The Sky This Week, 2012 December 4 - 11
Jupiter, Europa & Io, 2012 November 30, 03:49 UT
The Moon wanes in the morning sky this week, but she passes a number of bright objects as she moves through the rising stars of spring. Last Quarter occurs on December 6th at 10:31 am Eastern Standard Time. Luna begins the week passing six degrees south of the bright star Regulus on the morning of the 5th. On the 9th you’ll find her just a degree south of the star Spica in the pre-dawn sky. On the morning of the 10th she drifts four degrees south of yellow-hued Saturn. Finally, in the gathering twilight of the morning of the 11th, look for her slender crescent just two degrees south of the bright glow of Venus.
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.
As the Moon moves into the morning sky and the nights approach their longest duration for the year we are treated to one of the most dazzling sights in the natural world. By a coincidence of long-term cycles involving the precession of the Earth’s axis of rotation and others that vary over thousands of years we enjoy dark nights filled with the brightest stars, with the prominent constellation of Orion as the centerpiece. We discussed Orion at length last week, so you should be very familiar with him by now. Go outside at around 10:00 pm and locate the Hunter’s three "Belt Stars" to begin a journey around the winter sky. Follow an imaginary line through the three stars toward the southeast and you will run into the brightest star in the sky, Sirius, the "Dog Star". This star shines so brightly because of its proximity to us at a distance of just over eight light-years. Its distinctive blue color tells us that it is hotter and brighter than our Sun. From Sirius we can move in a clockwise direction to encounter the star Procyon, another nearby neighbor some 11 light-years distant. Continuing on this arc we next run into Pollux and Castor, the Twin Stars of Gemini; we then encounter the beautiful golden glow of Capella, the northernmost of these bright luminaries. Swinging to the southwest we pass the bright planet Jupiter and encounter Aldebaran, the fiery red-tinted "eye" of Taurus, the Bull. Finally we come to Rigel, the bright blue star that marks one of Orion’s knees. Rigel bears a superficial resemblance to Sirius, but physically the two stars are very different. Rigel is a "blue supergiant" star located over 100 times farther away than the Dog Star that shines with the equivalent luminosity of over 50,000 Suns! To put this another way, if Rigel were located at the distance of Sirius, it would appear as bright as the Full Moon in our sky! At the center of this "Great Winter Circle is Betelgeuse, the distinctive ruddy star marking Orion’s shoulder. Within the bounds of the circle are several other bright luminaries, and in this region you’ll find nine of the 25 brightest stars in the sky, a nice treat for light-deprived denizens of the Northern Hemisphere.
Early evening skywatchers can still glimpse ruddy Mars skulking along the southwestern horizon as evening twilight falls. The Red Planet is hanging tough against the advancing Sun as he moves into the din stars of the constellation Capricornus. Mars will remain barely visible in the evening sky until late January.
Jupiter is far and away the focal point of the evening, rising just before sunset and dominating the sky until the first glimmers of morning twilight. Old Jove reached opposition on the 3rd, so now is the optimal time to see him at his best. There is much detail to be seen in his turbulent cloud tops when viewed with a modest telescope, and even a simple spotting scope will easily show his four Galilean moons. If you point the telescope at him at around 10:00 pm EST on the evenings of the 4th and the 9th you should be able to see the famous Great Red Spot as it rotates across his disc, and on the night of the 6th you can watch his large moon Ganymede transit across the planet’s southern polar region trailing its ink-black shadow while the moon Callisto passes just to Jupiter’s north.
If you’re up by 6:00 am during the first few days of the week you can see three planets in the eastern sky neatly lined up like beads on a string. The highest is golden Saturn, some 20 degrees above the southwest horizon. Closer to the skyline is brilliant Venus, and halfway between Venus and the horizon is elusive Mercury. The latter planet will become lost in the solar glare by the weekend, but Venus and Saturn will remain behind, gradually growing farther apart for the remainder of the year.