Image Credit: NASA
Winter began with the solstice at 12:11 p.m. (EST), on December 21.
If you are like me, you were taught that the Winter Solstice is the shortest day of the year.
Even NASA tells us that in the Northern Hemisphere, the Winter Solstice is the shortest day of the year:
The Winter Solstice is the point during the year when the Northern Hemisphere is the most inclined away from the Sun. After the solstice . . . the days begin to lengthen.
So you might be surprised to learn that Patrick Di Justo writes in The Atlantic that is a fantasy that the Winter Solstice is the shortest day of the year:
Actually, that's a fantasy. The earliest sunset really comes in the first week in December, and the latest sunrise occurs in early January. Yet December 21 really is the shortest day of the year.
Di Justo explains it all has has to do with Earth's non-circular orbit and its tilt in relation to the Sun:
The Earth’s orbital eccentricity (the degree to which the orbit deviates from a perfect circle) makes the distance at perihelion—the point in a year where the Earth is at its closest to the Sun—about 91,402,000 miles. Every go around the sun, perihelion occurs around January 4, two weeks after the solstice. And that makes all the difference to the timing of the earliest sunrise.
Why? As Johannes Kepler discovered in the 1600s, when an object is closer to the Sun, it moves more quickly; when it is farther away, it moves more slowly. On average, the Earth orbits the Sun at 18.508 miles per second. But in December and January, as it gets closer to the Sun, the Earth speeds up, until, at perihelion, on or around January 4, it zooms along in its orbit at 18.664 miles per second. A paltry difference of .156 miles per second at first glance, but it means that near perihelion, the Earth is moving around the Sun just under an “extra” 13,500 miles each day.
In our provincial Earthbound way, we experience this speed change as the Sun moving farther to the east each day. Because of this, the Earth has to rotate a few seconds more each day to 'catch up' to the sun rise, and a few seconds even more to see the sun set. Over the course of the year, this difference of seconds—the “perihelion change”—adds up. On some days of the year, the Sun can rise and set nearly 8 minutes earlier or later than it “should” if the Earth's orbit were a perfect circle.
The Equation of Time reaches exactly zero on December 25.
Then there is the effect of the Earth's 23.5 degree tilt relative to the Sun and the ecliptic plane. The Northern axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly. You can see the effect in the picture above or perhaps more easily in the video below:
With the tilt as the Earth orbits the sun, the amount of sunlight that falls on any given location changes depending on whether that location is tilted toward the sun, as in summer, or away from the sun, as in winter. Because of this tilt, the sun can rise and set nearly 10 minutes later than it “should” if the planet were not tilted.
As explained by Di Justo, in early November, the orbital movement toward perihelion coincides with the Earth's tilt to put the Sun more than 16 and a half minutes ahead of where it “should” be if the Earth were in a not-tilted, perfectly circular orbit. This +16 minutes dwindles as the Earth continues its orbit: the Sun is 14 minutes ahead by mid-November, and only 10 minutes ahead by early December. This difference is large enough to cause the earliest sunset to occur during the first week of December.
Regardless of Di Justo's astronomical hijinks, many people feel the need to celebrate the Winter Solstice. It might be because the Winter Solstice marks the beginning of the return of the Sun. Just as likely is the celebration of holidays such as Christmas and Hanukkah.