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Volume xviii, Issue 2 ~ January 14 - January 20, 2010

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Sky Watch

by J. Alex Knoll

Celestial Navigating

This stuff is cosmic

As the sun sets a few minutes past 5:00 and darkness settles in, the celestial swan Cygnus appears overhead in the northwest, flying toward the horizon, its brightest star Deneb marking the bird’s tail. While the swan dips beneath the horizon by 10pm, it reappears in the northeast around 5am. Inverted, the constellation becomes the Northern Cross.

Trailing Cygnus and looking like a crown or a pointy-roofed house is Cepheus the king. Despite its distinct shape, Cepheus’s stars are no stand-outs. The brightest, Alderamin, is a mere magnitude 2.4, while the rest are no brighter than third magnitude. Yet this constellation deserves its royal status, as thousands of years ago its star Alderamin was the celestial pole star, and in another five thousand years it, and not nearby Polaris, will be again.

Earth spins at a 231⁄2-degree tilt, its north-south axis shifting over a 26,000-year cycle, called the precession of the equinoxes, and pointing at different pole stars. Twelve thousand years ago, Vega in the constellation Lyra was the Pole Star. In ancient Egypt, the pharaohs built their pyramids to face Thuban in Draco the dragon, the North Star of the time.

Unlike most constellations, Polaris’ Ursa Minor, the little bear, has no myths attached to it but came to be around 600bc. While others used the bright stars of Ursa Major to navigate, the great seafaring Phoenicians used Polaris as it was closer to true north and provided greater accuracy. Polaris comes its nearest to true north in 2095.

Below the little bear and Cepheus sits his queen Cassiopeia. Looking like the letter W, this constellation boasts no stars brighter than second magnitude. But these circumpolar stars tell time: When the white star Caph at the queen’s feet points to Polaris, it marks celestial midnight, with the constellation pivoting 360 degrees counter-clockwise every 24 hours.

Illustration: © Copyright 1925 M.C. Escher/Cordon Art-Baarn-Holland; Graphics: © Copyright 2009 Pacific Publishers. Reprinted by permission from the Tidelog graphic almanac. Bound copies of the annual Tidelog for Chesapeake Bay are $14.95 ppd. from Pacific Publishers, Box 480, Bolinas, CA 94924. Phone 415-868-2909. Weather affects tides. This information is believed to be reliable but no guarantee of accuracy is made by Bay Weekly or Pacific Publishers. The actual layout of Tidelog differs from that used in Bay Weekly. Tidelog graphics are repositioned to reflect Bay Weekly’s distribution cycle.Tides are based on National Oceanic and Atmospheric Administration and are positioned to coincide with high and low tides of Tidelog.

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