The Night Sky: December 2008

Contents

The Night Sky in December 2008

By Harry J. Augensen
Professor of Physics & Astronomy, Widener University

Moon’s Phases in Month
  "Full Moon before Yule" on the 12thNew Moon on the 27th

Stars and Constellations

The Sun’s apparent annual path in the sky (an effect of perspective created as Earth orbits about the Sun) takes it through the Ophiuchus/Sagittarius region during December, coinciding approximately with the celebrations of Christmas and Hanukkah.  On December 21^st at 7:04 am EST, the Sun reaches its southernmost position in 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.  High in the north, the constellation Cassiopeia is now looking like the letter “M”, and is followed in the northeast by the constellation Perseus.  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 3 days , thus creating periodic changes in brightness (the winking).

By 9 or 10 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. Just below Orion is the constellation Lepus, the Hare. This faint and often overlooked group contains the variable star R Leporis, which is listed in University of Illinois astronomer James Kaler’s book, The Hundred Greatest Stars (New York: Copernicus Books, 2002) as the reddest naked-eye star in the sky.  Also known as Hind’s Crimson Star after the 19^th century astronomer who discovered it, R Leporis is a pulsating giant star with a diameter of over 300 times that of the Sun.  Its deep red color is due to its unusually cool temperature (for a star) and also to the fact that its atmosphere contains carbon molecules which absorb a significant amount of the blue light.

Located in the east-northeast between Auriga and Orion is the constellation Gemini, which contains the “twin” stars Pollux and Castor.  After about 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 is magnificent in the sky at dusk, mimicking a brilliant yellow star above the southwestern horizon.  Over the past few months, Venus has gotten higher and brighter in the evening sky, and that trend will continue throughout December.  Venus sets about 3 hours after the Sun in early December, but just shy of 4 hours after it on New Year’s Eve.   As a visual bonus, Venus passes close to Jupiter during the first few days of December, and the duo of bright planets makes for a remarkable spectacle in the sky at dusk.

Jupiter’s days as an evening attraction may be numbered, but the Giant Planet still garners attention, especially since it is paired with even more brilliant Venus in early December.  They both reside low above the southwestern horizon at dusk.  While Venus increases its separation from the Sun during December, Jupiter continues to sink toward the evening twilight.  Jupiter sets around 7:30 pm at the beginning of December, but a little after 6:00 pm, or only about 1½  hours after sunset, by NewYear’s Eve.  On that date, Mercury passes very close to Jupiter to produce another eye-catching pairing.

Mercury is invisible at the beginning of the month, lost in the glare of the Sun.  It rapidly ascends into the evening sky, however, and by New Year’s Eve, it sets 1½ hours after the Sun.  Assuming you have clear view of the western horizon, spotting Mercury on this date should be easy:  Mercury looks like a modestly bright yellow star sitting just to the left of much brighter Jupiter.

 Saturn migrates from the morning sky into the late evening sky during December, rising around 12:30 am EST in early December, and by 10:30 pm at month’s end.   Saturn is easily picked out as it rises in the east, resembling a bright yellow star in the constellation Leo below the true bluish-white star Regulus. 

Saturn appears less bright than usual, however, because the normally bright but thin ring system is currently seen nearly edge-on (as even a small telescope will reveal), and thus they will add little to Saturn’s magnitude in the night sky.

Last year in December Mars reached opposition with the Sun and was therefore in prime position for evening viewing.  Mars was also closest to Earth, with a brightness rivaling that of Jupiter. This December, by contrast, Mars is on the far side of its orbit from Earth, reaching conjunction with the Sun in on the 5^th.  It is thus is not viewable at all.  Mars will very gradually re-appear in the morning sky during winter/spring of 2009, but eager Mars watchers will need to wait until late 2009/early 2010 to see Mars regain its former glorious stature in the evening sky.

The Geminid Meteor Shower is expected to reach its peak on the nights of December 13 and 14, as the Earth in its orbit passes through debris shed by the asteroid 3200 Phaethon.  The trails appear to radiate from the constellation Gemini, but they can be seen in nearly any part of the sky. This year a just-past-full Moon will wash out the sky contrast, allowing only the brighter meteors to be seen.

For more information on astronomy and weather, visit the Widener University Public Viewing Website at 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/

Some content for this article has been obtained from US Naval Observatory Data Services