The equator is the parallel of latitude on the Earth at 0 degrees. It marks the artificial boundary between the Northern and Southern Hemispheres.

If we extend the Earth's equator onto the celestial sphere, the equator is mapped to the celestial equator. On the days of the equinoxes, which occur around 21 March and 22 September each year, the Sun is directly overhead as seen by an observer on the equator. At these times, the path of the Sun (the ecliptic) crosses the celestial equator, and the length of daylight and night are equal.