equator, imaginary great circle around the earth, everywhere equidistant from the two geographical poles and forming the base line from which latitude is reckoned. The equator, which measures c.24,902 mi (40,076 km), is designated as lat. 0°. It intersects N South America, central Africa, and Indonesia. The celestial equator is the projection of the plane of the earth's equator on the celestial sphere (see equatorial coordinate system).

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 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. (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 is only important for detailed scientific studies... nobody is likely to move a road sign declaring "you are at the equator" because of this. 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:

• 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. Ecuador's capital, Quito, is 13 km south of the equator.
• Colombia - mostly through the jungles of the south.
• Brazil – including some islands in the mouth of the Amazon River and passing through Macapá, capital of Amapá state

Despite 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".

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.

Notes and References

• (IUGG/WGS-84 data)
• (IAU data)

See also

• Antarctic Circle
• Arctic Circle
• Intertropical Convergence Zone
• Prime Meridian

• Thermal equator
• Tropic of Cancer
• Tropic of Capricorn