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The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy

For December 2012, the Moon will be last quarter on December 6th. The first two weeks find the moon waning in the evening sky. The waning crescent moon passes four degrees south of Saturn on December 10th. The moon is only 2 degrees south of Venus on the morning of December 11th. The moon is new on December 13th, and will not interfere with the peak of the Geminid Meteor Shower. This major shower gives us about a meteor a minute in the morning hours, with the radiant in the northeast at sunset and high overhead by dawn. The waxing crescent moon passes six degrees north of faint Mars on December 15th, probably your last chance to catch the red planet for several months. The winter solstice occurs at 5:12 AM CST on December 21st, and winter begins on the shortest day of the year. The waxing gibbous moon passes a half degree south of Jupiter in the evening of December 26th, and covers the planet for observers farther south in Central and South America. The Full moon occurs on December 28th. This is the Long Night Moon, out for about 14 hours for local observers.

Jupiter dominates the evening sky for the next five months, brighter than any other planet now and imbedded among the Hyades and Pleiades Clusters in Taurus. Our feature photo shows Jupiter east of the V of stars that marks the Hyades, the face of Taurus the Bull. The tiny dipper shaped Pleaides, or "Seven Sisters" lie farther west in our photo. Jupiter reaches opposition on December 3rd, and rises in the NE at sunset. Any small telescope will show the four large Galilean moons around at 10X or higher power, but larger scopes will be needed to show the famed Great Red Spot, which seems to be shrinking currently.

Venus is still visible in the morning sky, but heading behind the sun in early 2013, not to emerge in the evening sky for several months. Likewise Mars and Mercury lie too close to the sun to be easily seen this month. Saturn rises about midnight in Libra, and the famed rings are now open about 10 degrees. They were edge on as seen from the Earth in 2009, and will be tilted 27 degrees toward us at solstice in 2017, when they will double the planet’s present brightness.

The square of Pegasus dominates the western sky. The constellation Cassiopeia makes a striking W in the NW. She contains many nice star clusters for binocular users in her outer arm of our Milky Way, extending to the NE now. Her daughter, Andromeda, starts with the NE corner star of Pegasus’’ Square, and goes NE with two more bright stars in a row. It is from the middle star, beta Andromeda, that we proceed about a quarter the way to the top star in the W of Cassiopeia, and look for a faint blur with the naked eye. M-31, the Andromeda Galaxy, is the most distant object visible with the naked eye, lying about 2.5 million light years distant.

Overhead is Andromeda’s hero, Perseus, rises. Between him and Cassiopeia is the fine Double Cluster, faintly visible with the naked eye and two fine binocular objects in the same field. Perseus contains the famed eclipsing binary star Algol, where the Arabs imagined the eye of the gorgon Medusa would lie. It fades to a third its normal brightness for six out of every 70 hours, as a larger but cooler orange giant covers about 80% of the smaller but hotter and thus brighter companion as seen from Earth.

Look at Perseus’ feet for the famed Pleiades cluster; they lie about 400 light years distant, and over 250 stars are members of this fine group. East of the seven sisters is the V of stars marking the face of Taurus the Bull, with bright orange Aldebaran as his eye. The V of stars is the Hyades cluster, older than the blue Pleaides, but about half their distance. Aldeberan is not a member, but about twice as close as the Hyades; distances in astronomy can be deceiving. Jupiter dominates the constellation Taurus this year, and now is retrograding just east of the Hyades.

Yellow Capella, a giant star the same temperature and color as our much smaller Sun, dominates the overhead sky. It is part of the pentagon on stars making up Auriga, the Charioteer (think Ben Hur). Several nice binocular Messier open clusters are found in the winter milky way here. East of Auriga, the twins, Castor and Pollux highlight the Gemini. You can associate the pair with Jason and the Golden Fleece legend, for they were the first two Argonauts to sign up on his crew of adventurers.

South of Gemini, Orion is the most familiar winter constellation, dominating the eastern sky at dusk. The reddish supergiant Betelguese marks his eastern shoulder, while blue-white supergiant Rigel stands opposite on his west knee. Just south of the belt, hanging like a sword downward, is M-42, the Great Nebula of Orion, an outstanding binocular and telescopic stellar nursery. It is part of a huge spiral arm gas cloud, with active starbirth all over the place

Last but certainly not least, in the east rise the hunter’s two faithful companions, Canis major and minor. Procyon is the bright star in the little dog, and rises minutes before Sirius, the brightest star in the sky. Sirius dominates the SE sky by 7 PM, and as it rises, the turbulent winter air causes it to sparkle with shafts of spectral fire. Beautiful as the twinkling appears to the naked eye, for astronomers this means the image is blurry; only in space can we truly see "clearly now". At 8 light years distance, Sirius is the closest star we can easily see with the naked eye from West Florida. You must be in south Florida to spot Alpha Centauri on June evenings. Below Sirius in binoculars is another fine open cluster, M-41, a fitting dessert for New Year’s sky feast.

Read past issues of the Sky at Night