Would you like to know the time of sunset without consulting an almanac or a newspaper or some website? If you live in or near Washington, from February through April you can, to within a minute or two, just by knowing the date. Even in January and May, the error increases only to about 5 minutes.
Because of a happy accident of D.C.’s longitude — near the middle of the time zone — and latitude — around 39 degrees from the equator — you can calculate sunset time from the date very easily.
I am writing this March 18, and the sunset time in March, to within about a minute, is the date in minutes past the hour.
What does this mean? On March 18, the formula gives sunset as 18 minutes past the hour, which is 7:18 p.m.
(You do need to know the hour of sunset before you start. Daylight Saving Time takes care of itself; because the sun still sets at the date in minutes past the hour, even if the hour is artificially changed.)
For February and April, the formula is the date in minutes past the half-hour. So on Feb. 18, sunset would be at 5:30 p.m. plus 18, which equals 5:48 p.m., and on April 18 you’d get 7:48 p.m. These times are all within about two minutes of the correct time.
In January and May, where the formula, as in March, is the hour plus the date in minutes, the error is up to about 5 minutes, but that’s still close enough to be useful.
In June and the first three weeks of July, we’re close enough to the solstice that the formula doesn’t work because the time of sunset changes little from day-to-day, but if you assume the sun sets at 8:32 p.m., you’ll be within 5 minutes of the right time throughout that period.
Sadly, for people in Philadelphia and New York and Boston or even Charlotte and Atlanta, the formula doesn’t work well, but for San Franciscans and Richmonders, it works nearly as well as for Washingtonians.
Of course, in the fall, sunset gets earlier each day, but the date still increases by one each day, so there’s no simple formula. You’ll just have to remember or look it up.
The author, David Policansky, is a retired scientist who previously served as associate director of the Board on Environmental Studies and Toxicology at the National Research Council.