The Sky This Week, 2013 December 10 - 17
|Orion and friends
Imaged 2011 DEC 31, Morattico, VA
The Moon moves from autumn's dim constellations to the bright stars of the Great Winter Circle this week. She also waxes through the gibbous phases to Full Moon, which occurs on the 17th at 4:28 am Eastern Standard Time. Her bright light drowns out all but the brightest object in the sky, but fortunately at this time of the year we have many bright objects to enjoy in her glow. December's Full Moon is variously known as the Cold Moon, the Long Night Moon, and the Moon Before Yule. She seems to draw more attention than usual at this time of the year due to her persistence in the sky on the year's longest nights as well as her high declination in the northern sky. Look for the Pleiades star cluster about six degrees north of Luna on the evening of the 14th. On the following night she passes just two degrees north of the bright star Aldebaran in Taurus, the Bull.
Luna’s presence puts something of a damper on the annual Geminid meteor shower, which should be at its peak in the early morning hours of the 14th. Over the past several years this has been the most dependable annual meteor display, with single observers in dark locations able to spot some 50 meteors per hour. This year the waxing gibbous moon will limit dark-sky visibility to the hour or two before dawn on the 13th, but for those of you willing to brave the chill it should be a fine display. This is one of the few annual showers whose radiant rises well before midnight, so if you're willing to peer through the moonlight you can expect to see a half-dozen meteors per hour after around 10:00 pm. The shower is active for several days on either side of the peak, so if you missed them on the night of the 13th/14th you should still see a few the next night. The Geminids seem to be a relatively new shower, having first been identified in the 1860's. Their hourly rate steadily increased from about a dozen per hour to around 80 by the close of the 20th Century. For the past dozen years they have stabilized at the current rate of 50-60 per hour. They are associated with the unusual asteroid (3200) Phaethon, the only meteor shower known that's not associated with a periodic comet.
In these last frantic weeks before the holidays many of us will be out running errands, visiting friends, and gathering with our families. The late evening hours are a perfect time for a short stroll before bed to work out the last of the day's kinks and to stop and have a few moments of peace under the stars. By 10:00 pm the winter constellations will be commanding your attention in the southeastern sky, led by the striking figure of Orion, The Hunter. Orion's stars blaze with a distinct bluish color with the exception of one, the star that marks the Hunter's shoulder, Betelgeuse. This star has a distinctive ruddy hue that sets it off from its companions. Betelgeuse is red because it is a highly evolved star that's reaching the twilight of its years. Changes in its internal structure have caused it to swell to gargantuan proportions; if it were to occupy the Sun's place in our solar system the Earth would be orbiting inside its outer layers! This vast surface area thus glows with a cooler red light than the surfaces of the younger blue supergiant stars that populate the rest of the Hunter's figure.
Surrounding Betelgeuse are a dozen more bright luminaries from several different constellations. In fact, nine of the 25 brightest stars in the sky may be found gracing our longest nights.
The brightest object in this part of the sky is not a star at all, but it is still a giant compared to Earth. Jupiter now rises shortly after sunset and is now well-placed for viewing by 10:00 pm. Old Jove is currently located among the stars of Gemini and beams a cheerful light golden glow from his lofty perch. After the Moon, Jupiter is far and away the most satisfying object to view in the telescope, providing a good target for instruments of almost any size. Owners of modest instruments of four inches or more aperture can watch the shadow of Jupiter's innermost moon Io cross the planet's face on the night of the 13th. On the following night you can watch the next moon, Europa, project its shadow on the planet's billowing clouds.
In the early evening Venus is now at her best showing for the year. She becomes visible in the southwest as soon as the Sun goes down, and by the end of evening twilight she is a beacon in that part of the sky. You'd better act quickly to catch her show, though. Over the next couple of weeks she will beat a hasty retreat toward the Sun, and by the year's end she will be wallowing in bright twilight.
The pre-dawn sky finds the rising stars of spring pushing the winter constellations over to the west. High in the eastern sky look for ruddy Mars diligently plodding eastward toward the star Spica. The red planet is gradually brightening as Earth catches up to him, and he's now second in brightness to the star Arcturus in this part of the sky.