Twice a year, Earth tilts just right, giving nearly equal amounts of day and night across the globe. One of those moments arrives Monday: the fall equinox, officially ushering in the arrival of autumn.

While equinoxes don’t always fall on the same date, they occur in mid-September. This year’s fall equinox happens Monday, Sept. 22, at 12:19 p.m. MDT, according to NASA . It marks one of only two days each year when the sun shines directly over Earth’s equator, creating nearly equal lengths of day and night.

“Observing and predicting equinoxes is one of the earliest skills in humanity’s astronomical toolkit,” NASA notes. The word itself comes from Latin — “aequus,” meaning equal, and “nox,” meaning night.

The September equinox signals the start of shorter days and longer, cooler nig

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