What Is the Equator?

The equator is the great circle on the surface of the Earth that lies in the plane passing through the planet's center perpendicular to its axis of rotation, dividing the globe into the Northern and Southern Hemispheres. It defines zero degrees latitude and serves as the origin from which all other parallels are measured. Because the Earth rotates, the equator is also the line of maximum tangential velocity at the surface, about 465 meters per second, and the line where centrifugal effects on the planet's shape are largest.

In geoscience the equator is a computational reference as much as a geographic one. Latitude, map projections, orbital inclination, and geomagnetic coordinates are all defined relative to it, and the distinction between the geographic equator, the geomagnetic equator, and the celestial equator matters in practice. The geomagnetic equator, the great circle perpendicular to the axis of the best-fitting dipole model of the field, is displaced from the geographic line by several degrees, and the dip equator, the irregular line along which the field is horizontal, departs from both. Each shifts over time with secular variation of the field.

Geodetic Definition and Dimensions

Modern geodesy models the Earth as an oblate ellipsoid of revolution rather than a sphere. In the WGS 84 reference system used by GPS, the semi-major axis, which is the equatorial radius, is 6,378,137 meters, and the inverse flattening is 298.257223563, giving a polar radius of about 6,356,752 meters. The equator is therefore roughly 21 kilometers farther from the center of the Earth than the poles, and its circumference is close to 40,075 kilometers against a meridional circumference near 40,008 kilometers. That bulge is a consequence of rotation, and it produces the oblateness term in the geopotential that dominates satellite orbit perturbations, causing nodal regression at every inclination except polar, and apsidal rotation at every inclination except the critical values near 63.4 and 116.6 degrees, where it vanishes.

Climate and the Intertropical Convergence Zone

The equator receives the most consistent annual solar irradiance of any latitude, with day length close to twelve hours year round and the Sun passing directly overhead twice a year at the equinoxes. The resulting surface heating drives a band of rising air where the northeast and southeast trade winds meet, the intertropical convergence zone, described by NOAA's JetStream reference on the convergence zone. This band produces near-continuous convective thunderstorms and the heavy rainfall associated with equatorial rainforest, and its seasonal migration north and south of the line sets the wet and dry seasons across the tropics. Satellite imagery of the intertropical convergence zone shows it as an almost unbroken cloud band circling the planet. The Coriolis parameter vanishes at the equator, which is why tropical cyclones do not form within a few degrees of it and why equatorial ocean and atmospheric dynamics follow their own set of trapped wave solutions.

Equatorial Orbits and Launch Geometry

The equatorial plane is the reference plane for orbital inclination, and a satellite in a circular orbit of zero inclination at an altitude of 35,786 kilometers has a period matching the Earth's rotation, holding a fixed position over a point on the equator. That geostationary belt carries most weather and communications satellites, including the Geostationary Operational Environmental Satellites, and access to its longitude slots and associated spectrum is coordinated internationally. Launch sites near the equator gain the largest velocity contribution from the Earth's rotation and require no plane change to reach an equatorial orbit, which is why facilities at low latitude, such as Kourou in French Guiana at about five degrees north, are preferred for geostationary missions.

Applications

The equator is used as a reference in a wide range of fields, including:

• Geodesy and satellite navigation, through latitude and reference ellipsoid definitions
• Cartography and map projection design
• Satellite mission planning, including geostationary slot allocation and orbit inclination
• Meteorology and tropical climate forecasting
• Oceanography, in the study of equatorial currents and El Niño dynamics
• Ionospheric research, particularly the equatorial electrojet and plasma bubbles that disrupt radio links
• Solar energy resource assessment and photovoltaic array siting