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December Starwatch

It's no accident that holidays featuring lights are celebrated in December. This month Venus and Jupiter--and possibly one comet--crank up the wattage for us.

In early December, Venus shines as bright as it ever gets. Hanging like a lantern over the sunset horizon, our brilliant sister planet begins the month as a thick crescent, its face about 30 percent lit. By the end of the month, the crescent has lengthened considerably and thinned to a mere 5 percent of the planet's disk. These changes happen because Venus is circling in for its next pass in front of the sun; soon it will drop from the evening sky.  

Over in the east, Jupiter, in Gemini, rises almost three hours after sunset on the 1st, but cuts the interval to about 20 minutes by month's end. Its giant globe out-radiates all the bright winter stars in its neighborhood, including Sirius, the brightest of all.

Morning viewers can watch Saturn angling its way ever higher above the eastern horizon. Look to the upper right of Saturn for the bright star Spica, in Virgo, and again to the upper right for Mars. The Red Planet approaches Spica all month long and will be high in the south at dawn.  

On the 1st, a waning crescent moon rises between Saturn and low, fast-dropping Mercury. After the new moon on the 2nd, our faithful satellite reappears in the evening sky and glides by Venus on the 5th. On the 18th, a large but waning moon rises near Jupiter, and on the 29th a waning crescent is back below Saturn. But this time Mercury is gone--all the way to the other side of the sun.

December's full moon, known to Algonquin Indians as the cold moon or the long nights moon, is the most distant full moon of the year. Fullness occurs at 3:28 a.m. CST on the 17th. The moon vaults extremely high across the sky that night for the simple reason that when we're near the winter solstice; our patch of the planet tilts away from the sun and, therefore, toward the full moon, which is opposite the sun.

The solstice occurs at 11:11 a.m. CST on the 21st. At that moment the sun reaches its southernmost point, over the Tropic of Capricorn. By then, due to irregularities in Earth's orbit, sunsets will already have started getting later, and it won't be long before we begin to notice the days lengthening again.

And we may just be treated to a holiday show by Comet ISON. 

Don't bet on it, though. As famed comet hunter David H. Levy puts it, "Comets are like cats; they have tails, and they do precisely what they want."

ISON, discovered last year, zipped around the sun on Thanksgiving and follows a steep northward trajectory on the return part of its journey. That's assuming, of course, it has survived the intense tidal forces trying to rip it apart as it skirted the sun. If it's bright enough to see, it will come into view in both the morning and evening skies. Because it moves so fast, you may need a finder chart to spot it.

But if you look about 45 minutes before sunrise on the 6th, you can use a pair of closely spaced stars to find its position. First find Spica, well up in the southeast, then look down and to the left for Saturn. Look left again, and slightly down, to see two closely spaced stars--one above the other--in the constellation Ophiuchus, the snake handler. They are Delta Ophiuchus and Epsilon Ophiuchus. Delta's the one on top.

ISON will be just to the upper right of Delta. The comet's tail will point away from the sun as the solar wind (a fast stream of particles from the sun) blows cometary dust and gas away from ISON's nucleus. University of Minnesota astrophysicists are among the many worldwide who are studying this comet, using big telescopes in Hawaii and Arizona.

ISON moves farther to our upper left each morning as it speeds away from us. It passes closest to Earth on the 26th, when it will be just shy of 40 million miles away. After that it will be goodbye, for good.

Comets may forsake us, but at least the stars and planets are perennial. 

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see:

Duluth, Marshall W. Alworth Planetarium: