The Sky This Week, 2012 December 26 - 2013 January 2
Jupiter, with Io's shadow & Ganymede
The last Full Moon of 2012 falls on the 28th at 5:21 am Eastern Standard Time. December’s Full Moon is variously known as the Cold Moon, Ice Moon, or Long Night Moon. The latter is particularly appropriate as we are just beginning to enter the time when the nights gradually begin to shorten after the winter solstice. Fortunately, Luna serves as our night-long companion to shed her pale light on the winter landscape as she reaches her highest declination for the year. On the evening of the 26th look for the Moon between the stars that mark the "horns" of Taurus, the Bull. By New Year’s Eve she has drifted to the east, rising just before Regulus, lead star of Leo, the Lion.
If you’ve been paying attention to the times of local sunset over the past week or two you will have noticed that the time of sunset has been gradually getting later. Our earliest sunsets occurred back on December 7, and by New Year’s Eve Old Sol will dip below the horizon some 10 minutes later than he did back then. However, the total length of day is still just one minute longer than it was at the solstice as the week begins, so what’s going on? The answer lies in noting the time of the latest sunrise, which is still getting later each morning and will continue to do so until January 4th. Once we’ve passed this marker the darkest days of the year will truly be behind us.
The bright Moon will do her best to wash out all but the brightest stars in the sky, but fortunately these long winter nights are also lit by nine of the 25 brightest stars in the sky. In some ways this is a good thing for those of you who may have received a planisphere or a beginner’s astronomy book for Christmas. It’s much easier to learn basic constellation patterns when only the brightest of their stars are visible. Quite often, under moonless skies away from city lights, constellation patterns can get lost among the plethora of fainter stars within their bounds, but the natural filter of moonlight helps to avoid the confusion. Everyone should be able to spot the striding figure of Orion rising shortly after sunset, and by the late night hours he and his attendant winter star patterns are well-placed for viewing. Orion’s three "belt stars" serve as a natural guidepost to other constellations in the region. Following the line of these stars down ad to the left will bring you to a bright blue sparkling star, Sirius. This star is very hard to miss as it is the brightest in the sky, and atmospheric turbulence often makes it twinkle furiously. In ancient Egypt the first sighting of Sirius just before sunrise signaled the onset of the annual flood of the Nile, and with it the Egyptian New Year. Today Sirius crosses the meridian at local midnight on New Year’s Eve, ushering our annual calendar change. With a little imagination you can make out a "stick-figure" dog among the stars surrounding Sirius, leaping up towards Orion, and this is indeed Canis Major, one of Orion’s two hunting canines. Now return to the "belt stars" and follow their line in the opposite direction. You’ll run into a prominent rose-tinted star, Aldebaran, which marks the ruddy eye of Taurus, the Bull. You should just be able to make out a sideways "V"-shaped group of stars to the right of Aldebaran which outlines the rest of the Bull’s face. These stars form a true cluster known as the Hyades which is the closest galactic cluster to the Sun. Bright Jupiter is currently perched just north of the Hyades, and if you look a bit farther north you’ll encounter the Hyades’ half-sisters in mythology, the Pleiades. Resembling a tiny version of the Big Dipper, the Pleiades are another nearby star cluster, although they are some three times the distance of their siblings.
Jupiter continues to be the dominant star-like feature in the winter night sky. The giant planet becomes visible in the eastern sky very shortly after sunset and beams down from nearly overhead by 10:00 pm. If you were lucky enough to find a new telescope under the tree this year set it up on the next clear night and enjoy the view of this massive distant world. Virtually any telescope will reveal the four bright moons discovered by Galileo in the winter of 1609 – 1610, and watching them move from night to night through ever-changing configurations can be a source of endless delight. If your telescope is four or more inches in aperture you can also follow the constant changes in Old Jove’s turbulent cloud belts. If your conditions and timing are right you may even catch a glimpse of the famous Great Red Spot, a storm in Jupiter’s atmosphere that has persisted for at least 300 years. Of course larger telescopes will reveal more features, but if you train yourself to have a patient eye you’ll be surprised at how much detail a good small-aperture instrument can offer!
Golden Saturn may be found in the wee hours before dawn in the southeastern sky. The ringed planet occupies a rather barren stretch of sky to the southeast of the bright star Spica. He’s high enough for a quick telescopic peak as the first rays of dawn begin to brighten the eastern horizon.
As the year draws to a close brilliant Venus edges closer to the horizon in the pre-dawn hours. I’ve been watching her sink lower into the barren trees from an upstairs window in my house for the past several weeks, and she is now only visible as morning twilight brightens the eastern horizon. Her time in our sky is now limited, and she’ll disappear into the solar glare shortly after the new year begins.