December 2007

by Deane Morrison

Like a bolt out of the blue, a comet burst on the scene in late October and immediately dominated the astronomical news. If you haven't seen it yet, Comet 17P/Holmes is a fuzzy, very unstarlike ball in the northern constellation Perseus. Having swung around the sun in May, the comet was headed back into the outer reaches of the solar system when it suddenly spewed prodigious quantities of dust and gas, multiplied its brightness a millionfold, and catapulted to celestial celebrityhood.

Comet Holmes is fading, but with a little luck it will be visible during the first half of December through binoculars. To find it, first find the brilliant star Capella, above Orion, and the Pleiades star cluster, northwest of Orion. The comet forms an almost equilateral triangle with the Pleiades and Capella. Look for it high in the sky, not in the direction of Orion; at midnight, the comet will be almost directly overhead. You may also find it by sighting about halfway along a line connecting Capella and another fuzzy spot, the Andromeda Galaxy, visible to the upper left of the Great Square of Pegasus.

Not to be left in the comet's dust, Mars takes the role of December's featured performer. After brightening all year, the planet finally reaches its pinnacle of glory on the 18th, when its irregular orbit brings it within 56 million miles of us--closer than it will be again until 2016. Look for it in Gemini, just above Orion. Compare it to Sirius, the brightest star in the night sky, shining below Orion in Canis Major. Not even the brilliant Dog Star can compete with the fiery glow of the Red Planet at its peak.

But as Mars approaches its big moment, a spoiler lurks ready to dim its luster. That would be the waxing moon, which begins flooding the evening sky with light starting around the 15th and keeps up the barrage for about two weeks.

Nevertheless, December's full moon, called the cold moon or the long nights moon by Algonquin Indians, makes amends of sorts. It rises about a half hour before sunset on the 23rd, and, as the sky darkens, Mars comes out just below it. That night the small red orb and the big white one will travel together, tracing a high arc across the sky.

If you have a good pair of binoculars and a tripod, you may even be able to spot Mars shortly before sunset by looking about a moon-width below and left of the moon. A small telescope ought to be able to pick out features of the Martian landscape such as the large dark area called Syrtis Major, which will be turned toward Earth in December. Earth sweeps by its colorful neighbor on the afternoon of the 24th. But despite being left behind, Mars will remain a fascinating object for months.

Saturn, in Leo, follows Mars by a few hours, rising by11:30 p.m. on the 1st and by 9:30 p.m. on the 31st. If you go out at midnight late in the month, you'll see the bright knot of winter constellations ornamented by Mars, with Saturn trailing to the east. Saturn's rings are only slightly tilted toward us, but they start to open up in the second half of December. Early risers who venture outside an hour or two before dawn will be greeted by Venus in the east, Mars in the west, and Saturn in the south.

The 22nd marks the onset of winter, which arrives with the solstice at 12:08 a.m., 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.