The shortest day of the year, in terms of daylight, is December 21, the winter solstice. But the days will actually begin to feel a bit longer starting this week, two weeks before the solstice. That’s because the earliest sunset of the year has already passed. It occurred this past Friday, December 7.
But shouldn’t the earliest sunset happen during the solstice? No.
Because of the Earth’s elliptical orbit around the Sun, the planet moves faster in its orbit during January (when we’re closest to the Sun) than in July, when we’re farthest away. Because of this motion, the Sun’s path through the sky, when charted on a day-by-day basis, appears to take a lopsided figure-8 pattern astronomers call an “analemma.” See the diagram below:
Because the Sun makes this odd pattern in the sky during the year, it is rarely on the meridian at exactly noon. More often than not, the Sun lines up with the meridian as much as 15 minutes before or after noon. Those who have sundials set up in their gardens are well aware of this discrepancy, known as “The Equation of Time.”
The Sun’s odd looping path also explains why the earliest sunset and the latest sunrise fail to exactly coincide with the solstices. Instead, the earliest sunset occurs about two weeks before the winter solstice, while the latest sunrise occurs about two weeks after the solstice. On December 21, the solstice, the Sun’s position on the analemma is at the very top of the figure-8, whereas on December 7, it’s slightly to the right of the topmost position, causing it to arrive at the eastern horizon a little sooner than on December 21. On January 2, on the other hand, it’s slightly to the left of the lowest position, causing it to arrive at the western horizon a bit later than on December 21.
A similar effect happens during the summer solstice, when the earliest sunrise arrives about a week before the summer solstice, and the latest sunset occurs about a week after the solstice.
Why two weeks in winter and only one in summer? Remember the Earth moves slower in its orbit around the Sun in the summertime because it’s farther away, and faster in the winter because it’s closer. In the winter “covers more ground” in its orbit.