Equator

The equator is an imaginary line that circles Earth at 0 degrees latitude, splitting the planet into the Northern and Southern Hemispheres. It sits halfway between the North and South Poles and measures roughly 21,639 nautical miles (40,075 kilometers, or 24,902 miles) around. The word can also describe a similar line on any other roughly spherical celestial body.

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

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 keeps daytime temperatures fairly steady 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 day and 12 hours of night.

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”).

By definition, the equator’s latitude is 0° of arc. It’s one of Earth’s five notable circles of latitude, alongside the Arctic and Antarctic polar circles and the Tropics of Cancer and Capricorn. It’s the only line of latitude that’s also a great circle—its plane passes through Earth’s center. When that plane is extended outward to the celestial sphere, it defines the celestial equator. Twice a year, during the March and September equinoxes, the equatorial plane lines up with the Sun; from Earth, the Sun appears to travel along the equator (or celestial equator) at those times.

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 is almost constant, though it’s 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. That extra velocity cuts the fuel needed to launch spacecraft eastward (in Earth’s rotation direction) into orbit and avoids costly maneuvers to flatten inclination for missions like geostationary orbit insertion.

The equator’s precise location isn’t 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 the equator shifted significantly between 48 and 12 million years ago, as sediment deposited by ocean thermal currents at the equator moved. Those deposits depend on Earth’s axis, which determines solar coverage. Changes in the axis also show up in the layout of volcanic island chains, created by shifting hot spots under the crust as the axis and crust move. This aligns with the Indian tectonic plate colliding with the Eurasian plate, causing the Himalayan uplift.

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)—the IAU 2009 value, also in the 2003 and 2010 IERS Conventions and the IERS 2003 ellipsoid. If the equator were a perfect circle, its length would be exactly 2π times that radius: 40,075.0142 km (24,901.4594 mi). The GRS 80 (Geodetic Reference System 1980), adopted 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 for 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 radius assumed. Under WGS-84, it’s 1,855.3248 meters (6,087.024 ft); under IAU-2000, it’s 1,855.3257 meters (6,087.027 ft)—a difference of 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.