Watch: Why December Has The Longest Days

Even though it has fewer hours of sunlight.

23 DEC 2015

If you're living in the northern hemisphere, your December days will consist of the fewest hours of sunlight and the most hours of darkness out of any other month. This occurs because of the way Earth's axis is tilted away from the Sun at the year's end. But as completely counterintuitive as it sounds, despite December having the least sunlight, it also features the longest day of the entire year. How? The latest episode of MinutePhysics explains.


While the average length of a day has been calculated to 86,400 seconds, each day actually has a slightly different length. You need to measure the time it takes for a line of longitude to rotate all the way around to face the Sun, and because Earth is rotating on its axis as it rotates around the Sun, you're looking at a roughly 361-degree rotation to measure the length of a day.

And then things get even more complicated because Earth's orbit isn't perfectly round - it's elliptical, which means it moves nearer and farther away from the Sun as it rotates around it, spinning faster the closer it gets and slower as it draws away. The peculiar way Earth's axial tilt and the eccentricity of its elliptical orbit combine is the reason December has the longest day, says Henry Reich in the video above.

And here's where things start to get real tricky. Because of how gravity affects Earth's spin when it's closest to the Sun, you need to add 8 seconds to the actual length of each day.

"Plus Earth's axis is tilted, which is what gives rise to the seasons, but it also means that at the time of the year when the tilt points towards or away from the Sun, narrower slices of longitude are aimed directly at the Sun," says Henry. "So as Earth moves in its orbit, it has to rotate slightly farther in order for a particular line of longitude to catch up with the direction of the Sun."

Confused yet? Don't worry, the video above has plenty of great diagrams to get you through this strange phenomenon. Watch the latest episode of MinutePhysics and prepare to feel really lost before everything falls into place in your head and makes you feel like a genius. And then you can tell everyone at Christmas why December 22 is one of the most fascinating days of the year.

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