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Winter Solstice Puzzle

by | Jan 6, 2020 | Intellectual Property

(This blog post is just a holiday interest story. It does not concern anything legal.)

The winter solstice – the shortest day of the year – usually occurs on December 21st. This is the day of the year when the northern hemisphere is tilted the farthest away from the Sun. Because of this tilt, a smaller portion of the northern hemisphere is illuminated by the Sun than at other times of the year, and the very top of the Earth receives almost no sunlight at all.

Since the winter solstice is the shortest day of the year, you might think that on this day, the Sun should rise later in the morning than any other day, and similarly, the Sun should set earlier in the evening than any other day. But this is wrong. In Washington DC., for example, the latest sunrise usually occurs around January 6th (about two weeks after the winter solstice, around 7:27 am). And the earliest sunset typically happens around December 5th (about two weeks before the winter solstice, at 4:45 pm). What’s going on?

The answer is: our clocks are not synchronized with the Sun. If our clocks followed the Sun, then the latest sunrise and the earliest sunset of the year would indeed occur exactly on the winter solstice. But if our clocks worked that way, we would have to admit something that we are not willing to admit. There are not exactly 24 hours in each solar day. In fact, the length of a solar day varies by nearly a minute during the year. In December, the length of a solar day is about 24 hours plus 30 seconds. But in September, the length of a solar day is about 24 hours minus 21 seconds. The cumulative effect of this difference – compared to the precise 24-hour clock that we use – is why we observe weird time shifts where the earliest sunset occurs a little sooner in the year than it should, and the latest sunrise occurs a little later in the year than it should.

Why does the length of a solar day vary during the year? The answer to this question is the interesting part of the story. To start, we must remember that the Earth experiences two simultaneous motions: it not only spins on its axis, but it also travels in orbit around the Sun. Because it takes about 365 days for the Earth to go all the way around the Sun, the Earth travels about 1/365 of its orbit every day. This daily progress in the Earth’s orbit is almost exactly one degree (1/360 of a circle). This means the Earth must spin one extra degree in order to line up with the Sun again each day. The result is that one complete cycle of sunlight and darkness – one single 24-hour day on average – represents a rotation of about 361 degrees, not 360 degrees.

Now, if the Earth’s orbit around the Sun was perfectly circular, then the length of a true solar day – the time from one true noon until the next – would be consistent throughout the year. However, the Earth’s orbit is not a circle, but an ellipse. This means the distance between Earth and the Sun varies during the year. The distance actually varies by about 3% (roughly 3 million miles). The speed at which the Earth travels around the Sun also varies by about 3% during the year. Due to gravitational attraction, the Earth’s speed is fastest when it is closest to the Sun, which coincides with the winter solstice. So, at this time of year, the solar days are longer because the Earth is moving faster and therefore farther in its orbit compared to other times of the year. It must therefore spin a little longer to line up with the Sun again. That extra spin takes several extra seconds to complete.

So now you know. The reason why the latest sunrise and the earliest sunset do not line up with the winter solstice is because the solar day in winter is about 30 seconds longer than 24 hours, and our clocks are not synchronized with the Sun. On average, there are 24 hours in each day. But no single day is average. The length of a solar day changes as the Earth orbits the Sun. And as we all suspected, the dark winter days truly are longer than summer days!

Clyde Findley is Special Counsel in the Intellectual Property practice at Berenzweig Leonard. He can be reached [email protected]

1 See https://www.timeanddate.com/sun/usa/washington-dc?month=12&year=2019. 

2 The Earth rotates in the same direction that it orbits the Sun.

3 This paragraph and the following paragraph are adapted from https://medium.com/the-philipendium/a-day-is-not-24-hours-c36ee96078c6.