Behold, the darkest evening of the year approaches! Well, perhaps it has already come. It depends on where you live. You see, the darkest evening of the year — that is, the evening on which the sun sets earliest — is not the evening of the winter solstice. In fact, the darkest evening can be weeks ahead of the solstice!
Over the course of a year, the sun travels through the constellations of the zodiac on a path called the ecliptic. At the summer solstice the sun is at its northernmost point on the ecliptic, in the constellation of Gemini.
As the months pass, the sun moves through Gemini, Cancer, and Leo so that by the autumnal equinox (in the northern hemisphere) it is in Virgo.
By the winter solstice the sun is at its southernmost point on the ecliptic. The sun takes 365.25 days to go all the way around the ecliptic (360 degrees); thus the sun moves through the zodiac at a rate of approximately 1 degree per day.
Imagine that you decide to track this. On the summer solstice, you note where the sun is in the sky at the time when at its highest point — when on the meridian, half-way between sunrise and sunset. You mark that time on the clock. Then, every week for a full year you observe the sun (or better, a shadow cast by a pole) at that exact same time.
Over the course of the year you will see the sun go from high in the sky at the summer solstice to low in the sky at the winter solstice and back to high again. But you will also see something else: the sun’s position (or that of the pole shadow) moves eastward and westward, tracing out a figure-8 pattern called the analemma. This is illustrated below, for the northern hemisphere, using the Stellarium app (ignore the stars — the stars will change over a course of a year, but I could not figure out how to do the analemma against a daylight sky). The red circles show the sun’s position week by week.
Why does the sun’s weekly position trace out a figure-8? Consider the sun at the summer solstice in Gemini. At that point it is moving 1 degree per day, almost directly eastward through the zodiac. But at the autumnal equinox, the sun is moving both eastward and southward, still 1 degree per day. This means that the sun’s rate of eastward movement will be less. Every star crosses the meridian every 23 hours, 56 minutes. The sun crosses every 24 hours, owing to its eastward motion through the zodiac that is contrary to the westward motion of rising and setting, but the 24 hours is an average. The variations caused by the sun’s motion southward and northward cause this time to fluctuate, meaning the sun runs ahead and falls behind of where it would be if it crossed the meridian every 24 hours. What is more, the Earth’s elliptical orbit around the sun causes these variations to be greater around the southernmost portion of its path, so that the southern lobe of the “8” is larger.
Looking at what the analemma looks like in the evening explains the darkest evening of the year.
Consider what it looks like in the evening at 40° N latitude:
Here we can see that the day on which the sun will hit the horizon, or set, earliest is not the day of the winter solstice. The day of the winter solstice is at the bottom of the “8”. The point of the evening analemma that is closest to the horizon indicates the day on which the sun will set earliest. It is arrowed, below, and is a couple of weeks from the solstice.
In fact, at Philadelphia in the US (which is at about 40° N) the sun sets about four minutes later on the solstice than it does on the day of earliest sunset, which is December 7 — two weeks earlier.
This effect is stronger at lower latitudes. Below is the evening analemma at 20° N latitude. The day of earliest sunset is now 3-4 weeks before the solstice.
And here is the evening analemma at the equator. The day of earliest sunset is about 7 weeks before the solstice, and the sunset is about fifteen minutes earlier. Keep in mind, however, that at the equator the day length does not change over the course of a year, like it does at latitudes away from the equator. The daylight period just shifts back and forth a little on the clock because of the analemma.
In contrast, look at 60° N latitude. The day of earliest sunset, the darkest evening of the year, is essentially the same as the solstice. But the day length varies dramatically over the course of a year, with the sun barely getting above the horizon at the winter solstice, and barely dropping below it at the summer.
So watch the sunset time for your locality. On what day does the sun set earliest? What is the darkest evening of the year where you live?
Oh, and one more thing… the analemma is featured in the new 2024 Vatican Observatory calendar. This image is in it, showing three analemmas at once: