The Northern Hemisphere will bid an official farewell to summer on the morning of Sept. 23 and welcome in the fall. At precisely 3:50 a.m. Eastern Daylight Time, the sun will be directly in line with Earth's celestial equator (the equator projected onto the sky) and day and night will last roughly the same amount of time. From there, the days will gradually grow shorter until the winter solstice, at which point they will begin their journey back in the unending cycle that marks our four seasons.
Welcome to the autumnal equinox.
While most of us realize that this special September day marks the start of sweaters-boots-and-pumpkin season (known to some as “fall”), there’s much more to know about the date than first meets the eye. Consider the following facts:
1. The term equinox comes from the Latin “aequus” for "equal" and "nox" for "night," since the equinox (both in the spring and the fall) is the point where day and night are equal.
2. That said, day and night equality is not exactly the case. While the very center of the sun does indeed set 12 hours after it rises, the day begins when the upper edge of the sun reaches the horizon (which happens a little before the center rises), and it doesn't end until the entire sun has completely set, explains The Old Farmer’s Almanac — meaning that the days are still a bit longer. It is a few days after the September equinox when the span of equal day and night occurs.
3. While the autumnal equinox usually falls on Sept. 22 or Sept. 23 every year, sometimes it goes askew. In 1931, it fell on Sept. 24. Why? Because the planet takes 365.25 days to travel around the sun, meaning that every so often, the Gregorian calendar and the sun’s orbit collaborate to push the equinox back a day — but not very often. The next fall equinox slated to happen on Sept. 24 will not be until 2303.
The Harvest Moon appears over the horizon in Washington, D.C. (Photo: CTDPIX [CC by SA 4.0]/Wikimedia Commons)
4. The full moon nearest the autumnal equinox is known as the "harvest moon." During this time of year the moon rises earlier in the evening, allowing farmers to work further into the evening. In years when the harvest moon occurs in October, it is called the “full corn moon” because it often coincides with the corn harvest.
5. Along with the vibrant flood of fall leaves ushered in by the September equinox, the sky often has a colorful display of its own. According to NASA, during the fall, geomagnetic storms happen twice as frequently than the annual average, meaning that it is prime time for viewing the aurora borealis.
6. Brilliant leaves and geomagnetic storms aren’t the only things that ramp up around the time of the autumnal equinox. The creature world responds as well. Case in point? Animals at high latitudes go through biological changes with the change of seasons. Take for example the male Siberian hamster, a rodent that experiences a swelling of the testes up to almost 17 times their normal size when the days begin to get shorter.
7. Between the years of 1793 to 1805, the fall equinox was the official start of each new year according to the French Republican Calendar. The French monarchy was abolished one day prior to the equinox in 1792, so the revolutionaries designed their new calendar to kick off on the equinox. By law, the beginning of each year was set at midnight, beginning on the day the autumnal equinox fell at the Paris Observatory.
8. The spring and fall equinoxes are the only two instances during the year in which the sun rises due east and sets due west. So if you aren’t that sure which way is which, use the equinox to note where the sun is rising and setting, set your inner compass, and never get lost again!
Editor's note: This story has been updated with new information since it was originally published in September 2014.