December 2006

by Deane Morrison

Three planets gather among the stars of Scorpius this month, and Venus makes a comeback in the evening sky. Even so, easy views of bright planets are few. As we enter the December installment of the theater of the seasons, it becomes clear that the stars are the real stars of the show.

From the eastern wing of the celestial stage comes the panoply of winter constellations, led by the lovely Pleiades star cluster. Nearby, the Hyades cluster fills in the face of Taurus, the bull, whose baleful eye glimmers via the bright star Aldebaran. Above Taurus, Auriga, the charioteer, carries a little she-goat named Capella, a brilliant and multicolored star. Moving further east, we see the Gemini twins, Pollux and Castor. The head of Castor is the site from which the Geminid meteors radiate, peaking this year on the night of the 14th.

Below Gemini are Procyon, in Canis Minor, and Sirius, in Canis Major, the night sky's brightest star. At the center of the whole entourage is Orion, whose dangling sword encompasses a major site of new star formation.

To the west, beyond the Great Square of Pegaus, is a faint cluster of stars known by its catalog name, M15. This is a globular cluster, an ancient grouping of stars nearly as old as our Milky Way galaxy. Recently, University of Minnesota astronomers have found cosmic dust among the stars of M15, a surprising finding because supernova explosions were thought to be the major sources of such dust. Now it appears that the deaths of smaller, humbler stars may have supplied the early dust that seeded the myriad stars like our sun, and produced dust more efficiently than the big guns. M15 can be seen in dark skies with a steady pair of binoculars.

Early risers can watch Mercury, Mars and Jupiter file into the eastern predawn sky. The three planets draw closer until the 10th, when they form a tight grouping just above the horizon about 45 minutes before sunrise. All three will fit in a circle just one degree in diameter, making this the closest trio of naked-eye planets since 1925 and until 2053. That same morning, Saturn will appear beneath a waning moon high in the southwest, next to Regulus, the heart of Leo.

As the month progresses, the trio of planets breaks up. Jupiter climbs higher and Mercury drops out of sight while Mars, the dimmest of the three, stays low in the sky. Late in December, as Scorpius rears its head over the horizon, Mars and Jupiter rise close to Antares, the red heart of the scorpion. As for Saturn, it rises earlier every day, starting about 10 p.m. on the 1st and ending the month with an appearance shortly after 8 p.m. Through most of December, the ringed planet moves westward against the background of stars, a motion that carries it farther from Regulus.

Venus, having passed behind the sun in October, now appears above the western horizon after nightfall. As our shining sister planet puts angular distance between itself and the sun, it climbs higher in the sky and sets later from night to night.

The full moon of December, called the cold moon or the long nights moon by Algonquin Indians, rises about a half hour before sunset on the 4th. As this moon wanes, it glides through Gemini and hangs as a fat crescent just below the bright star Spica, in Virgo, the morning of the 15th. A new moon arrives on the 20th, and, a wispy crescent of the next young moon comes out in the southwest to the left of Venus on the 21st.

The 21st is also the onset of winter, which arrives officially at 6:22 p.m. CST, the moment the sun reaches a point directly above the Tropic of Capricorn. Although this is the shortest day of the year, the earliest sunset will have already occurred about 12 days before and the latest sunrise won't happen until early January.

The University of Minnesota offers public star viewings at its Morris, Duluth and Twin Cities campuses.

For more information and viewing schedules see:

Morris: UMM 16'' Telescope schedule

Duluth: Marshall W. Alworth Planetarium

Twin Cities: Department of Astronomy