The Winter Solstice Has a Surprising Secret

If you live in the midlatitudes of Earth’s Northern Hemisphere—and there’s a pretty good chance you do—you’ve probably noticed the days getting shorter and the nights growing longer over the past few weeks. This process started at the time of the June solstice, was fastest during the equinox in September and culminated at 10:27 P.M. EST on December 21 (3:27 A.M. UTC on December 22).

At that time, the sun was at its southernmost point in the sky, or, if we take a more cosmic perspective, Earth’s northern axis was tipped the farthest from the sun that it gets all year. We call that moment the solstice, and many people consider it the beginning of winter.

(Note that at this same moment, Earth’s southern axis was tipped most toward the sun, so for people living south of the equator, the seasons are opposite, and yesterday can be thought of as the first day of summer. Living on an orb zipping around a star can be complicated, and it pays to keep an open mind to other peoples’ perspectives. But either way, Earth’s axial tilt is the reason for the season.)

There are two major effects we feel from this in the Northern Hemisphere. One is that the sun’s path across the sky is the lowest it will be all year. The sun doesn’t get up as high in the sky, so its light doesn’t heat the ground as efficiently, and our half of Earth gets colder. The second is that the time the sun is above the horizon—the length of daytime—is shortest, so there’s less time for it to warm us as well. This also cools our hemisphere, hence winter.

You’d think that if the solstice is the shortest day, then December 21 would have the latest sunrise and the earliest sunset. But—as is always true in the real world—things are more complicated than that.

If you check a table of the sunrise and sunset times for, say, Washington, D.C., you’ll find the latest sunrise around the time of the solstice is not on December 21 but actually on January 5, 2024 (at 7:27 A.M.), and the earliest sunset already occurred two weeks ago on December 7 (at 4:45 P.M.)! That’s a surprise.

What’s throwing off Earth’s timing? The culprit is its orbit—or, more accurately, the shape of its orbit. It’s not a circle but an oval—that is, an ellipse.

The fact that our planet’s orbit is not circular wasn’t known until early in the 17th century. Around 60 years before the start of that century, Nicolaus Copernicus had worked out that the sun, not Earth, was the center of the solar system and that all the planets orbited it. He still assumed those orbits were circular, however. So while conceptually his heliocentric model worked better than an Earth-centered one, it still didn’t accurately predict the positions of the planets. It was Johannes Kepler, using meticulously curated observations made by his mentor Tycho Brahe, who realized those orbits were, in reality, ellipses—a breakthrough insight that at last allowed astronomers to accurately predict planetary positions and better understand our local cosmic neighborhood.

Earth’s orbit is indeed elliptical but still very close to being a circle. The difference between our planet’s nearest and farthest points from the sun during the year is about five million kilometers, only around 3 percent of its average of 150 million km. What Kepler understood is that this slight difference means Earth’s velocity through space changes as well. It moves fastest when it’s closest to the sun (a point called perihelion) and slowest when it’s farthest away (aphelion).

By chance, at this moment in history, perihelion is in early January, not long after the December solstice. So right now Earth is moving slightly faster around the sun than average, and this is what’s messing with our sunrises and sunsets.

From our ground-based perspective, the time it takes for the sun to go all the way around the sky and come back to the same spot is called a solar day. If Earth were fixed in space but still allowed to rotate, the sun would rise, set and then rise again once every 23 hours and 56 minutes.

But it’s not fixed, of course, and instead orbits the sun, taking about 365 days to do so. That means our planet moves about one degree per day (because there are 360 degrees in a circle). This changes the length of the solar day because every day Earth has to spin an extra degree to get the sun back to the same position it was in the sky on the day before. This adds about 1360 of a day—roughly four minutes—to each day’s duration, bringing the length to the familiar 24 hours.

But that’s only on average. At this time of year, when Earth is approaching perihelion and moving faster around the sun, our planet has to spin a little bit more to catch up to our home star, making the day a little bit longer—about 30 seconds or so. It takes more time for the sun to appear to circle the sky once, so this means solar noon—when the sun is due south in the sky—is a little bit later in the day, according to a notional clock keeping an average time. Sunrise and sunset are symmetric on either side of noon, which means that they both happen later in the day as well.

And that, in turn, means the time of sunset on the solstice is later than it was the day before. So we’ve already experienced the earliest sunset. That was on December 7. Conversely, it also means that sunrise occurred a bit later than it did the day before and will continue to do so until around the time of perihelion; the latest sunrise isn’t until January 5.

If you’re having a hard time picturing this, happily, Henry Reich of Minute physics has an animated video explainer for you:

I know, I know. This is still confusing and weird. But it leads to an important point, one I make all the time: the universe is under no obligation to be simple. In many ways, it seems to be, until you start digging down a bit, and then all sorts of complications arise. We may wish everyday experiences such as the passage of time were simple, but nature has other plans.

And once you do see the mechanism of the heavens’ clockwork, you can truly see its beauty and how it profoundly affects everything in our life.