The sun also sets

About this time every year, as the sun sets well before 4:30 PM (I live near Boston), I start to pine for the days to get longer again. Which is too bad, because they won’t actually get longer, as every school child knows, until after December 21st, or thereabouts. But I have an ace in my back pocket that most people don’t know about. The sun will start setting later as of about December 9th or so. How can this be, you ask, if the days are still getting shorter? The balance is maintained by the fact that the sunrise will continue happening later and later until January 3rd, leaving the solstice snug on December 21st. But since I never wake up early enough to see the sunrise, then I am free to celebrate my own little pagan happy dance on December 9th.

While poking around the Christmas sale goodies at the Sky & Telescope site, I came across a dandy online sky chart that let me verify the rough time of sunset for my hometown. I got curious to see what the exact time of the earliest sunset would be, and soon turned up at the US Naval Observatory’s site where I could generate the sunset table for a year. Sure enough, 4:12 PM on December 9th ought to just about do it.

If you’re interested in why the earliest sunset is not on the shortest day of the year, the short unsatisfying explanation is that the Earth’s orbit around the sun is a slight oval and not a perfect circle. The long unsatisfying answer is here: Sunrise And Sunset, Position of the Sun. Beware, the answer includes the phrases “obliquity of the ecliptic” and “non-zero eccentricity.” The long satisfying answer involves lots and lots of drawings of the three dimensional geometry of planets which I am too tired to provide.

It’s easy to paint a picture of our ancient forebears clad in animal skins, grunting and chanting through quaint solstice rituals because they were afraid the sun would disappear altogether. But the people who built Stonehenge, for example, were clever and knew their astronomy well. They weren’t afraid the sun would disappear. They were just depressed because it was so damn dark all the time. I bet they celebrated December 9th just like me. May it come soon.