During an equinox, places around the globe experience nearly equal amounts of daytime and nighttime. The second occurrence this year happens on Sunday.
This Sunday, the autumnal equinox is upon us, and with it, a shift to shorter days, longer nights and the astronomical start of fall in the Northern Hemisphere.
Equinoxes occur when Earth reaches a point in its orbit where the sun shines directly on the Equator. This happens twice a year, in March and in September, and is a result of Earth’s spinning on an axis that is tilted 23.5 degrees from the plane of its orbit. During an equinox, places around the globe experience nearly equal periods of day and night.
Here are five interesting facts about the fall equinox.
Its timing can change from year to year.
The autumnal equinox always occurs in September, but its date can shift each year. That’s because Earth takes slightly longer than a calendar year — 365 days, 6 hours and 9 minutes, to be exact — to complete one full orbit around the sun.
Last year, the autumnal equinox was on Sept. 23. But this year, according to the National Weather Service, it falls on Sept. 22, at 8:44 a.m. Eastern time.
The astronomical start of autumn differs from its meteorological start.
The ever-changing dates of equinoxes and similar events cause the timing and length of the astronomical seasons to vary, which means they are not useful for scientists analyzing climate trends from year to year.
Instead, meteorologists rely on a separate system that establishes seasons by the months of the calendar. Meteorological fall began on Sept. 1 and will conclude at the end of November.
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