The Homely Truth About the Shortest Day

We passed the shortest day of the year last week. It’s the annual drama of encroaching darkness turning to growing light, the grand rebirth, the celestial, uplifting reminder that in any sphere of life, the gloom gives way to brightness.

I’ve always imagined the event as accompanied by an elegant symmetry. I thought that the darkness closed in evenly from both sides, that the sun rose a little later each morning and set a little earlier until the shortest day on December 21st, when the process neatly reversed at both ends of the day. The sequence, I thought, between early December and early January had the shape of a tall hourglass. The left “sunrise” side sloped in to the right during most of December and the right “sunset” side sloped left, each changing by a minute or two each day. On December 21st, they met at the narrow waist and reversed direction.

scienceblogs.com

scienceblogs.com

Well, it doesn’t work that way. Changes in sunrise and sunset times aren’t in synch. The sun doesn’t neatly rise later each morning until the 21st and then reverse course. It keeps rising later and later well beyond the 21st, past Christmas and into the first week of January. Imagine the left side of the hourglass sloping down and right until it’s well below the waist.

The timing of sunsets changes in the opposite way. Sunsets change direction, from happening earlier to happening later, about a week before the shortest day. Imagine the right side of the hour glass sloping inward not all the way down to the waist but only part way. Such an hourglass would have a weird, uneven tube descending from the upper left to lower right. It isn’t until early January that both sides would be moving apart from each other again.

The shortest day is the shortest only because the speed of the changes in the times of rising and setting vary from day to day. In early December, the sun rises later by a sizable couple of minutes every day, while sunset drags on at almost the same time, so the length of daylight shrinks until the 21st. After that, the changes in sunrise slow way down while it is sunset’s turn to pick up the pace, getting rapidly later (by about 7 minutes between the 21st and New Year’s Eve in New York) and lengthening the day.

So the shortest day grows out of a ragged process, not the aligned and symmetrical one we thought we were seeing. The universe spins in ways that we don’t or can’t grasp in detail, but we pull the meanings that we need from our approximations anyway.

 

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