The equator is the great circle farthest from the north-south spin axis of Earth. The equator is everywhere equidistant from the North and South poles on the surface of Earth, which is a sphere or a reference ellipsoid.
Although the equator is easy to define, it is not easily found and marked on Earth. The gravitational force is lower along the equator since it is the circle on the ellipsoid farthest from the center of mass of Earth. The equator marks the line above which the Coriolis force deflects movement toward the left of the direction of travel and below which the deflection is to the right. While these effects are real and influence weather patterns, ocean currents, and aircraft flight, they are so weak as to be almost immeasurable at the equator. Polaris, the polestar, can be used to find zero latitude, but atmospheric effects make it difficult to observe accurately at the equator.
However the equator is defined by any of more than 100 geodetic datums in use in the world, it is a physically definable origin for latitude. The geodetic equator is the circle on the surface of a reference ellipsoid equidistant from the poles of rotation of the ellipsoid. For different ellipsoids, the geodetic equator can be in different places on the physical surface of Earth.
Since there are various definitions for other latitudes, there are other equators found along the line representing 0° of those latitudes. There is a celestial equator, defined with respect to celestial latitude, an astronomic equator, a galactic equator, a geomagnetic equator, and others.
The equator has meanings in many contexts. The geostationary weather and communications satellites orbit Earth above the equator. The tropical sun's rays are at their highest at midday along the line. Along its course, tropical rain forests, massive river systems, and sparse populations are found on land. At sea, sailing ships can be trapped in the equatorial doldrums, and sailors still perform ceremonies for those crossing the line for the first time. Joseph Conrad, Mark Twain, and Herman Melville have used the equator in the titles of their books. There have been over a dozen films with “equator” in the title. There is a region known as Equateur in the Republic of Congo, the Equatorial Channel in the Indian Ocean, the countries of Equatorial Guinea and Ecuador, and the group of Pacific islands known as the Line Islands or the Equatorial Islands.
There are dozens of equatorial monuments in South America, Africa, and Asia, but they are often tourist facilities more than they are accurate markers of the location of the zero for the variously defined systems of latitude. For example, the Mitad del Mundo monument in Ecuador marks the approximate position of zero latitude for a system no longer in use in Ecuador or anywhere else in the world. On Ecuadorian topographic maps, the monument is responsibly placed several hundred meters above the line representing zero latitude for the Provisional South American Datum of 1956. GPS-equipped visitors to the Middle of the World will find the World Geodetic System of 1984 zero latitude a few hundred meters north of the monument.
Datums, Earth's Coordinate Grid, Geodesy, Latitude, Longitude, Poles, North and South
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