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The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy
Escambia Amateur Astronomers' Association

For December, the Moon will be full on December 2nd , the long night moon, up for 14 hours. The winter solstice occurs at 11:47 AM CST on December 21, 2009; this is the shortest day of the year. The first two weeks of December finds the moon waning in the morning sky, passing 5 degrees south of Mars on December 6th; both rise about 9 PM in the northeast. The last quarter moon is on December 9th, and the waning crescent moon passes seven degrees south of Saturn on December 10th. New moon is on December 16th. The Geminid Meteor shower peaks on the morning of December 14th, so the waning crescent moon will interfere little with the approximately a meteor a minute fall of cometary debris from the NE. The very slender waxing crescent moon passes just above Mercury on December 18th low in the SW. The first quarter moon is high overhead on Christmas Eve, and on New Year’s Eve, the moon is again full, a "blue moon", the second full moon in a month.

While the naked eye, dark adapted by several minutes away from any bright lights, is a wonderful instrument to stare up into deep space, far beyond our own Milky Way, binoculars are better for spotting specific deep sky objects. For a detailed map of northern hemisphere skies, about Halloween visit the website and download the map for December 2009; it will have a more extensive calendar, and list of best objects for the naked eyes, binoculars, and scopes on the back of the map. Also available as the next month begins is wonderful video exploring the December 2009 sky, featuring many different objects, available from the Hubble Space Telescope website at:

Venus lies too close to the Sun to see now, and will emerge into the evening sky in early 2010. Mercury joins Jupiter in the evening at midmonth, reaching greatest eastern elongation, 20 degrees east of the setting Sun, on December 18th, then rapidly retrogrades between us and the Sun by month’s end. While Jupiter is the brightest object in the SW evening skies, it too will be swallowed up by the Sun in early 2010. Saturn rises in Leo about 11 PM, but if you use a scope to look for the rings, you find them very thin, almost edge on. Its equinox occurred last August, and the rings will gradually open up to 27 degrees in the next seven years. But our attention should be devoted to that bright red object in the NE after sunset, Mars. The faster Earth overtakes Mars on January 29, 2010, when Mars will be at opposition, rising in the NE at sunset and closest and brightest for the next several years. Mars starts retrograding back westward in mid December, and will be magnitude -.8 with a disk about 13" of arc across by New Years, so that amateur telescopes will reveal its rapidly melting north polar cap and larger surface details. It will not however be nearly as close and large and bright as when it made headlines in August 2003. This view by Chris Vinson of the EAAA in December 2007 is a good approximation of what we can expect to glimpse this winter.

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. To the northeast, 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. Check it out on a clear December evening, and see it the gorgon is winking at you. If so, then instead of being as bright as Polaris, Algol fade to be only as bright as kappa Persei, the star just to its south. Look at Perseus’ feet for the famed Pleiades cluster to rise, a sure sign of bright winter stars to come; 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.

Yellow Capella, a giant star the same temperature and color as our much smaller Sun, dominates the NE sky until even brighter Mars rises about two hours later. 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 (closer to Capella, rising first about 7:30 PM as December begins) and Pollux highlight the Gemini. UWF alumni 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. As appropriate for the twins legacy, Castor is a spectacular binary star, split with good amateur telescopes at about 200X, and Pollux was recently found to have a planetary system in orbit about it as well.

South of Gemini, Orion is the most familiar winter constellation, dominating the eastern sky by 8 PM. The reddish supergiant Betelguese marks his eastern shoulder, while blue-white supergiant Rigel stands opposite on his west knee. The three stars in a row that mark his belt have a Christmas association in Latin America. As "Los Tres Reyes", they stand for the three kings, bringing gifts to the Christ Child. 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. In amateur telescopes, I rank it next of Saturn as the most beautiful thing in the sky. The bright diamond of four very hot, young stars that light it up are the trapezium cluster, visible even in 60mm refractors.

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