An equator is an imaginary line around the middle of a planet or other celestial body. It is halfway between the North Pole and the South Pole, at 0 degrees latitude. An equator divides the planet into a Northern Hemisphere and a Southern Hemisphere.

The Earth is widest at its Equator. The distance around the Earth at the Equator, its circumference, is 40,075 kilometers (24,901 miles).

The Earth's diameter is also wider at the Equator, creating a phenomenon called an equatorial bulge. The diameter of a circle is measured by a straight line that passes through the center of the circle and has its endpoints on the boundary of that circle. Scientists can calculate the diameter of latitudes, such as the Equator and Arctic Circle.

The Earth's diameter at the Equator is about 12,756 kilometers (7,926 miles). At the poles, the diameter is about 12,714 kilometers (7,900 miles). The Earth's equatorial bulge is about 43 kilometers (27 miles).

The equatorial bulge means that people standing at sea level near the poles are closer to the center of the Earth than people standing at sea level near the Equator. The equatorial bulge affects the ocean, too—sea levels are slightly higher in equatorial regions than near the poles.

The equatorial bulge is created by the Earth's rotation. As lines of latitude increase in size, a point has to travel faster to complete a circle (revolution) in the same amount of time. The rotational speed, or spin, at the Arctic Circle is slower than the spin at the Tropic of Cancer, because the circumference of the Arctic Circle is much smaller and a point doesn't have to travel as far to complete a revolution. The spin at the Tropic of Cancer is much slower than the spin at the Equator. Near the poles, the Earth's rotational speed, or spin, is near zero. At the Equator, the spin is about 1,670 kilometers per hour (1,038 miles per hour).

The Earth's gravitational pull is slightly weaker at the Equator due to its equatorial bulge.

The slightly weaker gravitational pull and momentum of the spinning Earth makes equatorial regions ideal places for space launches. It takes an enormous amount of energy to launch a satellite or other spacecraft out of the Earth's atmosphere. It takes less energy (rocket fuel) to launch in lower gravity. It also takes less energy to launch when the spinning Earth is already giving the satellite a push of 1,670 kilometers per hour (1,038 miles per hour).

The United States launches most spacecraft from the Kennedy Space Center in southern Florida, as close to the Equator as possible in the continental U.S. Other rocket-launching facilities near the Equator include Shaba North, Democratic Republic of Congo, and Gan Island, Maldives.

Recently, mobile launch platforms, such as Ocean Odyssey, have successfully launched satellites into orbit from the equatorial Pacific Ocean.

Equatorial Climates

Twice a year, during the spring and autumn equinoxes, the sun passes directly over the Equator. Even during the rest of the year, equatorial regions often experience a hot climate with little seasonal variation.

As a result, many equatorial cultures recognize two seasons—wet and dry.  The wet, or rainy, season often lasts most of the year. The long, warm, rainy season creates tropical rain forests. Some of the most expansive rain forests in the world are in equatorial regions: the Amazon rain forest of South America, the Congo rain forest of Central Africa, and the varied Southeast Asian rainforest stretching from India to Vietnam.

Humid weather means that equatorial regions are not the hottest in the world, even though they are among the closest to the sun. The water in the equatorial air cools it slightly.

Many cultures thrive in warm equatorial regions. The Fang people of Gabon, for instance, are successful farmers who take advantage of the warm temperature and long rainy season to cultivate crops such as corn, yams, and plantains. The Fang also raise livestock that have adapted to the climate, such as goats and chickens.

Not all equatorial regions are hot and humid, however. Mount Kilimanjaro, Tanzania, is only 330 kilometers (205 miles) from the Equator, but its elevation creates a climate with cool, dry weather and even alpine glaciers.

The Andes are another equatorial region lacking the hot, humid climate often associated with the Equator. The mountain range includes a desert with almost no rain (the Atacama), as well as some of the tallest peaks on Earth. Here, too, cultures have thrived for thousands of years. The Aymara people of the Altiplano of Bolivia, Peru, and Chile, are primarily an urban people who identify strongly with the innovative navigational successes of their ancestors. In the 20th century, the Aymara helped build railroads through the high, equatorial Andes.

Many plant and animal species thrive in equatorial climates. The Amazon and Congo rain forest ecosystems, for example, are amazingly rich in biodiversity. A single hectare (2.47) of rain forest in Brazil may contain 750 species of trees and twice that many species of insects. The equatorial savanna of Kenya includes mammals such as lions, cheetahs, and elephants. The chilly equatorial Andes are famous for its camelid species: llamas, alpacas, vicunas, and guanacos.

equator
Kenya is one of 13 nations that straddle the Equator.

Bulging through Ecuador
Mount Chimborazo, Ecuadornot Mount Everestis the highest point on Earth. Earth's equatorial bulge pushes Mount Chimborazo, near the Equator in the Ecuadorian Andes, further from the center of the Earth.

However, elevation is measured from sea level, not the center of the Earth. Mount Everest is 8,848 meters (29,035 feet) above sea level, while Mount Chimborazo is just 6,310 meters (20,702 feet) above sea level.

Crossing the Line
Sailors have elaborate rituals and celebrations when they cross the Equator, which they call crossing the line. Sailors who have never crossed the line are called pollywogs. Pollywogs are usually the target of embarrassing practical jokes.

Short Sunsets
The time it takes for the sun to set and rise at the Equator is the fastest on Earth. The transition from day to night takes only a few minutes.

alpine glacier
Noun

mass of ice that moves downward from a mountain.

Altiplano
Noun

high plateau in the Andes Mountains of South America.

Arctic Circle
Noun

paralell of latitude that runs 66.5 degrees north of the Equator.

Noun

layers of gases surrounding a planet or other celestial body.

Noun

all the different kinds of living organisms within a given area.

camelid
adjective, noun

family of two-toed mammal, including camels, alpacas, and llamas.

celestial body
Noun

natural object in space, such as a planet or star. Also called an astronomical object.

circumference
Noun

distance around the outside of a circle.

Noun

all weather conditions for a given location over a period of time.

Noun

agricultural produce.

culture
Noun

learned behavior of people, including their languages, belief systems, social structures, institutions, and material goods.

diameter
Noun

width of a circle.

Noun

community and interactions of living and nonliving things in an area.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

equatorial bulge
Noun

difference between the equatorial and polar diameters of a planet.

Noun

period in which daylight and darkness are nearly equal. There are two equinoxes a year.

fuel
Noun

material that provides power or energy.

gravitational pull
Noun

physical attraction between two massive objects.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

humid
Adjective

containing a large amount of water vapor.

innovative
Adjective

new, advanced, or original.

Noun

distance north or south of the Equator, measured in degrees.

livestock
noun, plural noun

animals raised for sale and profit.

momentum
Noun

speed, direction, or velocity at which something moves.

Noun

art and science of determining an object's position, course, and distance traveled.

Northern Hemisphere
Noun

half of the Earth between the North Pole and the Equator.

Noun

fixed point that, along with the South Pole, forms the axis on which the Earth spins.

orbit
Noun

path of one object around a more massive object.

Noun

large, spherical celestial body that regularly rotates around a star.

Noun

area of tall, mostly evergreen trees and a high amount of rainfall.

rainy season
Noun

time of year when most of the rain in a region falls.

Noun

any area on Earth with one or more common characteristics. Regions are the basic units of geography.

revolution
Noun

orbit, or a complete journey of an object around a more massive object.

rocket
Noun

device that moves through the atmosphere by release of expanding gas.

Noun

object's complete turn around its own axis.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

savanna
Noun

type of tropical grassland with scattered trees.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

Noun

period of the year distinguished by special climatic conditions.

seasonal variation
Noun

differences in temperature, climate, and weather between seasons of the year.

Southern Hemisphere
Noun

half of the Earth between the South Pole and the Equator.

Noun

fixed point that, along with the North Pole, forms the axis on which the Earth spins.

spacecraft
Noun

vehicle designed for travel outside Earth's atmosphere.

species
Noun

group of similar organisms that can reproduce with each other.

thrive
Verb

to develop and be successful.

Tropic of Cancer
Noun

line of latitude 23.5 degrees north of the Equator.

urban
Adjective

having to do with city life.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.