The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy

For December 2011, the Moon will be first quarter on December 2nd. The first week find the moon waxing in the evening sky. The waxing gibbous moon passes five degrees north of Jupiter on December 6th. The full moon is on December 10th, the long night moon by tradition. The waning gibbous moon passes eight degrees south of Mars on December 17th, with both rising about 11 PM. The last quarter moon occurs in the morning sky on December 18th, and it passes south of Saturn on December 20th. The winter solstice occurs at 11:30 PM CST on December 21st, and winter begins on the shortest day of the year. The waning crescent moon passes 3 degrees south of Mercury in the dawn on December 23rd. The new moon occurs on December 24th. The waxing crescent moon passes six degrees north of Venus in the SW sky on December 27th.

Venus dominates the evening sky for the next six months. She is still on the far side of the sun, and telescopically appears as a small, gibbous disk. Mercury passes from evening to morning sky as December begins, with the crescent moon south of it on December 23rd in the dawn. Mars is rising in the NE in Leo about midnight, and will come to opposition next spring. Jupiter is still well placed in the southern sky through March of 2012. Saturn rises about midnight near Spica in Virgo, and the famed rings are now open about 8 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. Saturn is in the morning sky in Virgo east of Spica now, but comes to opposition and moves into evening sky by late March of 2012.

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.

The famed Pleiades cluster

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. Our photo feature for December is this fine view of the Pleiades by Bob Gaskin. The clouds of dust that accompany this young cluster in its tour of the Galaxy are beautiful, and recall the poem "Locksley Hall" by Alfred Lord Tennyson, in which he notes, "Many a night I saw the Pleiads, rising thro’ the mellow shade, Glitter like a swarm of fire-flies tangled in a silver braid." Bob’s fine photo certainly conveys the verse…

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.

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. The pair are associated 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. Below Sirius in binoculars is another fine open cluster, M-41, a fitting dessert for New Year’s sky feast.

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