When Is the Earliest Sunset of the Year?

If you open your latest edition of the Farmers’ Almanac, (you have yours, right?) you’ll see that the earliest sunset for the year occurs on December 7, 2024 (at 40° North latitude). But wait, if the winter solstice, which happens on December 21, is the shortest day of the year, how can this be?

Due to a discrepancy between our clocks, which mark off nice, neat 24 hours every day, and the actual rotation of the Earth on its axis, we experience a phenomenon where the earliest sunset of the year does not coincide with the shortest day of the year, but occurs nearly 2 weeks prior in the mid latitudes.

Whereas the astronomical day goes from solar noon to solar noon (a description which marks the highest point of the Sun in the sky at a given latitude, regardless of the date), these solar days aren’t exactly 24 hours every time. They’re close—most everywhere but polar regions—but in the weeks approaching the solstice, an astronomical day is more like 24 hours and 30 seconds.  The only time our days are exactly 24 hours long are between a solstice (when we experience the longest or shortest day of the year) and an equinox (when day and night are more or less of equal length.)

Why The Discrepancy?

The reason for the discrepancy in time is twofold. First, the Earth is tilted 23.5° on its rotational axis relative to the plane of our path around the Sun. In the winter, those of us in the Northern hemisphere are tilted away from the Sun.

Second, the orbit of the Earth around the Sun isn’t a perfect circle. It’s a bit of an oval, or ellipse, and at some points in the orbit we are closer to the Sun, causing the Earth to speed up a bit in the winter, and slow down in the summer. That’s enough to put our clocks a bit out of sync around the solstices, to the tune of about half a minute per day.

And so, because the time on our wristwatches or on our smartphones doesn’t match exactly with the time we observe by looking at the actual movement and position of the Sun in the sky, the earliest sunset of the year falls 2 weeks prior to the solstice at latitudes of 40° N.

While the Sun sets a little earlier at the 40° N. latitude on December 7th (in New York, sunset occurs at 4:35 p.m.), the day itself will be longer in hours (9 hours and 27 minutes) than on December 21st, the shortest day of the year (9 hours and 20 minutes); it’s just that the sunset will come earlier on the clock.

When Is The Earliest Sunrise?

If you are wondering when the earliest sunrise occurs, the pattern is nearly a mirror image of what happens in the winter. The earliest sunrise in 2024 arrives on June 13, about a week before the summer solstice, which is June 21. The latest sunset happens a week after that, on June 27 (at 40° North latitude).