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