When is the fall equinox, and why does it happen?

When does fall begin in the Northern Hemisphere? Whereas a meteorologist might tell you Sept. 1, the beginning of meteorological fall, an astronomer will inform you that seasons pivot on solstices and equinoxes — waymarkers of Earth's journey around the sun that change where, and for how long, sunlight strikes the planet.

This year, the autumn equinox occurs on Monday, Sept. 22 at 2:19 p.m. EDT (18:19 UTC), according to Time and Date, and it brings roughly equal day and equal night to the entire planet. North of the equator, it's been summer, with more daylight than darkness, so September's equinox marks the first day of fall; south of the equator, it's been winter, with more darkness than daylight, so the equinox brings spring.

September's equinox marks the exact time when the midday sun somewhere on Earth crosses the equator, going southward. That's because it's a transition point, with the Northern Hemisphere tipped toward the sun before the equinox and the Southern Hemisphere after it. It's when Earth's axis is tilted neither toward nor away from the sun, resulting in an almost equal amount of daylight and darkness at all latitudes.

Almost, but not quite — although the word "equinox" comes from the Latin words "aequus" (equal) and "nox" (night), it's a bit of a misnomer. Because light rays from the sun bend, the sun can appear to be above the horizon when, in fact, it has already set. That effect is increased at higher latitudes because the sun takes longer to rise and set. According to the National Weather Service, that means a 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.

That's difficult to measure, and the equinox is generally hard to see and appreciate — but there's a way. If you go outside at sunset on Sept. 22, you'll notice that the sun dips below the horizon due west — something that occurs only on the September and March equinoxes.

The changing direction of shadows at the equinox was noticed by many ancient cultures. At Chichén Itzá in Mexico, seven triangular shadows resembling a snake appear to move down the steps of the Pyramid of Kukulcán on the equinox. At Angkor Wat in Cambodia, the equinox sunrise projects a shadow that aligns exactly with its causeway entrance.

It will remain fall in the Northern Hemisphere until the winter solstice — the shortest day of the year — on Dec. 21. After that, the days will grow steadily longer with the next equinox heralding the arrival of spring on March 20, 2026.