An equinox is an event that occurs when a planets subsolar point passes through its equator. On Earth, there are two equinoxes every year: one around March 21 and another around September 22. During an equinox, the subsolar point is directly above the equator, and the day and night are of equal length. The word "equinox" is derived from two Latin words - "aequus" (equal) and "nox" (night). The March equinox is the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere, while the September equinox is the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere. Equinoxes are caused by the tilt of the Earths axis, which causes different points on Earth to receive more or less sunlight at different times of the year. If the Earth were not tilted, the Sun would always appear to be directly above the equator, and there would be no seasons.