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The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy

For December, the waning crescent moon passes just above bright Venus in the dawn sky on December 3rd. It lies just above Mercury an hour before sunrise on December 5th. It is new the next day. The waxing crescent lies below Saturn in twilight on December 8th, and above it on December 9th. Note we are losing Saturn into the Sun’s glare quickly. The peak for the Geminid meteor shower is the evening of December 13th and the following morning.

About 9-10 p.m., as Gemini is rising in the NE, look for "earthgrazers", zapping through the top of our atmosphere from the NE to SW, some stretching almost all the way across the sky. As the moon sets by 11 p.m., you should see still more meteors, perhaps up to one a minute in the predawn hours. That same morning, the almost first quarter moon passes south of red but fading Mars. The winter solstice and the shortest day occur at 4:23 p.m. on December 21st. The full moon, the "Long Night Moon" occurs on December 22nd. The last quarter moon is on December 29th.

Mercury and Jupiter play tag in the predawn skies, with the pair only a degree apart on December 21st. Venus is the bright planet to the upper right of them, dominating the dawn skies as the year ends. Mars is the only evening planet by month’s end, moving rapidly eastward in Aquarius, but fading fast. Saturn was briefly visible just after sunset early, but gone by midmonth behind the Sun. But our feature graphic for this month is the trajectory of Comet Wirtengen. Normally a faint periodic comet, coming back to perihelion every 5.4 years, but this time around, it will pass within seven millions miles of us on December 16th, and MAY become visible with the naked eyes as it passes between the Pleiades and Hyades star clusters then.

The square of Pegasus dominates the western sky. South of it are the watery constellations of Pisces (the fish), Capricorn (Sea Goat), Aquarius (the Water Bearer), and Cetus (the Whale). Below Aquarius is Fomalhaut, the only first magnitude star of the southern fall sky. It is the mouth of Pisces Australius, the Southern Fish.

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. Comet Wirtanen should be photobombing the clusters between December 15-20th! 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. 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. The Geminid shower’s radiant is just north of Castor, with the peak on December 14th, so perhaps you can get lucky and capture a comet and a meteor in the same wide angle shot then!

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


Prepare for a wonderful Christmas Comet

Get ready for what astronomers are predicting to be a bright, possibly naked eye, Christmastime comet: 46P Wirtanen.

As this comet is slowly closing in on perihelion this December 16 (closest pass by the sun) it will also swing closely by Earth four days later in December and it perhaps might be as bright as 3rd magnitude, or even a bit brighter. That is easily naked eye visible and certainly should be a treat in binoculars.

At 11 million miles from Earth, this will be the 10th closest pass of any comet ever known to Earth when it speeds rapidly by on December 19. But its speed of course is not so fast on any given night as you will see it with the naked eye or even telescopes. As you gaze at this comet among the distant stars telescopically, you WILL be able to watch its position change hour-to-hour as it moves rapidly north-north-eastward each night from far southern skies, into high northern skies by December.

From December 15-18 Comet Wirtanen will be moving between the two magnificent star clusters the Hyades and Pleiades in Taurus, the bull and will be likely naked eye at that time. Note the beautiful rendition offered by astronomers at the Vatican Observatory Foundation for this grouping in the sky, clearly visible with binoculars and a wonderful photographic opportunity.

In early November 46P will be very low and not visible for northern observers, in the constellation of Fornax; however within a few weeks of late November, it will be moving ever-more-quickly into northern skies particularly after the first week of December, where it will move slowly northward through the constellation of Cetus, the Whale.

This comet was discovered in 1948 by astronomer Carl Wirtanen and has a very short orbit around the sun - some 5.4 Earth years to complete one orbit. Since discovery, it has not passed as close to the Earth as in 2018.

Excellent daily updates, including the times of rising, setting and when highest can

Since the comet will be naked eye and quite high in northern skies throughout mid- to late-December, this is an excellent opportunity for all to witness a magnificent miracle in the world of cometary science as this glowing object will pass overhead about one hour before midnight for most of the last two weeks of December. Telescope, binoculars, naked eye and camera equipment will all benefit from the study of this object.

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