EQUATOR

An equator is an imaginary circle around the middle of a planet or other rotating body, defined as the set of points equidistant from the two poles. For Earth, the equator is the zero-degree latitude line that divides the planet into the Northern and Southern Hemispheres. It corresponds to the plane perpendicular to the Earth's rotation axis that passes through the planet's center, and it is a great circle—the largest possible circle on the globe.

Measurements: The Earth's equator has a circumference of about 40,075 kilometers (24,901 miles). The equatorial radius is about 6,378 kilometers (3,963 miles). The Earth is an oblate spheroid, so the equatorial radius is larger than the polar radius.

Climate and geography: Regions along the equator are typically hot year-round, with high humidity and substantial rainfall, supporting tropical rainforest and other tropical ecosystems. Because the Sun's path is near overhead at noon most days, day length is close to 12 hours throughout the year, with only small seasonal variations.

Significance: The equator is a fundamental geographic reference used to define latitude (0°) and to describe global climate zones. It also serves as a basis for navigation, satellite tracking, and coordinate systems. The concept applies to any rotating planet or moon with an axis of rotation.

Name and history: The term equator comes from Latin aequator, from aequare, meaning "to make equal" or "line of equal distance".