Today is the summer solstice for those of us in the northern hemisphere, meaning that summer is here and that today will feature the greatest amount of daylight of any of 2007′s 365 1/4 days.
One quirk of this phenomenon that no one but me cares about is that the solstice, despite being the “longest day of the year” (an inapt but popular description), includes neither the earliest sunrise nor the latest sunset. Thanks to the vagaries of astronomy, the earliest sunrise actually occurs about six days before the solstice and the latest sunset is observed about six days after the solstice. The difference is marginal (less than a minute at each end of the day), but worth mentioning.
The graph below is a rough depiction of the sunrise and sunset times in my hometown of Concord, New Hampshire.
A close look at the graph reveals that the trough of the sunrise line (blue) indeed falls a few days before the solstice (dashed vertical line), while the peak of the sunset curve (red) falls a few days later. (The notches represent changes resulting from Daylight Savings Time.) The distance between the sinusoidal curves is greatest at the solstice despite that day offering neither the earliest sunrise (5:06 a.m. on June 16) or the latest sunset (8:30 p.m. on June 27).
The solstice nonetheless includes “about” 15 hours, 24 minutes of sunlight. A breakdown including seconds would be required to tease out the meaningless differences I’m highlighting here.
Sunrise and sunset times change very slowly in the days and weeks on either side of both the summer and winter solstices. For example, in Concord, between May 22 and July 11 — a period of 51 days — the sunrise time remains within a ten-minute range (5:16 a.m. to 5:06 a.m. and back). Similarly, the sunset time ranges from 8:20 to 8:30 and back again between June 2 and July 21, a 50-day span.
Just thought you’d like to know. Wear sunscreen.