Happy first day of fall!
The autumnal equinox (sometimes called the September equinox or southward equinox) is the moment when the Sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the September equinox may occur anytime from September 21 to 24.
At the equinox, the Sun as viewed from the equator rises due east and sets due west. Before the Southward equinox, the Sun rises and sets more northerly, and afterwards, it rises and sets more southerly.
The equinox may be taken to mark the end of astronomical summer and the beginning of astronomical autumn (autumnal equinox) in the Northern Hemisphere, while marking the end of astronomical winter and the start of astronomical spring (vernal equinox) in the Southern Hemisphere.