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The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy

For December 2016, the waxing crescent moon will pass 7 degrees north of Mercury in twilight on December 1st. On December 3, it passes 6 degrees north of brilliant Venus, well up in the SW an hour after sunset. On December 5th it passes 3 degrees north of reddish Mars in the southern evening sky. It is first quarter moon on December 7th. The peak of the Geminid meteor shower this year on December 13th is only a day before the Full Moon, the "Long Night" Moon, so most of these shooting stars will be lost in the glare. The moon reaches last quarter phase on December 21st, the winter solstice. Winter begins at 9:44 AM on this the shortest day of the year. On December 22, the waning crescent moon passes 2.3 degrees north of Jupiter in the dawn sky. Christmas dawn finds the crescent moon midway between the two largest planets, Jupiter west of it, and Saturn in the SE dawn. The slender crescent moon passes 3.6 degrees N of Saturn on the morning of December 27, and the moon is new on December 29th. Only eight more new moons until the spectacular solar eclipse of August 21, 2017, an event that will give us 82% coverage here in North West Florida.

Mercury makes an appearance in the evening sky at the start of December, reaching greatest eastern elongation, 21 degrees behind the Sun on December 11th. Because it is so far south, it will be hard to spot in the SW evening twilight this time, however. Venus dominates the evening sky, with the waning gibbous disk setting over two hours after the sun. 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 southern evening sky this month, fading as the earth leaves it farther behind. Jupiter rises about 3 AM as December begins, NW of the bright star Spica in Virgo. Saturn is lost in the sun’s glare all month, to reappear in the dawn sky by January 2017.

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. 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. 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.

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. 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.

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