What Is The Autumnal Equinox?

Fall begins slightly after the midpoint of September. The arrival of fall may be met with joy that cooler temperatures and vividly colored leaves will soon be the norm. For others, the arrival of fall is met with some trepidation that shorter days will soon pave the way for less sunlight and the inevitable arrival of winter.

Fall begins on Saturday, September 23, 2023, in the Northern Hemisphere on what is known as the autumnal equinox. The National Weather Service reports that the 2023 autumnal equinox will arrive at 2:50 a.m. EDT on September 23. Those who are especially anxious for fall to begin can wake up in the wee hours of the morning and enjoy their first comforting spiced treat of the season.

Equinoxes are days during the year when the hours of sunlight and darkness are just about equal, or lasting 12 hours each. At the moment the equinox occurs, the sun will be directly over the Earth’s equator, which contributes to this even distribution of day and nighttime hours. National Geographic describes the equinox as “the event in which a planet’s subsolar point passes through its Equator. It is when solar declination is 0 degrees.” Solar declination refers to the latitude of Earth where the sun is directly overhead at noon. The equinoxes are the only times when both the northern and southern hemispheres experience roughly equal amounts of daytime and nighttime.

After the autumnal equinox, the subsolar point continues to move south as the southern hemisphere tilts toward the sun. Around December 21, the subsolar point hits the Tropic of Capricorn (23.5 degrees S), which marks the winter (December) solstice, or the day with the fewest hours of daylight during the year in the northern hemisphere.

It’s important to note that the equinoxes and solstices are opposite in the hemispheres. The northern hemisphere’s fall equinox occurs on the same day as the southern hemisphere’s vernal (spring) equinox.

The autumnal equinox is a time when hours of daylight and nighttime are equal, something that will not occur again until the arrival of spring.