You may have noticed that the sunset has been getting later, even in advance of the Winter solstice today. We called Caroline Simpson, a professor of Physics at Florida International University, to explain.

The short answer, she said, has to do with the tilt of the earth and its orbit around the sun.

“The earth is on an elliptical orbit around the sun,” she said. ”And an ellipse is kind of a squashed circle. And the sun is not in the middle of the ellipse. The sun is off to one side of the ellipse. The sunrise-sunset is partly complicated by this elliptical orbit issue, and then further complicated by the fact that the earth’s on a 23 ½ degree tilt.”

It also has to do with the fact that we use clocks now to tell time, and not sundials.

“If our orbit were perfectly circular, that sunrise-sunset thing probably would line up with the solstices. But when you add the combination of the tilt and the ellipticity of the earth’s orbit… where the sun actually is in the sky—that marks something we call apparent solar time. That’s the kind of time kept by sundials. However, because of all these complications, we long ago decided to stick with having 24 hours be the time it takes the earth to rotate once on its axis. And that solar time is based on pretending that the earth’s orbit is circular and that its axis is not tilted,” she said.

So there you have it. A tilted planet, an elliptical orbit, and NOT using sundials means that the Winter solstice is the shortest day of the year, but it’s not the day with the earliest sunset. Here in Southwest Florida, that was November 22 through Dec 7, when the sun set at 5:34. On the solstice, December 21, it will set at 5:39.

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