The upcoming autumnal equinox (it's happening on Sept. 22!) is still a bit of a mystery to me. I know it happens twice a year, and it's a day when the time of daylight and nighttime are roughly equal. But what really causes the equinox, and why isn't the fall equinox on Sept. 21?
I ask the question because there seems to be some confusion as to the exact date the fall equinox occurs, likely because it can fluctuate. The equinox, you see, occurs at the exact moment the sun crosses our Earth's equator — that imaginary line that divides the northern and southern hemisphere. This year, that moments happens to fall on Sept. 22, but Sept. 23 is also a common date. Less frequently, the autumnal equinox will fall on Sept. 21 or 24. And when I say less frequently, I mean it: The last Sept. 21 equinox was 1000 CE, and the next two will be in 2092 and 2096. While the last Sept. 24 equinox was more recent (1931), we'll have to wait even longer for the next one (2303).
If you're looking for a more scientific explanation of why the equinox occurs when it does, here's a condensed backstory that'll make your brain explode.
The Earth moves in two different ways — it spins on its vertical axis (once a day), and it orbits the sun (once a year). This gives us our seasons. When those two motions intersect, we have an equinox.
Just twice a year, as the equinox happens, the sun crosses our equator, moving from one hemisphere to the other (northern to southern, or vice versa). This changes the directions that sunlight falls on our planet. On Sept. 22, sunlight will shift in favor of the southern hemisphere and mark the start of spring, sending the northern hemisphere into the darkness of fall and winter. This continues for three months, culminating with the shortest day of the year: Dec. 21 (the solstice). After this date, the sun begins heading north again, approaching spring (and more sunlight) for the northern hemisphere.
I'm sure the explanation goes far beyond that, but this ain't NASA. Just remember to enjoy our sweet sunlight while we have it — because after tomorrow, we're all on our way to hibernation.
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