Autumnal Equinox is today!
The autumnal (fall) equinox is the first day of the fall season.
As the earth travels around the Sun in it orbit, the north to south position of the sun changes over the course of the year due to the changing orientation of the earth’s tilted rotation axes. The dates of zero tilt of the earth’s equator correspond to the spring equinox and the autumn equinox.
Equinoxes occur when the axis of rotation of the earth (i.e. the line from the north to south poles) is exactly parallel to the direction of motion of the earth around the sun. This happens on just two days of the year, the spring and autumn equinoxes. This means that day length is exactly the same (12 hours) at all points on the earth’s surface on these days (except right at each pole, where it will be about to change from permanent light to dark, or vice versa).
Seasons are opposite on either side of the equator, so the equinox in September is known as the autumnal (fall) equinox in the northern hemisphere, and is considered the first day of fall. At the same moment in time in the southern hemisphere, it is known as the vernal (spring) equinox, and marks the first day of spring.