The earliest sunset

Living at 42 degrees north latitude (Boston, Massachusetts) I am jealous of the winter sunshine. I am sorry to see it depart and I am happy to see it return. If you’re like me and you live at a similar latitude, you’ll be glad to know that today is the earliest sunset. In Boston on December 9th, the sun sets at 4:11:32 PM. On December 10th, it sets exactly two seconds later, and on the 11th it sets a further four seconds later still.

But wait! Isn’t December 21st the shortest day? Yes it is, but because of some astronomical fudge factors, the latest sunrise, earliest sunset, and shortest day do not occur on the same day. The latest sunrise occurs on January 3rd. The fudge factor that throws things off comes from the fact the length of a single day is a surprisingly complicated notion. As Bertrand Russell said, “Everything is vague to a degree you do not realize until you have tried to make it precise.” The length of a day was originally defined as the time from one noon (when the sun reaches its zenith in the sky) to the next. But this definition gives a different length of day depending on where you are on the planet, what season it is, and where the earth is in its elliptical orbit around the sun. To keep our clocks regular and responsible, we have defined a uniform “mean tropical day” in which we imagine a fictional sun passing overhead everyday at exactly 12 PM. But the real sun isn’t typically overhead then. Sometimes it is ahead of and sometimes it’s behind this fictional sun. This variance in the occurrence of sundial noon and wristwatch noon is called the equation of time. If you’re curious about the astronomical details, be sure and check out Bob Urschel’s amazing Analemma pages. If, like me, you enjoy knowing when the sunsets will start getting later for you, download the SunGraph program from the same site.

2 thoughts on “The earliest sunset”

  1. Do you think that the latest sunrise and earliest sunset times may have something to do with the christian advent and twelth night? Would the latitude make any difference?

Comments are closed.