Equator

The equator is the line of latitude that splits Earth into the Northern and Southern Hemispheres. It is an imaginary circle at 0 degrees latitude, running roughly 40,075 kilometers (24,902 miles) around the planet, and sits halfway between the North and South Poles. The word can also describe a similar line on any other roughly spherical celestial body.

In spatial geometry, especially in astronomy, the equator of a rotating spheroid—like a planet—is the parallel where latitude is set to 0°. It is an imaginary line on the spheroid, equally distant from its poles, cutting it into northern and southern halves. More precisely, it is where the spheroid meets a plane that is perpendicular to its rotation axis and lies midway between its geographic poles.

Equatorial seasons and climate

On Earth, near the equator, the noontime Sun appears almost directly overhead—never more than about 23° from the zenith—every day of the year. This gives the equator a fairly steady daytime temperature year-round. During the equinoxes (around March 20 and September 23), the subsolar point crosses the equator at a shallow angle, sunlight hits perpendicular to Earth’s rotation axis, and every latitude gets nearly 12 hours of daylight and 12 hours of night.

Etymology

The name comes from the medieval Latin aequator, from the phrase circulus aequator diei et noctis, meaning “circle equalizing day and night,” rooted in the Latin aequare, “to make equal.”

Overview

By definition, the equator’s latitude is 0° of arc. It is one of Earth’s five notable circles of latitude, alongside the Arctic and Antarctic Circles and the Tropics of Cancer and Capricorn. The equator is the only line of latitude that is also a great circle—its plane passes through the globe’s center.

When that plane is projected onto the celestial sphere, it defines the celestial equator. In the cycle of Earth’s seasons, the equatorial plane aligns with the Sun twice a year, during the March and September equinoxes. To someone on Earth, the Sun then appears to travel along the equator (or celestial equator).

Locations on the equator have the shortest sunrises and sunsets because the Sun’s daily path is nearly perpendicular to the horizon for most of the year. Daylight length stays almost constant, though it is about 14 minutes longer than nighttime due to atmospheric refraction and the fact that sunrise begins (and sunset ends) when the Sun’s upper limb, not its center, touches the horizon.

Earth bulges slightly at the equator. Its average diameter is 12,742 km (7,918 mi), but the equatorial diameter is about 43 km (27 mi) larger than the polar diameter. Sites near the equator, like the Guiana Space Centre in Kourou, French Guiana, make good spaceports because they have the fastest rotational speed of any latitude—460 meters (1,510 feet) per second. This extra speed reduces the fuel needed to launch spacecraft eastward (in the direction of Earth’s rotation) into orbit and avoids costly maneuvers to flatten inclination for missions like geostationary orbit insertion.

Precise location

The equator’s precise location is not fixed. The true equatorial plane is perpendicular to Earth’s rotation axis, which drifts about 9 meters (30 feet) over a year. Geological samples show that the equator shifted significantly between 48 and 12 million years ago, as sediment deposited by ocean thermal currents at the equator moved.

These deposits depend on Earth’s axis, which determines solar coverage of the surface. Changes in the axis can also be seen in the layout of volcanic island chains, created by shifting hot spots under the crust as the axis and crust move. This matches the Indian tectonic plate colliding with the Eurasian plate, which is uplifting the Himalayas.

For exact length, the International Association of Geodesy (IAG) and the International Astronomical Union (IAU) use an equatorial radius of 6,378.1366 km (3,963.1903 mi), codified as the IAU 2009 value. This radius also appears in the 2003 and 2010 IERS Conventions and the IERS 2003 ellipsoid. If the equator were perfectly circular, its length would be exactly 2π times that radius—40,075.0142 km (24,901.4594 mi).

The GRS 80 (Geodetic Reference System 1980), approved by the IUGG in 1979, has an equatorial radius of 6,378.137 km (3,963.191 mi). The WGS 84 (World Geodetic System 1984), used in cartography, geodesy, and GPS, also uses 6,378.137 km. For both GRS 80 and WGS 84, the equator’s length is 40,075.0167 km (24,901.4609 mi).

The geographical mile is defined as one arc-minute of the equator, so its value depends on the assumed radius. Under WGS 84, the distance is 1,855.3248 meters (6,087.024 ft); under IAU-2000, it is 1,855.3257 meters (6,087.027 ft).

The difference is less than one millimeter over about 1.86 km (1.16 mi). Earth is commonly modeled as a sphere flattened by 0.336% along its axis, making the equator 0.16% longer than a meridian (a great circle through both poles). The IUGG standard meridian is 40,007.862917 km (24,859.733480 mi) to the nearest millimeter; one arc-minute of that is 1,852.216 meters (6,076.82 ft), which explains the SI standardization of the nautical mile.