equator in Galician: Ecuador terrestre
equator in Central Khmer: ខ្សែអេក្វាទ័រ
equator in Korean: 적도
equator in Hindi: भूमध्य रेखा
equator in Croatian: Ekvator
equator in Ido: Equatoro
equator in Indonesian: Khatulistiwa
equator in Icelandic: Miðbaugur
equator in Italian: Equatore
equator in Hebrew: קו המשווה
equator in Javanese: Katulistiwa
equator in Kannada: ವಿಷುವದ್ರೇಖೆ
equator in Georgian: ეკვატორი
equator in Swahili (macrolanguage): Ikweta
equator in Kurdish: Ekwator
equator in Latin: Linea aequatorialis
equator in Latvian: Ekvators
equator in Lithuanian: Pusiaujas
equator in Lingala: Mambénga (monkɔlɔ́tɔ)
equator in Lombard: Equatuur
equator in Hungarian: Egyenlítő
equator in Macedonian: Екватор
equator in Malayalam: ഭൂമദ്ധ്യരേഖ
equator in Marathi: विषुववृत्त
equator in Malay (macrolanguage): Khatulistiwa
equator in Dutch: Evenaar
equator in Japanese: 赤道
equator in Norwegian: Ekvator
equator in Norwegian Nynorsk: Ekvator
equator in Polish: Równik
equator in Portuguese: Linha do Equador
equator in Romanian: Ecuator
equator in Quechua: Chawpipacha
equator in Russian: Экватор
equator in Albanian: Ekuatori
equator in Sicilian: Equaturi (tirrestri)
equator in Simple English: Equator
equator in Slovak: Rovník
equator in Slovenian: Ekvator
equator in Silesian: Růwnik
equator in Serbian: Екватор
equator in Serbo-Croatian: Ekvator
equator in Sundanese: Hatulistiwa
equator in Finnish: Päiväntasaaja
equator in Swedish: Ekvator
equator in Tagalog: Ekwador
equator in Tamil: நிலநடுக் கோடு
equator in Telugu: భూమధ్య రేఖ
equator in Thai: เส้นศูนย์สูตร
equator in Vietnamese: Xích đạo
equator in Turkish: Ekvator
equator in Ukrainian: Екватор
equator in Urdu: خط استوا
equator in Venetian: Equatore
equator in Wolof: Yamoo
equator in Samogitian: Posiaus
equator in Chinese: 赤道

Synonyms, Antonyms and Related Words

Antarctic Zone, Arctic Circle, Arctic Zone, Frigid Zones, Torrid Zone, Tropic of Cancer, Tropic of Capricorn, Variable Zones, anklet, aphelion, apogee, armlet, astronomical longitude, autumnal equinox, band, belt, bisector, boundary, bracelet, celestial equator, celestial longitude, celestial meridian, center, cincture, cingulum, circle, climate, clime, collar, collarband, colures, core, diameter, diaphragm, divider, earring, ecliptic, equinoctial, equinoctial circle, equinoctial colure, equinox, fascia, fillet, finger ring, furnace, galactic longitude, geocentric longitude, geodetic longitude, girdle, girt, girth, great circle, halfway mark, heart, heliocentric longitude, hell, hoop, horse latitudes, inferno, interior, kernel, latitude, line of demarcation, longitude, longitude in arc, mean, median, meridian, middle, midmost, midriff, midst, neckband, necklace, nose ring, nucleus, orbit, oven, parallel, partition, perigee, perihelion, period, prime meridian, quoit, ring, roaring forties, small circle, solstitial colure, steam bath, subtropics, the line, thick, thick of things, trajectory, tropic, tropics, vernal equinox, waist, waistline, wristband, wristlet, zodiac, zone
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