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. 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.