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Volume 14, Issue 37 ~ September 14 - September 20, 2006

Sky Watch

by J. Alex Knoll

Finding Light in Dark, Moonless Nights

Brilliant single stars and a river with plenty

Sunset, around 7:15 and creeping more than a minute earlier each night, unveils dark skies this week, as the late-rising moon wanes from Thursday’s last quarter to new.

Every 181⁄2 years or so, the moon’s phase, its orbit around earth and earth’s own orbit combine, adding up to a moon that travels much farther north than typical. At our latitude along Chesapeake Bay, Friday’s moon reaches its most northern declination in 37 years. Look for the moon directly overhead as the sun rises at 6:50am. For several days following, the moon will appear close to the zenith at sunrise, dropping a bit farther to the south each day.

This week, two of the three stars of the Summer Triangle pass the celestial zenith. As darkness settles, Vega, the fifth-brightest star in the heavens, stands at the zenith. As the stars and constellations rotate westward, Deneb moves to the zenith at 9pm.

These late-summer evenings find the Big Dipper hovering above the north horizon, never quite setting from view but as low as the constellation gets from our Bay-area vantage.

The Little Dipper, too, is at its farthest point north, with Polaris, the North Star, about midway above due north at 9pm. Follow an imaginary line arching toward the west horizon and you’ll find Arcturus, the fourth-brightest star in the sky, a red giant in the constellation Boötes.

Sunset finds the teapot-shaped Sagittarius, the last of summer’s zodiacal constellations, low in the south. Sagittarius marks the southern end of the Milky Way, our home galaxy. Staring at Sagittarius, you are looking to the very center, whereas we are perched in an outer arm of the spiral, some 25,000 light years from the center.

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