‘Welcome to astronomical fall!’ Skilling on Autumnal Equinox

Weather Blog

The Autumnal Equinox took place at 8:30 a.m. this morning. Welcome to astronomical fall, 2020!! It’s now here!!

There are features of each year’s two equinoxes which are unique to them. Before getting into some of the reasons, it’s worth posting the definition of equinox: “The time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of equal length (about September 22 and March 20).”

If you look at Chicago sunrise/sunset times, it appears day and night aren’t of equal length. But they actually are.

Here’s why it doesn’t appear day and night are equal. Questions about this come up each equinox. EarthSky’s Bruce McClure has produced an excellent explanation. Read it here

As anyone who visits this page knows, the work of climatologist Dr. Brian Brettschneider is endlessly fascinating to me. His post and map of the length of days on the equinox across the continent on the equinox is a great example. Check it out below and you’ll see how similar the length of day is everywhere on the continent. The difference that is illustrated in this graphic amounts to about 6 minutes and has to due with the refraction of the sun which occurs with latitude (i.e. distance from the equator).

Also of interest is a post from National Weather Service headquarters which points out that the “terminator” on a full disc weather satellite image—that’s the dividing line between sunshine and darkness—is perfectly vertical. That would not be the case at other times of the year—there would be a “slant” to the terminator.

Watch this animation of the full disc satellite imagery showing this perfectly north to south “terminator” traversing the disc of the planet:

By comparison, here’s the angled-appearance of the terminator as animated on the first day of winter. It slopes from northwest to southeast because in winter, days in the northern hemisphere are much shorter than in summer. Days end much more quickly because of this in winter.

Thanks to/and credit for this winter satellite animation goes to CIMSS at the University of Wisconsin-Madison.

Equinox myth: Let’s again debunk the myth that eggs unqiuely stand on their ends on the equinox. Any egg that you can get to stand on its end on an equinox—-and there are eggs that will do this—will stand on its end at times of the year other than the equinox. There are NO physical properties of the equinox which make this happen uniquely on the equinox. The “egg standing on its end on the equinox” is a wives tale which doesn’t go away.


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