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The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy

For December, the Moon will be last quarter on December 3rd. The waning crescent moon passes two degrees south of Jupiter in the morning sky on December 4th, and occult Mars for southern hemisphere viewers on December 6th. It passes a degree north of Venus on the morning of December 7th. New moon is December 11th, and the waxing crescent moon will be setting about three hours after sunset on December 14th, not a problem for observing the best meteor shower of the year, the Geminids, peaking the following morning.

The first quarter moon is December 18th. The winter solstice occurs at 10:48 PM on December 21st, and winter begins on the shortest day of the year. The full moon occurs on Christmas Day, and is the "long night moon", staying up 14 hours as opposed to the ten hours of daylight we now get. If you stay up to welcome 2016, check out the waning gibbous moon rising 2 degrees north of Jupiter, both rising just before midnight. Great way to ring in 2016.

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 November 30th visit the website and download the map for December; 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.

Mercury makes an appearance in the evening sky at the end of December, reaching greatest eastern elongation, 20 degrees behind the Sun on December 29. Because it is so far south, it will be hard to spot in the SW evening twilight this time, however. Venus dominates the dawn, with the bright side of the crescent moon passing in front of Venus in the daytime sky, starting at 11:30 AM on December 7th, and Venus coming out from behind the dark side of the moon about 1:02 PM. If you know where to look, this can be observed in broad daylight with the naked eyes and binoculars, and should make for some good telescopic photography.

Mars remains in the morning sky this month, with the moon passing close to it on December 6th. Jupiter has moved into eastern Leo now, and rises about midnight at midmonth. Saturn is lost in the sun’s glare all month, to reappear in the dawn sky by January 2016.

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 Pleiades, but about half their distance. Their appearance in November in classical times was associated with the stormy season, when frail sailing ships stayed in port. Aldeberan is not a member of the Hyades, 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. 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. Below Sirius in binoculars is another fine open cluster, M-41, a fitting dessert for New Year’s sky feast.

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