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Published: June 18, 2009
Q. The summer solstice occurs on June 21 this year, and that will be the longest day of the year. Will it also have the earliest sunrise of the year? -- M.U.
A. No. In Winston-Salem, sunrise will be at 6:04 a.m. Eastern Daylight Time on June 21, and sunset will be at 8:41 p.m. Earlier this month, from about June 11 to June 14, the sun was rising at 6:03 in the morning, but it was also setting earlier, at 8:38 or 8:39, so the days were slightly shorter than on the solstice.
A similar situation occurs in December, when the year's earliest sunset comes a few days before the winter solstice.
The United Kingdom's National Maritime Museum explains why on its Web site at www.nmm.ac.uk/explore/astronomy-and-time/time-fact... "The reason for this has to do with the slight variation in the length of ‘natural' days throughout the year, which results from a combination of the elliptical shape of the Earth's orbit around the Sun and the tilt of the rotation axis of the Earth. (The longest natural day is about 51 seconds longer than the shortest.)
"However, for a clock to work, all days measured need to have a fixed, equal length. Each is therefore fixed at the average length of a natural day (this is where the ‘mean' in Greenwich Mean Time comes from). By averaging out the length of each day like this, the clock time at which the sun reaches its highest point slowly drifts backwards and forwards as the months progress. There is a knock-on effect on the times of sunrise and sunset. The earliest sunrise occurs a number of days before the longest day and the latest a number of days after the shortest."
By the way, the local sunrise and sunset times cited in this answer came from the Astronomical Applications Department of the U.S. Naval Observatory.
Astronomy buffs can find a wealth of information and data on the department's Web site, at aa.usno.navy.mil/index.php.
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