The equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it 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 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. They are also the only places in the world where the sun can go directly from the zenith to the nadir and from the nadir to the zenith. 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 center). North or south of the equator day length increasingly varies with latitude and the seasons.
The Earth bulges slightly at the equator. It has an average diameter of 12,750 kilometres (7,922 mi), but at the equator the diameter is approximately 43 kilometres (27 mi) greater than the polar diameter.
Locations near the equator are good sites for spaceports, such as the 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. 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 9 metres (30 ft) 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 no 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 4,690 metres (15,387 ft), at 00°00′00″S, 77°59′31″W, on the south slopes of Volcán Cayambe (summit 5,790 metres (18,996 ft)) 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 the Prime Meridian and heading eastwards, the equator passes through:
|Co-ordinates||Country, territory or sea||Notes|
|Atlantic Ocean||Gulf of Guinea|
|São Tomé and Príncipe||Ilhéu das Rolas|
|Atlantic Ocean||Gulf of Guinea|
|Republic of the Congo|
|Democratic Republic of the Congo|
|Lake Victoria||Passing through some islands of Uganda|
|Indian Ocean||Passing between Gaafu Dhaalu and Gnaviyani atolls, Maldives|
|Indonesia||The Batu Islands, Sumatra and the Lingga Islands|
|Gulf of Tomini|
|Indonesia||Kayoa and Halmahera islands|
|Pacific Ocean|| Passing just north of Waigeo island, Indonesia |
Passing just south of Aranuka atoll, Kiribati
Passing just south of Baker Island,Uninited States Minor Outlying Islands
|Ecuador||Isabela Island in the Galápagos Islands|
|Brazil||Including some islands in the mouth of the Amazon River|
Despite its name, no part of Equatorial Guinea's territory lies on the equator. However, its island of Annobón is about 155 kilometres (100 mi) 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 Peru.
 "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 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 metres (20,925,656 ft), subsequently refined by the IUGG to 6,378,137 metres (20,925,646 ft) 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 metres (131,479,713.54 ft) in WGS-84 and 40,075,035.535 metres (131,479,775.38 ft) in IAU-1976 and IAU-2000.
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 1,855.3248 metres (6,087.024 ft) or 1,855.3257 metres (6,087.027 ft) 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 metres (131,259,392.77 ft), one arc minute of which is 1,852.216 metres (6,076.82 ft), explaining the SI standardization of the nautical mile as 1,852 metres (6,076 ft), more than 3 metres (10 ft) short of the geographical mile.
 See also
|Wikimedia Commons has media related to: Equator|
- 1st parallel north
- 1st parallel south
- Antarctic Circle
- Arctic Circle
- Intertropical Convergence Zone
- Thermal equator
- Tropic of Cancer
- Tropic of Capricorn
- ↑ "List of Unofficial US Navy Certificates". Navy.mil. 2005-11-07. http://www.history.navy.mil/faqs/faq92-3.htm. Retrieved 2008-07-13.
- ↑ 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 and other applications that require precise measurements. 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.