The equator is an imaginary line that runs from east to west on the Earth's surface (diagram) and is exactly halfway between the North and South Poles (the northernmost and southernmost points on the Earth). The Equator also divides the Earth into the Northern Hemisphere and the Southern Hemisphere and is an important line of latitude for navigational purposes as it is 0˚ and all other measurements head north or south from it.
Because the latitude of Earth's equator is 0˚ it is an important feature on the Earth for geography as well as navigation and exploration in that it is the starting point for studies of the planet's features based on latitude. For reference, the corresponding line of longitude is the Prime Meridian.
Geography of the Earth's Equator
The equator is the only line on the Earth's surface that is considered a great circle. This is defined as any circle drawn on a sphere (or an oblate spheroid as the Earth is classified) with a center that includes the center of that sphere. The Equator thus qualifies as a great circle because it passes through the exact center of the Earth and divides it in half. Other lines of latitude north and south of the equator are not great circles because they shrink as they move toward the poles. As their length decreases, they do not all pass through the center of the Earth.
Since the Earth is an oblate spheroid an ellipsoid, it is slightly squished at the poles and bulges at the Equator as a result of gravity and its rotation. Thus, its diameter at the Equator is 26.5 miles (42.7 km) larger the polar diameter of 7,899.80 miles (12,713.5 km).
Like its diameter, the Earth's circumference is also slightly larger at the Equator because of the equatorial bulge. For example, at the poles the circumference is 24,859.82 miles (40,008 km) but at the equator it is 24,901.55 miles (40,075.16 km).
In addition, because the Earth is an oblate spheroid and it is larger at the Equator, it also rotates faster on its axis there than anywhere else. This is because when one is at the equator the circumference is roughly 24,000 miles or 40,000 kilometers (for simplicity) and it takes 24 hours for the Earth to make one full rotation. Therefore, to find the speed of the Earth's rotation, divide 24,000 miles or 40,000 km by 24 hours to get 1,000 miles per hour or 1,670 km per hour. As one moves north or south in latitude from the Equator the Earth's circumference is reduced and thus the speed of rotation decreases slightly.
Climate and the Equator
The equator is distinct from the rest of the globe for its physical environment as well as its geographic applications. The largest of these distinctions however is its climate as it mainly has the same climatic patterns all year round. The dominant patterns are warm and wet or warm and dry all year round. Much of the equatorial region is also characterized as being humid. These climatic patterns at the Equator are because it receives the most incoming solar radiation.
As one moves away from the Equatorial regions, solar radiation is less (though it moves north and south from the Equator) and other climates are able to develop. However, due to the tropical climate at the Equator, the region is one of the most biodiverse on the planet. It features a plethora of plants and animals and is home to the largest areas of tropical rainforests in the world.
Countries along the Equator
In addition to the dense tropical rainforests along the Equator, the line of latitude crosses through the land and water of 12 countries. Some of these countries are sparsely populated, but others, like Ecuador have large populations and have some of their largest cities on the Equator. For example, Quito, Ecuador's capital, is within .6 miles (1 km) of the Equator and as such, the city's center features a museum and monument marking the Equator.
To learn more about the Equator and nearby areas, read "The Equator, Hemispheres, Tropic of Cancer and Tropic of Capricorn" on this website.
Pogge, Richard. (14 October 2006). Lecture 21: Rotation & Revolution of the Earth. Retrieved from: http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/movearth.html
Wikipedia. (16 August 2010). Quito - Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Quito
Wikipedia. (10 August 2010). Equator - Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Equator