U of M December Starwatch
Published on Saturday, 01 December 2012 13:50
Written by Deane Morrison, U of M Extension
Finding the king of planets is easy. Look in the east after sunset; it’s the brightest thing around. The later the hour, the farther west Jupiter will have moved, but it still dominates all other objects. It has no rival until Venus rises in the predawn sky.
Jupiter travels in company with Aldebaran, the eye of Taurus. And the night of the 25th–appropriately–the giant planet makes a fetching ornament for a waxing moon.
If Jupiter is high enough, the knot of beautiful winter constellations–Taurus, Auriga, Orion, Gemini, Canis Minor and Canis Major–will all have risen. Because they are close to the planet, you can compare Jupiter’s color and brightness to those stars. For example, Aldebaran is rather yellow or even orangish, but Betelgeuse, in Orion’s northeast corner, is red. Then just a little farther away than Betelgeuse, but above Jupiter, is multicolored Capella, in Auriga, the Charioteer. And Sirius, the brightest star, shines a brilliant white from Canis Major, below Orion. Jupiter is so bright these days because it reaches opposition on the Dec. 2. That’s the moment when Earth laps an outer planet in the race around the sun and the sun, Earth and the planet all line up. It’s called opposition because the planet appears opposite the sun in the sky, just like a full moon. Also like a full moon, Jupiter rises at sunset and stays up all night. As oppositions of Jupiter go, this is a nice one. It’s happening against a very dark sky and the planet is relatively close to us, so it’s very big and brilliant. And not just the day of opposition, but for months on either side of it. It takes Jupiter almost 12 years to orbit the sun. This means that every year it advances about 30 degrees eastward against the background of stars–one-twelfth of the sky–and it takes roughly a year to move through each of the 12 constellations of the zodiac. If Jupiter were stationary, we would catch up to it every 12 months and opposition would be an annual event. But because Jupiter’s forward motion is in the same direction as Earth’s, it takes Earth about 13 months to catch up to it again. And this means there won’t be an opposition next year; we’ll have to wait until Jan. 5, 2014, for the next one. Venus has a last hurrah in the morning sky this month before it sinks into the sun’s foreglow. Between the 9th and 11th, a waning crescent moon sweeps by Spica, the jewel of Virgo, then Saturn and finally Venus, in the predawn hour. Saturn has just passed Venus and continues to climb; by month’s end it’ll be well up in the southeast at dawn. Snow sparkles under a full moon the night of the 27th-28th. Algonquin Indians called this the full Cold Moon; some tribes knew it as the full Long Nights Moon. It sojourns that night in Gemini and reaches perfect fullness at 4:21 a.m. on the 28th. The winter solstice arrives at 5:12 a.m. Dec. 21. “Solstice” means “sun standing still,” and we can see this in the times of the year’s earliest sunsets and latest sunrises, which get stalled. In Minneapolis, the sun sets at 4:32 p.m. from Dec. 3 to 15 and rises at 7:51 a.m. from Dec. 27 to Jan. 7. Due to irregularities in Earth’s orbit, these events don’t coincide with the day of the solstice. The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see: Duluth, Marshall W. Alworth Planetarium: www.d.umn.edu/planet. Twin Cities, Minnesota Institute for Astrophysics (during fall and spring semesters): www.astro.umn.edu/outreach/pubnight. Check out the astronomy programs at the University of Minnesota’s Bell Museum ExploraDome: www.bellmuseum.umn.edu/ForGroups/ExploraDome/index.htm.