aequator

Aequātor comes from aequare (to make equal), from aequus (equal). The equator is the imaginary line around the Earth where day and night are equal in length throughout the year. At the equinoxes, every point on Earth has roughly equal day and night. But on the equator, this equality holds year-round. The equator is the line of perpetual equinox.

The concept of the equator predates the word. Greek astronomers knew that the celestial equator — the projection of the Earth's equator onto the sky — divided the heavens into northern and southern halves. The terrestrial equator was understood by the time of Eratosthenes (third century BCE), who calculated the circumference of the Earth with remarkable accuracy. The word 'equator' appeared in medieval Latin as the line was mapped onto geographic knowledge.

Equatorial as an adjective describes anything relating to the equator or the tropics. Equatorial climates, equatorial regions, equatorial Guinea. The word implies heat, humidity, and dense vegetation — the characteristics of the zone where the sun is most directly overhead. In astronomy, an equatorial mount is a telescope mount aligned with the Earth's axis, allowing it to track stars as they move across the sky.

The equator crosses thirteen countries. Ecuador is named for it — the country is the Equator in Spanish. Kenya, Uganda, the Democratic Republic of Congo, Republic of Congo, Gabon, São Tomé and Príncipe, Indonesia, Colombia, Brazil, and several smaller nations all straddle the line. The equalizer runs through some of the most biodiverse and some of the most politically unstable regions on Earth.