The Homely Truth About the Shortest Day

Last week we passed this year’s shortest day of the year. (It varies by a day or two in different years.) December 21st takes center stage in the annual drama of encroaching darkness turning to growing light, the grand rebirth, the celestial and uplifting reminder that in any sphere of life, the gloom gives way to brightness.

I always thought the event took place with an elegant symmetry. The darkness seemed to close in evenly from both sides, the sun rising a minute or two later each morning and setting a minute or two earlier in the evening up until December 21st, when the process neatly reversed itself.

scienceblogs.com

scienceblogs.com

But I wasn’t paying attention, it turns out. The changes in the times of winter sunrise and sunset aren’t in synch at all. In reality, the sun keeps rising later and later well beyond December 21st, past Christmas and even into the first week of January. And the times of sunsets change in the opposite way.  Instead of setting earlier right up until the 21st, the sun switches gears about a week before the shortest day and begins to set later and later.

It isn’t until early January that both sunrise and sunset move apart from each other again, lengthening the days. The shortest day is the shortest only because the speed of the changes in the times of rising and setting up to the 21st are such that the duration of daylight does decline until the infamous day when the world is darkest and yet the renewal begins.

So the shortest day grows out of a ragged process, not the aligned and symmetrical one I had assumed. The lesson is that we sometimes pull the meanings we need from our approximations of astronomy with no harm done.

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