Earliest Sunset is History

Yes!! The earliest sunset of 2009 is history, and the evenings are getting longer. According to data from the weather station at the Bonaire airport, the earliest sunset was on Tuesday or Wednesday. The Sun sets a whole second later tonight. Yippie!

It has been hard to squeeze in those after work bike rides before it gets dark, but it will be getting easier starting today. The total daylight hours in each day is still decreasing until the shortest day around Dec. 21. The sunrises are still getting later and later each day and that counters the later sunsets in the evening. The latest sun rise here on Bonaire comes around January 24 or so.

If you live further North (in the northern hemisphere) the earliest sunset occurs closer to the shortest day on Dec. 21. For example, in northern New Jersey, where I lived as a kid, the earliest sunset occurs around Dec. 8, more than two weeks from now.
Believe it or not, I first noticed this phenomenon by chance, many years ago during my daily after work bike rides. My goal has always been to be back home by sunset, and I had a pretty good feel for just how hard I had to ride to get home on time. One year I suddenly noticed that the sun seemed to be setting distinctly later the first week of December than it had in November. That got me curious.

There was no Internet, back in those pre-historic days, so the next year I observed every
sunset that I could (we lived on the sea side back then) and recorded the times on a calendar. Lo and behold, the earliest sunset was indeed in late November here on Bonaire.

The next step was to simulate the sunset on an early computer planetarium program called SkyGlobe. This shareware program came in the mail on a 5 1/4" floppy disk. The simulated sunsets matched well with my observed sunsets. I've since checked this sunset timing thing out on a number of Web sites. One year I even printed up a set of calendars with the sunrise and sunset times for every day of the year. (It doesn't take much to amuse us, here on our little desert island.)

Most of the Web sites round off the sunset and sunrise times to the nearest minute. The US Naval Observatory gives the data to the tenth of a minute, which allows one to interpolate the earliest sunset pretty well. I was really psyched this year, to see the data from Bonaire to the nearest second. Hooray Bonaire.

If you are wondering about the science behind all these musings, check out this brief article at Sky and Telescope. It is talking about summer sunrises, but the underlying principle is the same.