Tuesday, October 25, 2011

December 2011 - A Great Month for Planets

All of the bright planets are on good display in December.-

Eclipse awaits observers in western North America, the Pacific, and Asia on December 10th. And all of us can enjoy a rich variety of planetary sights this month.The nightly planet parade begins at dusk with Venus, which appears ever higher and more prominent in the southwest as the month progresses. Jupiter rules the southeast and south these evenings. Mars brightens impressively all month, rising in late evening and shining high in the south at dawn. Saturn trails Mars by three or four hours and glows in the southeast at dawn. Last but not least, Mercury springs up far to Saturn’s lower left to put in a fine dawn showing in the second half of December.
Venus, blazing at magnitude –3.9, appears higher in the southwest after sunset each week, on its way to the peak of a superb apparition in 2012. Skywatchers at mid-A total lunar northern latitudes see Venus about 9° above the southwest horizon 45 minutes after sunset on December 1st and about 18° high at the corresponding time on December 31st. Telescopes show its 12- or 13-wide disk as only slightly out of round — decreasing from 89% to 83% lit over the course of the month.
Jupiter comes into view almost halfway up the southeastern sky at dusk. During December, Jupiter remains impressive but dims a little, from magnitude –2.8 to –2.6, and shrinks a little in telescopes, from 47½ to 43½ wide. It hangs at the border of Aries and Pisces, shining highest in the south around 8 p.m. and setting in the early hours of the morning. Jupiter retro-grades westward against the stars for most the month, barely inching from Aries into Pisces on the 4th. On December 26th the planet halts and resumes direct motion (eastward relative to the stars). Uranus, in western Pisces, and Neptune, in Aquarius, are still well-placed for observation right after dark. For finder charts, see page 53 of the September issue or SkyandTelescope.com/uranusneptune.
Mars rises above the eastern horizon around 11:30 p.m. at the beginning of December and more than an hour earlier by December 31st. Mars brightens dramatically in December, from magnitude +0.8 to +0.2, plainly showing its striking orange-yellow hue to the naked eye. It moves from south-central to southeastern Leo, and its motion slows. That’s an indication that it will begin retrograde motion in early 2012 as it heads towards its March 3rd opposition to the Sun. This month, however, Mars is only going through quadrature, 90° west of the Sun on December 2nd. It reaches the meridian (shining highest in the south) during or before the 1st light of dawn.
Mars finally appears large enough in amateur telescopes that you’re likely to see a few surface features on good nights. The rusty desert world grows from 7.1 to 8.9 wide in December, with its shrinking north polar cap and dissipating north polar hood of clouds tilting towards us. Saturn rises with Spica to its right a few hours after midnight in December, the steady planet slightly outshining the twin-kly star. Their separation grows from 4.7°to 6.1° during the month. They’re about halfway up the southeast sky as morning twilight comes on, and if you fail to see any surface features on Mars these December dawns, just turn your telescope on Saturn.
The tilt of  the rings increases from 14° to almost 15° from edgewise this month — the most open view of the rings we’ve had since 2006. Saturn’s globe is currently less than 17 wide at its equator, but the rings span almost 38 by month’s end.
Mercury is the night’s fi nal planetary attraction, emerging low in the growing light of dawn. The swift planet goes through inferior conjunction with the Sun on December 4th but climbs, rapidly brightening, into the southeast dawn sky in the next few weeks. Mercury glows at magnitude –0.4 when it reaches greatest elongation from the Sun on December 23rd, and telescopes show its disk to be 6.6 wide and 63% illuminated. Around this date Mercury rises in a fully dark sky more than 1½ hours before the Sun.
The gibbous Moon shines near Jupiter on the evenings of December 5th and 6th. The full Moon experiences a total eclipse on December 10th — visible from the American West across the Pacific, Australia, and Asia (see page 58). At dawn on December 19th, the thick waning lunar crescent forms a lovely arc with Spica and Saturn; the next morning it’s below them.
A slimmer crescent makes triangles with Mercury and Antares (use binoculars for the latter) 45 minutes before sunrise on the mornings of December 22nd and 23rd. At nightfall on December 26th a slender waxing crescent moon shines to the right of Venus — always a lovely sight.
The Sun reaches the December solstice at 12:30 a.m. EST on December 22nd (9:30 p.m. PST on the 21st). This is the moment when the Sun is farthest south for the year and begins its six-month return north-ward, marking the start of winter in the Northern Hemisphere and summer in the Southern Hemisphere. 



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