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Volume 15, Issue 2 ~ January 11 - January 17, 2007

Sky Watch

by J. Alex Knoll

Location, Location, Location

Mild weather aside, winter trudges on, with short days and long, dark nights. Regular early risers, however, may have noticed a shift in the break of dawn this past week. Solstice, nigh a month behind, marked the year’s shortest day from sunrise to sunset. But more than six weeks separate the year’s earliest sunset from the latest sunrise, just one more result of earth’s elliptical orbit. Last Friday, January 5, the sun rose at its latest: 7:25:16, Annapolis time.

I specify this as a mea culpa after an astute reader questioned my column last month regarding the time of the earliest sunset of the year, 4:45:51. Puzzled, I realized my time was correct — for Washington, D.C.

Since 1993 I have relied on the same Macintosh-based software program, Expert Astronomer, both to generate my star maps and to calculate astronomical times. No worries there, as these things are written in the stars. Geometry, physics and computing can accurately predict the time of sunrise for the year 2929 — on Mars (5:17am, Mean Martian Time).

For 14 years, I have used Washington as the point of reference for my Sky Facts, never imagining the difference 30 miles can make. But the interactions of two moving spheres — the sun and earth — are strange and complicated.

In Washington, December 7th’s earliest sunset was 4:45:51, whereas in Annapolis it was 4:43:39. Farther north, in Baltimore, the sun set at 4:43:09, while at the mouth of the Patuxent, in Solomons, it wasn’t until 4:45:15.

Last week’s latest sunrise spanned several minutes, too: Washington 7:27:02; Annapolis, 7:25:16; Baltimore, 7:26:34; Solomons, 7:23:17. For the record, I’ll base my sky maps and my times of celestial events hereafter from my backyard in downtown Annapolis, latitude N 38:58:22.6; longitude W 76:30:4.16.

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