When is the Darkest Day of Winter?

November 19, 2018
Horizon Sunset

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The darkest afternoon—our earliest sunset—is upon us. For most of us, the darkest day of winter will feel like December 8.

Whoa, hold on. That must be a mistake. Isn’t that a bit early? 

No, mistake. It’s a strange, reliable yearly sequence. First comes the earliest sunset on December 8 this year, if you live around latitude 40 degrees.

Then comes the Winter Solstice on December 21. This is the shortest day, with the fewest minutes of sun. But by then, in typical US, European, and Canadian locations, the Sun is already setting three minutes later.

The next milestone is the darkest morning with the latest sunrise. This is the first week of January. If you’d like to check off further milestones, you could add the statistically coldest week of winter (somewhere between January 16 and 25), and then winter’s midpoint, which used to be Groundhog Day but now falls two days later.

 Why doesn’t the earliest sunset coincide with the solstice on December 21? It has to do with how the Sun syncs up with our clocks and calendars, and with our tilted axis, and with the fact that Earth’s path around the sun is an ellipse and not a circle, and that we therefore speed up and slow down in our orbit. Earth is closest to the Sun on January 2, which makes us whip around it at our year’s fastest speed during December, January, and February.

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This in turn makes our solar day—the moment from when it’s highest in the sky one day to when it’s highest up the next daylonger than average right now. With Earth currently moving faster in its orbit, we have to spin slightly more than one full turn to precisely face the Sun the next day. Result: Our actual solar days are now 24 hours and 30 seconds long. Meanwhile our clocks dumbly and steadily still use the “average” value of 24 hours on the nose. This produces a discrepancy. 

As a result of this and one or two other factors, the real Sun and what our clocks say are a bit out of sync. Bottom line: If you’re winter-weary even though winter hasn’t even started yet, you can celebrate. Since more of us are Sun-aware in the afternoon than in the early morning, in practical terms the darkest day of winter is next Tuesday.

By Wednesday, December 9, afternoons will already be brighter. Sure, it’s just a few seconds at first. But we’ll take it.

(If you live in Australia or anywhere in the southern hemisphere, ignore everything I just said. Better yet, reverse it.)

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe

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