Why Isn’t The Earliest Sunset On The Shortest Day?

The sun shines through pine needles. This Friday marks the summer solstice - the first day of summer. Credit: Bob King
The sun shines through the needles of a red pine shortly before sunset. Counter to our intuition, the earliest sunsets of the year happen in early December before the winter solstice. This season, the solstice happens on Weds. Dec. 21 at 4:44 a.m. Credit: Bob King

The dark days are upon us. Welcome to the earliest sunsets of the year this week and early next. Counter to intuition, the sun sets earliest before the winter solstice, not on it. Shouldn’t the sun set earliest and rise latest on the shortest day of the year? Well no, and it’s because our clocks keep steady, uniform time compared to the more “elastic” timekeeping system the sun follows.

A man-made clock will always measure the interval from one noon to the next as exactly 24 hours, but the solar clock can vary by a fraction of a second up to a half-minute from one noon to the next due to the tilt of Earth’s axis and the planet’s changing orbital speed.

The two most obvious solar motions -- rising and setting and traveling across the sky with the seasons -- are caused by Earth's spin and revolution. Credit: Tfr000 / Wikipedia
The rising and setting of the sun is caused by Earth’s spin, while the sun’s eastward movement across the sky with the changing seasons is  caused by Earth’s revolution around the sun. Credit: Tfr000 / Wikipedia

Each day, the sun rises in the east and sets in the west because of Earth’s rotation, but there’s a second more subtle shift happening behind the scenes. Earth’s revolution around the sun causes the sun to glide a short distance eastward (to the left for observers in the northern hemisphere) with each passing day. You can sense the sun’s eastward drift by noting that constellations that are first visible at low in the east at dawn climb into a dark sky a month later. Why? The sun has moved off to the east and no longer punishes them with its glare. If the sun moves east, the constellation is now farther west and higher up in a dark sky.

While talking about the sun moving in the sky, let me caution that it’s not the sun doing the moving — it only looks that way. Rather it’s the Earth that moves around the sun that makes the sun – and constellations – appear to move across the sky. If this sounds a bit confusing, millions of your distant ancestors never figured it out, assuming instead that the Earth sat still at the center of the solar system with everything else, including the stars, revolving around it.

Earth’s elliptical orbit causes our distance from the sun and orbital speed to vary during a year. This weeked we’re both closest and moving fastest. Illustration: Bob King
Earth’s elliptical orbit causes our distance from the sun and orbital speed to vary during a year. This in turn makes the sun appear to move more quickly across the sky (eastward), delaying December sunrises and causing sunsets to grow later early on in the month. Illustration: Bob King

When our planet’s closer to the sun, as we are in December and January, the sun appears to move eastward a little bit faster, covering more ground each day than it does in other seasons. This makes sense because the closer you are to something, the less it has to move for you to notice. Move a pencil held near your eye an inch and the shift is obvious, but the same pencil seen from a block away moved an inch isn’t.

The sun’s greater eastward shift delays the sunrise, making for later sunrises. But a later sunrise also means the sun arrives at the sunset point later. So now we have two things happening: the sun begins to set later at the same time it’s rising later.

In late November and early December, the sun is still sinking to its southernmost point in the sky ( a.k.a. the winter solstice), so its daytime arc is getting shorter and shorter, lower and lower. This southward slide pairs up with the sun’s flight east, delaying sunrise even more. Through early December, the southward-sinking sun also causes the sun to set earlier. But beginning around the Dec. 6, the the sun’s speedier eastern motion overpowers the tiny bit of southern travel remaining (see animation below), causing it to slow down, “stall” for about a week and then start setting later. And that’s why the earliest sunsets occur in early December and grow later by mid-month before the solstice.

The sun's path, shared by the moon and planets, is called the ecliptic. Because of Earth's tilted axis the sun swings high and low along the ecliptic with the changing seasons. Click image to see an animated version. Credit: Durham University
On the winter solstice, the sun reaches its lowest point in the sky on its yearly path called the ecliptic. Immediately after, it begins to move north again, and the amount of daylight begins to increase. Because of Earth’s tilted axis the sun swings high and low along the ecliptic with the changing seasons. Click image if it doesn’t animate. Credit: Durham University

Starting at the solstice, the sun begins moving north again. Its arc steepens and days begin to get longer. But the sun’s extra eastward movement still keeps it rising later for a time until that steady northward creep cancels out the eastern ‘bump.’ Result? Sunrises start coming earlier.  This happens in early January from the mid-northern latitudes, though the dates vary a bit depending on your exact latitude.

With later sunsets and earlier sunrises working in tandem, by mid-January keen-eyed skywatchers begin to notice a increase in the amount of daylight, so that by early February, winter doesn’t seem as dark as it was in December. Make sense? If not, drop me a comment! We don’t often think about it, but the sun is always on the move across the sky, and the Earth’s distance from it changes throughout the year if only by a small percentage. Keeps things lively, doesn’t it?

If you’d like additional details about how the Earth and its motion affect sunrise and sunsets around the solstices, click here. To see the lay of time for the sun’s rising and setting for your city, click here and type in your location.