World map showing the equator in red
In tourist areas, the equator is often marked on the sides of roads
The equator marked as it crosses
Ilhéu das Rolas, in São Tomé and Príncipe.
The 'equator' is an imaginary line on the
Earth's surface equidistant from the
North Pole and
South Pole. It thus 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 heavens 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 constant 12 hours of day and night throughout the year, while 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.
Equatorial climate
Temperatures near the equator are high all year round (except at altitude). 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 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 is the only point on 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:
★
São Tomé and Príncipe – passing through
Ilhéu das Rolas, an
islet in this archipelago
★
Gabon
★
Republic of the Congo
★
Democratic Republic of the Congo
★
Uganda – including some islets in
Lake Victoria
★
Kenya
★
Somalia
★
Maldives – misses every
island, passing between
Gaafu Dhaalu Atoll and
Gnaviyani Atoll
★
Indonesia – crosses many islands, most notably
Sumatra,
Borneo,
Sulawesi, and
Halmahera
★
Kiribati – misses every island, passing between
Aranuka and
Nonouti Atolls in the
Gilbert Islands
★
Baker Island (unincorporated territory of the
United States) – passes through territorial waters (NB the equator also passes through the
exclusive economic zones around
Howland Island and
Jarvis Island, but not through their territorial waters)
★
Ecuador (literal translation of its official name is "Republic of the Equator") – including
Isabela Island in the
Galápagos Islands
★
Colombia
★
Brazil – including some islands in the mouth of the
Amazon River and passing through
Macapá, capital of
Amapá state
Contrary to its name, no part of
Equatorial Guinea's territory lies on the equator. However, its island of
Annobón is about 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".[
[1]]
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 .336% along its axis. This results in the equator being about .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.
References
★
Geodetic Reference System, , H., Moritz, Bulletin Geodesique, (IUGG/WGS-84 data)
★
Computational Spherical Astronomy, , Laurence G., Taff, John Wiley and Sons, , (IAU data)
See also
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