Thursday at 4:49, the Sun will set on the shortest day of the year (only one second less of daylight than Wednesday). My magnificent 8th-grade English teacher, Mrs. Sullivan, said at this point a couple of decades ago, “We gotcha! Every day will be longer than the one before for the rest of the school year.”
She’s correct of course, but there’s an aspect of darkness where we haven’t yet hit rock bottom: late sunrise. Thursday, sunrise will be about about 7:23 am. Friday, it will actually be a bit later. Sunrise will keep getting later until January 5, when it will be 7:27. After that, we turn the corner, with sunrise getting earlier and earlier every day for six months (not counting the daylight-savings shift).
But on the other end of the day, sunset will get later at a faster pace than sunrise is getting later, meaning we get more daylight every day starting after tomorrow. Also, you may not have noticed, but today and Thursday are not the earliest sunsets of the year. Those came back around December 6. Every day for the last two weeks, sunset has been getting a tiny bit later, even as daylight got a tiny bit shorter (because sunrise was moving later at a faster rate).
So here’s one way to look at the Winter Solstice: it’s the first day on which sunset’s daily move later becomes faster than sunrise’s daily move later.
The reason things don’t work out as neatly as you might imagine is a tiny bit complicated, but not too much.
The relevant facts:
1) The Earth rotates on its axis in the same direction it revolves around the sun.
2) The Earth’s orbit is not perfectly circular, and so at times it moves faster around the sun than other times.
Take solar noon, the point of the day when the sun is at its highest point in the sky. In DC, I believe, it will always be due South at this time. When the Earth spins 360 degrees, guess what, we’re not back at solar noon yet, because the Earth also moved along its orbit a bit. We need to spin a tiny bit more to get back to solar noon.
Now remember the elliptical orbit of the Earth. At times we’re closer, and at times we’re further away from the Sun. Newton explained how the Sun’s pull on us is related to its distance from us. When we’re closer, the Sun moves us faster in our orbit. At this time of year, we’re closer to the sun than during most of the year. That means we move around the sun faster, but we don’t rotate faster. Consequently it takes more rotation than normal to get from solar noon to solar noon.
In other words, in December, the Earth’s solar day is a couple minutes longer than the average solar day, which is 24 hours. Our clocks, however, are set to the average solar day, so solar noon moves a bit later, as measured by our clocks, every day. (Today solar noon is 12:06, but seven days from now it will be 12:10.) Ironically, the shortest days of the year in the Northern Hemisphere (measured by daylight) are some of the longest days of the year on the Planet Earth (measured by solar noon-to-solar noon).
If we set our clocks by the solar clock, with 12:00 always falling at solar noon, sunrise & sunset would move neatly like you expect: today and tomorrow would have the latest sunrises and earliest sunsets, and both would start spreading out equally from there. Instead, we make the best approximations we can.
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UPDATE: It occurs to me that some readers may have been looking for a political point in this post. I didn’t have one. I just thought most people would be unaware of these facts, and a few would be interested. If you want a political point, here it is: I should be allowed to illuminate my increasingly dark mornings with whatever kind of lightbulb I want.
