Science & Technology

What Is the Winter Solstice, Anyway?

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Every year, generally on December 21, the Northern hemisphere experiences the winter solstice — the longest night of the year. But why does it happen? How is it so predictable? And what does it mean?

The Dark Night Rises

It's actually pretty easy to envision exactly how the solstices come around each year. Imagine standing 20 feet from a flashlight shining head-on as you spin a basketball on your finger. If the basketball is balanced perfectly — that is, if its axis of rotation is perpendicular to the beam of light — then the upper and lower halves will get the same amount of light as they spin. But if the basketball is slightly tilted so that it spins around an angled axis, the ball won't get the same amount of light everywhere anymore, and either the very top of the ball or the very bottom will never be illuminated.

You probably see where this is going: that tilted basketball is the Earth and the flashlight is the sun. In the Northern Hemisphere, the winter solstice happens on the day that the northern half is leaning furthest from the sun. That also means that when it's the winter solstice in one hemisphere, it's the summer solstice in the other. Because the Earth stays tilted in the same direction even as it orbits, the solstices are always six months away from each other, seeing as they come up at halfway points on the journey around the sun.

Long Dark Night of the Soul

These days, the winter solstice doesn't have a lot of impact on our lives other than driving up the electric bill and signaling that days are about to start getting longer. But historically, it's been a big part of religious practices around the world. Stonehenge is a great example of one of the ways ancient peoples would build their lives around the longest night of the year — each of those giant stones weighed about 25 tons (22.5 metric tons) apiece, and there's a pretty good chance they were there to "catch" the sun before the long night. Every year on the solstice, the sun sets directly between two of the stones, and that's almost certainly not a coincidence. Fortunately, most of our modern solstice traditions involve more hot cocoa than 30-foot slabs of stone.

4 Things You Probably Didn't Know About the Winter Solstice

Written by Reuben Westmaas December 21, 2017

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