The autumnal equinox, also known as the September equinox, occurs the moment the sun crosses the celestial equator. During the day this occurs, the length of night and day are nearly equal, as the sun is shining directly perpendicular to the equator. It is for that reason we call it an “equinox,” which is derived from the Latin word for “equal night.” The September equinox occurs on either the 22, 23 or 24 of the month, with this year’s equinox occurring on Sept. 23 at around 3:50am EST. For the Northern Hemisphere, this signals the first day of fall, while in the Southern Hemisphere, it signals the first day of spring. Earth’s axis is typically tilted at a 23.4 degree angle, which means either the southern or northern hemisphere is more tilted towards the sun. During that time period and until the equinox, that hemisphere receives more sun than the other, causing the varying seasons of winter and summer.