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Planet Earth is a roundish planet. In order to map it, geographers overlay grid of lines of latitude and longitude. Latitudinal lines wrap around the planet from east to west, while longitude lines go from north to south.
The equator is an imaginary line that runs from east to west on Earth's surface and is exactly halfway between the north and south poles (the northernmost and southernmost points on the Earth). It also divides the Earth into the northern hemisphere and the southern hemisphere and is an important line of latitude for navigational purposes. It is at 0° latitude, and all other measurements head north or south from it. The poles are at 90 degrees north and south. For reference, the corresponding line of longitude is the prime meridian.
Earth at the EquatorUser:Cburnett / CC BY-SA 3.0 / Wikimedia Commons
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) 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.
Earth is an oblate spheroid that is slightly squished at the poles, which means it bulges at the equator. This "pudgy basketball' shape comes from a combination of Earth's gravity and its rotation. As it spins, Earth flattens just a bit, making diameter at the equator 42.7 km larger than the diameter of the planet from pole to pole. Earth's circumference at the equator is 40,075 km and 40,008 km at the poles.
Earth also rotates faster at the equator. It takes 24 hours for the Earth to make one full rotation on its axis, and since the planet is bigger at the equator, it has to move faster to make one full rotation. Therefore, to find the speed of Earth's rotation around its middle, divide 40,000 km by 24 hours to get 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.
The Climate at the Equator
The equator is distinct from the rest of the globe in its physical environment as well as its geographic characteristics. For one thing, the equatorial climate remains much the same year-round. The dominant patterns are warm and wet or warm and dry. Much of the equatorial region is also characterized as being humid.
These climactic patterns occur because the region at the equator receives the most incoming solar radiation. As one moves away from the equatorial regions, solar radiation levels change, which allows other climates to develop and explains the temperate weather in the mid-latitudes and the colder weather at the poles. The tropical climate at the equator allows an amazing amount of biodiversity. It features many different species 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 the land and water of 12 countries and several oceans. Some land areas 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 a kilometer of the equator. As such, the city's center features a museum and monument marking the equator.
More Interesting Equatorial Facts
The equator has special significance beyond being a line on a grid. For astronomers, the extension of the equator out to space marks the celestial equator. People who live along the equator and watch the sky will notice that the sunsets and sunrises are very fast and the length of each day remains fairly constant through the year.
Sailors of old (and new) celebrate equator passages when their ships cross the equator heading either north or south. These "festivals" range from some pretty raucous events onboard naval and other vessels to fun parties for passengers on pleasure cruise ships. For space launches, the equatorial region offers a bit of a speed boost to rockets, allowing them to save on fuel as they launch eastward.
Edited and updated by Carolyn Collins Petersen.