Light at the End of the Solstice: Sunrise, Sunset, and Celestial Mechanics

Most people are familiar with the idea that the length of the day varies with the seasons. Especially for those in the more northern latitudes, it’s an obvious feature of the winter that the sun rises later and sets earlier than in the summer. Throughout history and across northern cultures, the winter solstice is celebrated to herald the imminent return of light; after the shortest day, things can only get brighter!

One would expect that following the winter solstice — the shortest day — the sun will start both rising earlier and setting later in the day in order to make it longer. That is, we’d expect the sun to be highest at noon and have sunrise and sunset start moving away symmetrically on either side as the days get longer. What surprises many, and left me at a partial loss to explain when asked about it, is the fact that this is not so. In fact, the sun continues to rise later in the morning for days following the winter solstice — and begins setting later days before it! The reason is truly celestial.

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