The Night Sky: December 2011
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The Night Sky in December 2011
By Harry J. Augensen
Professor of Physics & Astronomy, Widener University
Full "Long Night" Moon on the 10th New Moon on the 24th
Stars and Constellations
On December 22nd at 12:30 am EST, the Sun reaches its southernmost position in western Sagittarius, marking the official beginning of winter in the Northern Hemisphere and summer in the Southern Hemisphere. At this time of year, the Northern Cross, known officially as the constellation Cygnus, stands upright on the northwestern horizon in early evening, with its bright star Deneb shining at the top of the cross. Somewhat below Deneb lies the even brighter Vega, still visible low in the western sky. Fomalhaut, the white jewel of autumn, is now getting low in the southwest. High in the southwest are the four stars comprising the Great Square of Pegasus.
Looking toward the north, the constellation Cassiopeia is now high up, looking like the letter "M." Following Cassiopeia in the northeast is the constellation Perseus, which passes overhead during the late evening hours. The most famous star in Perseus is Algol, known as the "Demon Star" in medieval Arabic culture, because it frequently seemed to "wink." Modern astronomers have found that Algol actually consists of two stars which eclipse each other in their mutual orbit over a cycle of about 3 days, thus creating periodic changes in brightness (hence the winking). Algol is listed in University of Illinois astronomer James Kaler’s book, The Hundred Greatest Stars (New York: Copernicus Books, 2002), which gives a distance to the stellar pair of 93 light years. One of the stars is a blue-white in color, with a surface temperature of 12,000 K (twice as hot as our Sun), while the other is an orange subgiant star with a temperature of "only" 4000 K. The stars are in a tight orbit, separated by only 7 percent of the distance between the Earth and Sun.
By 9 pm, the brilliant star groups of winter move into fine position for viewing. Capella, the yellow-white star in the constellation Auriga, the Charioteer, is ascending in the northeast. High in the east, to the right of Auriga, is Taurus, the Bull, which contains the bright orange-red star Aldebaran. Also part of Taurus is the compact Pleiades star cluster, or Seven Sisters. Dominating the winter night sky is the constellation Orion, the Hunter, with its two very bright stars, reddish Betelgeuse and bluish-white Rigel. Most of Orion’s stars are hundreds of light years from our solar system. Located in the east-northeast between Auriga and Orion is the constellation Gemini, which contains the twin stars Pollux and Castor. After 9 pm, the brightest star in the night sky emerges in the southeast, below Rigel. This is blue-white Sirius, the "dog star" of the constellation Canis Major.
Naked-Eye Planets In the Evening and Morning Sky
Venus continues to slowly climb into better view with each passing week, glowing like a brilliant yellow star above the western horizon at dusk. On December 1st, Venus sets less than two hours after sunset, and by New Year’s Eve, Venus is setting about two and a half hours after the Sun, and so should be easily visible in the west from about 30 minutes to nearly two hours after sunset.
Though not as bright as Venus, Jupiter is still an impressive sight. And Jupiter graces the sky for a much longer time interval than Venus, from sunset until the early morning hours. Once night falls, Jupiter radiates like a brilliant cream-colored star in a region of the sky just above Cetus and on the border between Aries and Pisces, two relatively faint zodiacal constellations. Jupiter is already high up in the southeast as darkness falls; in mid-December it transits the meridian at around 8:30 pm, and sets in the west at 3:00 am EST.
Mars, which resembles a bright orange-red star in the constellation Leo, rises in the northeast at around 11 pm EST in mid-December. Orange-red Mars contrasts with bluish-white Regulus, Leo’s brightest star, which lies to the west of Mars. Mars brightens noticeably this month, roughly doubling in brightness between the 1st and the 31st. Mars will continue to increase in brightness over the next several months, until it reaches opposition and maximum brightness in early March.
Saturn lies in the early morning sky, rising around 3:30 am on the 1st and by 2 am on New Year’s Eve. Saturn resembles a bright star located near Spica, the brightest star in Virgo. Spica’s bluish white color contrasts well with the dull yellow color of Saturn.
Mercury reaches inferior conjunction with the Sun on the 4th, when its orbit carries it between the Sun and Earth. Within a week afterwards, the fast-moving Mercury vaults up into the morning sky, rising over 90 minutes before the Sun. Mercury reaches its maximum elongation from the Sun on the 23rd, when it rises around 5:30 am, or over 90 minutes before sunrise. Look for what appears to be a bright yellow star low above the eastern horizon about 30 minutes to an hour before sunrise.
Information on lunar phases and rise/set times of Sun and planets is obtained from the US Naval Observatory Data Services at http://www.usno.navy.mil/astronomy/. Additional information comes from Astronomical Calendar 2011 by Guy Ottewell (Raynham, MA: Universal Workshop, 2011)Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.
For more information on astronomy and weather, visit http://www.widener.edu/stargazing/, then click on Web Links & Resources. A set of free sky maps can be obtained at http://www.skymaps.com/