There are two equinoxes every year – in September and March
– when the Sun shines directly on the Equator and the length of day and night is nearly equal. Seasons
are opposite on either side of the Equator, so the equinox in September is also known as the autumnal (fall) equinox
in the Northern Hemisphere, and is considered the first day of fall. In the Southern Hemisphere, it is known as the vernal (spring) equinox
and marks the first day of spring.