The Equator
The equator explained — 0° latitude, 40,075 km circumference on WGS-84, the 21 km equatorial bulge, the 13 countries crossed, and the Coriolis-zero zone.
By Steve K.. Published . Last updated .
The equator is the great circle on Earth's surface equidistant from the geographic poles — the unique parallel where latitude is 0°. On the WGS-84 ellipsoid its circumference is exactly 40,075.017 km. The equator passes through 13 countries on land and defines the boundary between the Northern and Southern Hemispheres.
The equator is the most physically significant parallel on Earth. It is the cross-section perpendicular to the rotation axis, the line of maximum solar irradiance, the boundary between the climate-driving hemispheres, and the largest of all parallels. This article runs the exact numbers, identifies the 13 countries crossed by the equator on land, explains the meteorological and oceanographic consequences, and contrasts the equator with the other named parallels. The companion pillar /learn/what-is-latitude covers the latitude axis; /learn/the-prime-meridian is the longitude counterpart.
Geometry
The equator is the unique great circle equidistant from both geographic poles. Because Earth is an oblate spheroid, the equatorial plane is the cross-section perpendicular to the rotation axis where the surface is furthest from the centre.
| Property | Value | Source |
|---|---|---|
| Latitude | 0° exactly | Defining |
| Length (WGS-84 ellipsoid) | 40,075.017 km | NGA STND 0036 |
| Length on a perfect sphere of radius 6,371 km | 40,030.2 km | For comparison |
| Length difference (oblate vs sphere) | +44.8 km | Due to equatorial bulge |
| Equatorial radius a (WGS-84) | 6,378,137 m (exact) | NGA STND 0036 |
| Polar radius b (WGS-84) | 6,356,752.3142 m | Derived from a × (1 − f) |
| a − b (equatorial bulge) | 21,385 m | Centrifugal flattening |
| 1° of equatorial circumference | 111.320 km | Equator divided by 360 |
| 1 arcminute of equatorial circumference | 1,855 m | Equator divided by 21,600 |
The equatorial bulge — 21.385 km of "extra" radius vs the poles — was predicted by Newton in Principia (1687) from the centrifugal effect of Earth's rotation on a self-gravitating fluid. The full ellipsoid parameters live on /learn/wgs84-explained and /reference/wgs84-parameters. The actual flattening was confirmed by Maupertuis's 1736 Lapland expedition, settling a long Cartesian-vs-Newtonian dispute about Earth's shape.
The 13 countries crossed by the equator on land
The equator crosses 13 countries on land plus a number of oceanic stretches. The terrestrial crossings:
| Country | Continent | Approximate equator length on land (km) | Notable equator-line landmark |
|---|---|---|---|
| São Tomé and Príncipe | Africa (offshore Atlantic island) | ~25 | Ilhéu das Rolas (uninhabited rock at 0°00′) |
| Gabon | Africa | ~600 | Equator monument near Lopé National Park |
| Republic of the Congo | Africa | ~250 | — |
| Democratic Republic of the Congo | Africa | ~2,500 | Major Congo basin crossing |
| Uganda | Africa | ~250 | Famous Equator marker at Kayabwe (1 hr S of Kampala) |
| Kenya | Africa | ~550 | Equator marker at Nanyuki (Mt Kenya district) |
| Somalia | Africa | ~500 | — |
| Maldives | Asia (oceanic) | ~30 | Just south of Addu Atoll, the country's southernmost atoll |
| Indonesia | Asia (archipelago) | ~3,300 | Crosses Sumatra, Borneo (Kalimantan), Sulawesi; 'Equator Bridge' at Pontianak |
| Kiribati | Oceania | ~70 | Crosses several atolls; nation spans both hemispheres |
| Ecuador | South America | ~600 | Mitad del Mundo monument near Quito (the country named for the equator) |
| Colombia | South America | ~1,300 | Crosses Amazonian region |
| Brazil | South America | ~2,200 | Crosses Amazon basin from Macapá eastward |
Indonesia has the longest equator crossing on land (~3,300 km), because the archipelago straddles the equator across most of its east-west extent. Ecuador is the country named after the equator (Spanish ecuador); its capital Quito sits at 0°13' S, just 24 km south of the equator itself. The Mitad del Mundo ("Middle of the World") monument outside Quito is at the most-visited equator marker on Earth; the actual 0° latitude is a few hundred metres from the monument's position (the monument was built on the colonial-era estimate of the equator location).
Solar position and climate at the equator
Standing at the equator on either equinox (March 20 / 21 or September 22 / 23), the Sun is exactly overhead at solar noon. On the solstices, the Sun is 23.4° off vertical (north on June solstice, south on December solstice). Day length is approximately 12 hours all year.
| Date | Solar zenith angle at noon (Sun's distance from vertical) | Day length | Notes |
|---|---|---|---|
| March 20-21 (March equinox) | 0° (Sun directly overhead) | 12 h 06 min | Day = night globally |
| June 20-21 (June solstice) | 23.4° N of vertical | ~12 h 07 min | Sun is overhead at Tropic of Cancer |
| September 22-23 (Sept equinox) | 0° (Sun directly overhead) | 12 h 06 min | Day = night globally |
| December 21-22 (December solstice) | 23.4° S of vertical | ~12 h 07 min | Sun is overhead at Tropic of Capricorn |
| Annual mean solar elevation at noon | ~78° (very high) | 12 h ± 7 min | Highest yearly solar irradiance on Earth |
The "12 h plus 6-7 minutes" excess of day over night at the equator is due to atmospheric refraction (which bends sunlight around the horizon) and the Sun's angular radius (the Sun's upper limb crosses the horizon before the Sun's centre). At higher latitudes the day-night ratio varies dramatically through the year; at the equator it's essentially constant.
The Intertropical Convergence Zone (ITCZ)
The equator is the climatological centre of a meteorological band called the Intertropical Convergence Zone, where the northern and southern hemisphere trade winds meet.
| ITCZ feature | Value | Significance |
|---|---|---|
| Typical position | ±5° of the equator | Drifts north in northern summer, south in southern summer |
| Mean position (annual) | ~5° N (over oceans) | Slightly north of the geographic equator over the Pacific and Atlantic |
| Rainfall in ITCZ | 2,000-5,000 mm/year | Among the wettest regions on Earth |
| Tropical cyclone formation | None within ~5° of equator | Coriolis force too weak for cyclone spin |
| Solar irradiance (annual mean) | ~340 W/m² (top of atmosphere) | Among the highest globally |
| Equatorial sea-surface temperature (warm pool) | 28-30 °C in western Pacific | Drives ENSO climate variability |
The equator is the climate-driving zone of Earth: the band of greatest solar input, the rising arm of the Hadley cells, the nursery for tropical rainforests, and the source region for trade winds that dominate Atlantic and Pacific weather. Equatorial ocean upwelling along the Pacific coast of South America drives the El Niño – Southern Oscillation (ENSO), the largest year-to-year climate-variability signal on Earth.
The Coriolis effect at the equator
The Coriolis force is zero at the equator and grows linearly with sin(latitude). The consequences ripple through meteorology, oceanography and aviation.
| Phenomenon | At the equator | At higher latitudes | Consequence |
|---|---|---|---|
| Coriolis parameter f = 2Ω sin(φ) | 0 | ~10⁻⁴ s⁻¹ at 45° | No Coriolis spin at the equator |
| Tropical cyclone formation | None within ~5° of equator | Frequent | Cyclones need rotation; can't spin up at equator |
| Cross-equatorial weather systems | Inhibited; storms cannot crossing without dissipating spin | — | ITCZ is meteorological boundary |
| Ocean currents | No geostrophic balance | Geostrophic flow | Equatorial currents driven by wind only, not by pressure gradients |
| Ballistic trajectories | No deflection | Right (N) or left (S) | Artillery / spaceflight corrections grow with latitude |
The "water spins differently in northern vs southern hemisphere" folklore is real for very large scales (tropical cyclones, ocean currents) where the Coriolis force has time to act. It is not real for bathtub-scale drains, where the Coriolis force is ~1 millionth of the dominant turbulent and geometric forces. Demonstrations of "equator-line bathtub direction reversal" near tourist equator markers are stage tricks, not physics.
Gravity at the equator
Surface gravity is slightly weaker at the equator than at the poles for two compounding reasons.
| Source | Effect on g at equator vs pole | Magnitude |
|---|---|---|
| Larger radius from Earth's centre at equator | Gravitational attraction is weaker (inverse-square) | ~0.34% reduction |
| Centrifugal effect of Earth's rotation | Effective gravity reduced by Ω²r | ~0.34% reduction |
| Combined effect: g (equator) vs g (pole) | g_equator ≈ 9.7803 m/s²; g_pole ≈ 9.8322 m/s² | ~0.53% weaker at equator |
| Practical consequence | A 1 kg mass weighs ~5.3 g less at equator vs pole | Detectable by precision scales (spring scales) |
| Free-fall acceleration impact on space launches | Equatorial launches benefit from rotational velocity (~465 m/s eastward) | ~10% lower launch energy required vs pole |
The Equatorial launch advantage is why the European Space Agency operates from Kourou (5° N) and SpaceX considered sea platforms near the equator. Earth's rotation gives an "free" ~465 m/s eastward velocity at the equator (cos(0°) × 463 m/s); the contribution drops as cos(latitude) at higher latitudes (~230 m/s at 60°, zero at the poles).
The other named parallels
For comparison, the equator's position in the system of named parallels (each covered in its own /learn article — Tropics, Polar Circles, the /learn/north-pole, and /learn/south-pole):
| Parallel | Latitude (2026 epoch) | Set by | Circumference (km) |
|---|---|---|---|
| Equator | 0° | Earth's rotation axis | 40,075.017 |
| Tropic of Cancer | +23.4365° N | +ε (current obliquity of ecliptic) | ~36,788 |
| Tropic of Capricorn | −23.4365° S | −ε | ~36,788 |
| Arctic Circle | +66.5635° N | +(90°−ε) | ~15,977 |
| Antarctic Circle | −66.5635° S | −(90°−ε) | ~15,977 |
| North Pole | +90° N | Rotation axis | 0 (point) |
| South Pole | −90° S | Rotation axis | 0 (point) |
The equator is the only one set by Earth's rotation axis alone; the other named parallels are set by the axial tilt (obliquity ε) combined with the rotation axis. As ε drifts (currently decreasing by 46.81 arcseconds per century), the tropics and polar circles shift, but the equator stays at exactly 0°.
Common misconceptions
Related
- Parallels of Latitude— The family of circles the equator anchors
- The Tropic of Cancer— The northern boundary of the tropical belt
- The Prime Meridian— The longitude analogue — the reference for 0° longitude
- What Is Latitude— The angular axis the equator anchors at 0°
- Why the Earth Is Not a Sphere— The oblateness that gives the equator its bulge
- WGS 84 Explained— The reference ellipsoid whose semi-major axis defines the equatorial circumference
- My Location— Your current latitude — measured against the equator
- Methodology— How content is sourced and verified
Frequently asked questions
What exactly is the equator?
The equator is the great circle on Earth's surface that is everywhere equidistant from the two geographic poles. By convention it defines 0° latitude, and it lies in the equatorial plane — the plane perpendicular to Earth's rotation axis passing through the centre of mass. Its length on the WGS 84 ellipsoid is 40,075.017 km, slightly longer than any meridian (40,007.863 km) because Earth bulges at the equator.
How much wider is the Earth at the equator than at the poles?
About 42.8 km wider in diameter, or 21.4 km in radius. The WGS 84 ellipsoid defines the equatorial semi-major axis as 6,378,137.0 m exactly and the polar semi-minor axis as 6,356,752.314 m. The difference is the equatorial bulge, a consequence of Earth's rotation: centrifugal force at the equator slightly counteracts gravity, stretching the planet into an oblate shape with flattening f = 1/298.257223563.
Is the magnetic equator the same as the geographic equator?
No. The magnetic equator (technically the dip equator) is the curve where the magnetic field is horizontal — where a freely suspended magnetised needle points exactly along the horizon. It wanders north and south of the geographic equator by up to about 12° depending on longitude and shifts a few kilometres per year because the geomagnetic field is generated by fluid motion in Earth's outer core. The NOAA World Magnetic Model publishes updated dip-equator positions every five years.
Does water really drain in opposite directions north and south of the equator?
No. The Coriolis parameter at the equator is exactly zero (f = 2Ω sin 0 = 0), and elsewhere it is far too small to overcome the initial conditions and basin shape that actually determine which way a sink or toilet drains. At hurricane scales (hundreds of kilometres, hours of duration) the Coriolis force genuinely controls rotation direction; at toilet scales (a metre, a minute) it does not. The 'demonstrations' tourists watch near equator markers in Ecuador and Kenya are sleight of hand.
How many countries does the equator cross?
Thirteen sovereign states have land or island territory the equator crosses, distributed across three continents and the Pacific: in South America, Ecuador, Colombia, and Brazil; in the Atlantic, the islands of São Tomé and Príncipe; in Africa, Gabon, the Republic of the Congo, the Democratic Republic of the Congo, Uganda, Kenya, and Somalia; in Asia, Indonesia and the Maldives; and in the Pacific, Kiribati. The equator also crosses the Atlantic, Indian, and Pacific Oceans.
Why is the equator usually hot but the hottest places aren't on it?
The equator receives the most consistent solar energy across the year because the Sun is never far from directly overhead at noon. But the highest surface temperatures on Earth are recorded in subtropical deserts (the Sahara, Death Valley, the Lut Desert in Iran) at roughly 25–35° latitude. The equatorial belt is dominated by the Intertropical Convergence Zone, a band of rising air, persistent cloud cover, and heavy rainfall that moderates the temperature; the subtropical highs are dominated by descending dry air that allows the surface to bake.
How long is the equator?
The equator is 40,075.017 km long on the WGS-84 ellipsoid — Earth's longest circle. The polar (meridional) circumference is 40,007.863 km, 67.154 km shorter. The difference comes from the equatorial bulge: Earth's equatorial radius is 21.385 km longer than the polar radius. One degree of equatorial circumference is 111.320 km; one arcminute is 1,855 m.
What is the equator made of?
The equator is a geometric line on the WGS-84 ellipsoid, not a physical feature — it is the great circle where latitude = 0° by definition. Equator monuments on the ground (Quito's Mitad del Mundo in Ecuador, the line painted on a road near Kayabwe in Uganda, the Pontianak Equator Monument in Indonesia) mark its position symbolically. The 0° line itself is invisible — only a marker can show where it is.
Sources
- NGA — Department of Defense World Geodetic System 1984 (NGA.STND.0036) · https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84 · Accessed .
- NASA — Earth Fact Sheet — equatorial and polar radii, oblateness · https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html · Accessed .
- NOAA — The Intertropical Convergence Zone (NOAA SciJinks) · https://scijinks.gov/itcz/ · Accessed .
- NOAA NCEI — World Magnetic Model — magnetic equator (dip equator) · https://www.ncei.noaa.gov/products/world-magnetic-model · Accessed .
- IERS — IERS Conventions (2010) — reference systems and pole · https://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36.html · Accessed .
- USGS — Geographic Names Information System — equatorial features · https://www.usgs.gov/tools/geographic-names-information-system-gnis · Accessed .
- IAU — IAU Working Group on Cartographic Coordinates and Rotational Elements · https://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2015reprint.pdf · Accessed .
Cite this article
APA format:
Steve K. (2026). The Equator. Coordinately. https://coordinately.org/learn/the-equator
BibTeX:
@misc{coordinately_theequator_2026,
author = {K., Steve},
title = {The Equator},
year = {2026},
publisher = {Coordinately},
url = {https://coordinately.org/learn/the-equator},
note = {Accessed: 2026-06-05}
}