While meteorologists like to call fall at the beginning of September, astronomical autumn starts later, on the autumnal equinox. This year, it falls on Sunday, Sept. 22 at 8:44 a.m. EDT (12:44 UTC), according to the National Weather Service.
At this specific time, Earth’s axis is tilted side-on to the sun, meaning there’s an almost equal amount of sunlight and darkness across the globe.
This celestial geometry marks the changing of the seasons — from summer to autumn in the Northern Hemisphere and from winter to spring in the Southern Hemisphere. The word “equinox” is Latin for “equal night,” one of two days per year when the lengths of daylight and darkness are the same across the globe.
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