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University of Minnesota

Starwatch December 2010

November 18, 2010

Deane Morrison

This full moon was called, for good reason, the cold moon by some Algonquin tribes, the long nights moon by others. It shines at a time when northern dwellers are beset by both freezing weather and the longest, darkest nights of the year.

The lunar eclipse begins at 12:32 a.m. CST on the 21st, when a high and bright moon encounters the Earth’s umbra, or dark inner shadow. Totality lasts from 1:41 to 3:53 a.m., with the moon in deepest shadow at 2:17 a.m. The show ends at 4:02 a.m. as the last sliver of moon emerges from the umbra.

The eclipse unfolds among the wonderfully bright stars of Taurus, Gemini, Orion, and other winter constellations. As the lunar disc darkens, you may notice that dimmer stars, as well as the Pleiades and Hyades star clusters in Taurus, appear to brighten. 

Both the Pleiades and the Hyades are known as “open clusters” because their stars are only loosely bound to each other by gravity. This leads to stars being pulled away by passing stars or gas clouds and lost to the cluster; eventually, most open clusters dissipate. At about 100 million years of age, the Pleiades are quite young as open clusters go, while the Hyades are an elderly 625 million years old. 

Winter arrives just as the day of the eclipse is fading. The solstice occurs about an hour after sundown, at 5:38 p.m. on the 21st. At that moment the sun reaches a point directly over the Tropic of Capricorn and we experience the shortest day of the year. 

December also brings the annual Geminid meteor shower, predicted to peak at 1 a.m. on the 14th. This should be a good year for the shower, despite a pesky waxing moon that sets shortly before 1 a.m. The Geminids radiate from near the star Castor, in Gemini, just northeast of Orion. The meteors represent the fiery deaths of dust from a bizarre object called Phaethon, which is built like an asteroid—that is, has a similar mineral composition—but acts like a comet by shedding dust. 

All month long, Jupiter glows like a beacon in the southwest below the Great Square of Pegasus. Meanwhile, in the southeast, Sirius, the brightest of stars, rises earlier each evening. Try comparing these two objects in the first or last week of the month, when the moon won’t interfere. Can you see the color difference? Also see if you can make out the ruddy hue of Betelgeuse, the star at Orion’s northeast shoulder, and the orange beauty of Aldebaran, the “eye of the bull” in Taurus.

On the morning side, brilliant Venus rules the predawn sky, rising more than three hours before the sun. The bright star above Venus is Spica, the jewel of Virgo, and above Spica is Saturn. If you have a telescope, this is a good month to see Saturn’s rings. They open to 10 degrees from horizontal late in December, and you may spot a dark “break” where the planet’s shadow falls across its glittering girdle.

The University of Minnesota offers public viewings of the night sky at its Morris, Duluth and Twin Cities campuses. For more information and viewing schedules, see: Morris, UMN 16-inch telescope schedule:

Duluth, Marshall W. Alworth Planetarium:

Twin Cities, Department of Astronomy (during fall and spring semesters):

11/18/10 Contact: Deane Morrison, University Relations, (612) 624-2346,

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