equinox

Time when the Sun is directly overhead at the Earth's Equator and consequently day and night are of equal length at all latitudes. This happens twice a year: 20 or 21 March is the spring, or vernal, equinox and 22 or 23 September is the autumn equinox.

The variation in day lengths occurs because the Earth is tilted on its axis with respect to the Sun. However, because the Earth not only rotates on its own axis but also orbits the Sun, at the equinoxes the two bodies are positioned so that the line on the Earth separating day from night passes through both of the Earth's poles.