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Winter Solstice

December 20, 2016

If you don’t like winter and you live in the northern hemisphere, then give a little thanks to the laws of orbital mechanics. Because of Earth’s lopsided path around the Sun, winter is the shortest season north of the equator — several days shorter than summer.

Earth’s orbit around the Sun isn’t a perfect circle. Instead, it’s an ellipse, which looks like a flattened circle, with the Sun slightly away from the center. That means our distance to the Sun changes. And that’s where the laws of orbital motion come into play.

Johannes Kepler devised those laws more than four centuries ago. One of them says that if you draw a line from the center of the Sun to the center of a planet, as the planet orbits the Sun that line will sweep out equal areas over equal periods of time.

To do that, a planet must move fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. Earth is closest to the Sun in early January — the start of winter — and farthest at the start of summer, in early July. So Earth moves around the Sun in a hurry during winter, making the season shorter.

This winter, for example, starts tomorrow, at 4:44 a.m. Central Standard Time. That’s the time of the December solstice, when the Sun stands farthest south in the sky for the year. And winter ends 89 days later. By comparison, this past summer lasted almost 93 days — a longer season thanks to the science of orbits.


Script by Damond Benningfield



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