The September equinox marks the official start of astronomical autumn in the Northern Hemisphere. (Image courtesy of Anton Petrus via Getty Images)
When does autumn begin in the Northern Hemisphere? A meteorologist might tell you it's September 1, the start of the meteorological fall season, while an astronomer might tell you that the seasons change at the solstices and equinoxes — milestones on Earth's path around the sun that change the location and duration of sunlight that hits the planet.
This year, the autumnal equinox occurs on Monday, Sept. 22, at 2:19 p.m. EST (18:19 UTC), according to Time and Date, bringing roughly equal lengths of day and night across the planet. North of the equator, it was summer, when daylight hours were longer than darkness, so the September equinox marks the first day of fall; south of the equator, it was winter, when daylight hours were longer than darkness, so the equinox coincided with the arrival of spring.
The September equinox marks the exact time when the midday sun at some point on Earth crosses the equator, moving south. This is because it is a transition point: the Northern Hemisphere is tilted toward the Sun before the equinox, and the Southern Hemisphere is tilted after it. At this time, the Earth's axis is tilted neither toward nor away from the Sun, resulting in nearly equal amounts of daylight and dark light at all latitudes.
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Almost, but not quite — Although the word “equinox” comes from the Latin words “aequus” (equal) and “nox” (night), the name is a bit of a misnomer. Because of the refraction of the sun’s rays, the sun may appear to be above the horizon when it has actually set. This effect is magnified at higher latitudes, as sunrise and sunset last longer. According to the National Weather Service, this means that the day on the equinox lasts about 12 hours, 6 minutes at the equator; 12 hours, 8 minutes at 30 degrees from the equator; and 12 hours, 16 minutes at 60 degrees from the equator.
It's hard to measure, and equinoxes are usually hard to see and appreciate, but there is a way. If you go outside at sunset on September 22, you'll notice the sun dipping below the horizon due west — something that only happens on the equinoxes in September and March.
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The changing direction of shadows at the equinox has been observed in many ancient cultures. At Chichen Itza in Mexico, seven triangular shadows resembling a snake seem to move along the steps of the Kukulkan pyramid at the equinox. At Angkor Wat in Cambodia, the sunrise at the equinox casts a shadow that exactly lines up with the entrance to the temple.
In the Northern Hemisphere, autumn will last until the winter solstice, the shortest day of the year, on December 21. After that, the days will gradually become longer, and the next equinox will herald the arrival of spring on March 20, 2026.
Jamie Carter, Social Link Navigator, Live Science Contributor
Jamie Carter is a freelance journalist and regular Live Science contributor based in Cardiff, UK. He is the author of The Beginner's Guide to Stargazing and lectures on astronomy and nature. Jamie writes regularly for Space.com, TechRadar.com, Forbes Science, BBC Wildlife magazine, Scientific American and many other publications. He edits WhenIsTheNextEclipse.com.
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