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The Sky This Week, 2011 December 6 - 13

A Pacific eclipse, our earliest sunset, and bright winter lights

 Four for the price of one!
Jupiter, Callisto, Ganymede & Europa
imaged 2011 DEC 3, 02:33 UT

The Moon spends the week among the bright stars of the rising winter constellations. Full Moon occurs on the 10th at 9:36 am Eastern Standard Time. December’s Full Moon is variously known as the Cold Moon, Frost Moon, Long Night Moon, or Moon Before Yule. Residents of eastern Asia, Australia, and the western Pacific Ocean will see a total eclipse of the Moon at this time. Residents of Alaska and Hawaii will see the eclipse in its entirety, while folks in the western U.S. will see varying stages of it before local moonset, with Washington state getting the best view from the "lower 48". Luna begins the week a few degrees to the northeast of Jupiter on the evening of the 6th. On the 8th she is only three degrees south of the Pleiades, and over the next several evenings drifts through the heart of the Great Winter Circle.

December 7th marks the beginning of the series of phenomena associated with the winter solstice. This is the evening of the year’s earliest sunset, which in the Washington, DC area occurs at 4:46 pm EST. From this evening onward Old Sol will set a little bit later on successive nights. The change is very incremental at first, but by the time the solstice occurs on the 22nd sunset will be four minutes later. By the end of the year sunset will occur at 4:58 pm. The trade-off comes with the time of latest sunrise. That won’t occur until January 4th, 2012, when the Sun peeks over the horizon at 7:27 am. The shortest day of the year still falls halfway between these dates on the solstice itself, marking the astronomical beginning to the winter season.

The year’s longest nights are accompanied by the brightness of the winter Moon and a bevy of colorful bright stars that give our natural satellite some tough competition. Nine of the 25 brightest stars in the sky are contained within a large oval asterism that surrounds the familiar outline of the constellation Orion. This asterism is popularly known as the Great Winter Circle and consists (moving clockwise around the sky) of Sirius, southernmost member and brightest star in the entire sky; Procyon, Pollux and Castor (the Gemini twins); Capella; Aldebaran; and Rigel, the star marking one of Orion’s feet. Almost set in the middle is the star with the funny name that marks one of Orion’s shoulders, the red supergiant star Betelgeuse. Take the time to admire the varying colors of these stars. Betelgeuse and Aldebaran have noticeably ruddy tints while canella shines with a lustrous gold hue. Rigel and Sirius burn bright blue while Procyon is almost pure white. It’s almost like having a string of holiday lights hanging in the night sky!

Another bright object will probably catch your attention shortly after sunset in the southwest. This is the dazzling glimmer of the planet Venus, now pushing herself ever-higher in the evening sky. By the end of the week she sets about half an hour after the end of evening twilight, and she will continue to beckon from the western horizon well into the new year.

Jupiter also shines brightly in the early evening sky. He becomes visible within minutes after sunset high in the east and makes his way toward the meridian by 8:30 pm. While he’s somewhat dimmer than Venus he still stands out as the brightest object in the sky at this hour, setting the stage for the arrival of Orion and his cohorts. Telescopically Jupiter is the sky’s most rewarding target after the Moon, and he is now optimally placed for observing at a decent hour. Small telescopes should easily show his four bright Galilean moons and prominent equatorial cloud belts. Instruments with apertures of four inches or more should begin to bring out some of the more delicate details in his turbulent clouds.

Mars comes up just before midnight now, accompanying the stars of the constellation of Leo, the Lion into the sky. The best time to view the red planet is still just before dawn, and moderate-aperture instruments should reveal an extensive north polar ice cap and dusky features on the planet’s growing disc.

Saturn rounds out the night’s planet parade near the bright star Spica. Both objects may be seen in the gathering light of dawn in the eastern sky.

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