Welcome the Sun!

Posted by jns on 7 December 2006

This is the day of the year, 7 December, when I celebrate my own festival of light to welcome the return of the sun.

No, it is not the shortest day of the year, the day with the least amount of sunlight where I am (about 39 degrees north, 76 degrees, 46 minutes west — but the effect only depends on latitude), because it is not the Winter Solstice, which occurs about 21 December.

It is, however, the earliest sunset of the year, a more interesting inflection point. Since I rarely experience sunrise, at least by choice, this is psychologically much more important. Beyond today, the day will appear to me ever so slowly to be getting longer again because after today the sun will start going down later in the evening.

The effect is hardly noticeable at first,^* but by the time we get to the Solstice the day-to-day change in sun-setting time will be noticeably larger. I was happy when I learned about this, the pre-Solstice early-sun-setting day, because it explained for me the feeling I’d always had that once we got past the Solstice it seemed as though the days started getting longer very quickly.^#

The reason for the phenomenon is tougher to explain than to comprehend; I looked at three different versions (one, two, and three), none of which struck me as entirely satisfactory, but feel free to have a go. To make a long story short, I can point out that if the earth weren’t tilted then this curious misalignment of times wouldn’t happen. But then, neither would the seasons, and neither would the apparent position of the sun’s zenith in the sky^** change from day to day.^##

Regardless of all that, I’m always happy to see the sun starting to linger longer at the end of each day.
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^*For those with a calculus vocabulary, the curve of earliest sunset times as a function of date has just passed an extremum and the derivative is still very near zero.

^#Finally, this gives you something to do with those previously useless reports in the newspaper or in the nightly weather forecast that give you sunset and sunrise time: plot the curves for yourself and see when the minima and maxima in sunset and sunrise occur at your latitude.

^**Known as the “analemma”, the figure-8 shape found on precision sun-dials and on globes of the Earth.

^##How much it changes day-to-day depends on one’s latitude and is described by the grandly named “Equation of Time”.