Contents

The Night Sky in December 2010

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

Moon’s Phases in Month
  New Moon on the 5th               Full "Long Nights" Moon on the 21st

Stars and Constellations

 The Full Moon in December is often referred to as the "Full Cold Moon," the "Full Moon before Yule," and, because it usually falls closest to the winter solstice (when days are shortest), the "Full Long Nights Moon."

The Sun’s apparent annual path in the sky takes it through the constellations of Ophiuchus and Sagittarius during December. This is, of course, merely an effect of perspective created as Earth orbits about the Sun. On December 21st at 6:38 pm EST, the Sun reaches its southernmost position in Sagittarius, marking the official beginning of winter in the Northern Hemisphere. This also coincides approximately with the celebrations of Christmas and Hanukkah.

The stars of autumn dominate the early evening, but a famous trio from the summer – Altair, Vega, and Deneb – can still be seen before they vanish into the impending winter twilight. These three stars, of course, comprise the Summer Triangle, and in mid-December they set at about 10 pm, 11 pm, and 1 am, respectively. Also still visible, but getting low in the southwest on December evenings is Fomalhaut, the solitary white jewel of autumn. It sets at around 9:30 pm in midmonth. High in the southwest are the four stars comprising the Great Square of Pegasus. High up in the north, the constellation Cassiopeia now resembles the letter "M," and is followed in the northeast by the constellation Perseus.

By 9 or 10 pm, the brilliant star groups of winter move into prime position for viewing. Capella, the yellowish star in the constellation Auriga, the Charioteer, is ascending in the northeast. High in the east, to the right of Auriga, are the stars of Taurus, the Bull, which in early Greek mythology represented the head of the Minotaur, a half-human, half-bull creature. In later legends, Taurus represented a white bull which Zeus changed himself into in order to carry off Europa, the daughter of the King of Phoenicia, to the Isle of Crete. Taurus is officially the second constellation of the Zodiac (after Aries), and contains numerous objects of interest, the most obvious being the bright orange-red star Aldebaran. Aldebaran is a red giant star with a diameter of over 40 times that of our Sun and a luminosity of about 150 suns.

Perhaps the most famous member of Taurus is the Pleiades, or Seven Sisters, a compact star cluster shaped like a miniature dipper. Most people can indeed make out about seven or eight of the Pleiades stars, but these are only the very brightest members. Research telescopes have identified at least 500 member stars. The brightest and most visible members are intrinsically several hundred times more luminous than our Sun, but they appear faint because of their immense distance of over 400 light years from our solar system. The Pleiades cluster is relatively young, only 100 million years old compared with the Sun’s age of 4.6 billion years.

The other notable cluster in Taurus is the Hyades, which, at a distance of about 150 light years, is one of the nearest and most studied star clusters in the sky. The cluster appears to form a distinct "V" shape, with Aldebaran at the left tip of the V, but in reality Aldebaran is located at less than half the distance to the Hyades, and is not physically associated with it. There are over 300 member stars in the cluster, most of them too faint to be seen without optical aid. Astronomers estimate the age of the Hyades at about 625 million years.

Just below Taurus is brilliant Orion, with its distinctive shape and many prominent stars, including its two brightest, reddish Betelgeuse and bluish-white Rigel. Located in the east-northeast between Auriga and Orion is the constellation Gemini, which contains the bright star pair Pollux and Castor. Rising not far to the right of this pair is Procyon, a yellow-white star in the constellation of Canis Minor. 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

Mercury is at its greatest angular distance (elongation) from the Sun on the 1st, setting a little over an hour after sunset. Still, it will be quite low in the southwest, and only those fortunate enough to have an unobstructed view in that direction will have any chance of seeing it. Look for what appears to be a bright yellow star low in the southwest about a half-hour after sunset. On the 6th, an event known as a lunar occultation occurs, when the thin crescent Moon passes directly in front of Mercury. By midmonth, Mercury vanishes into the evening twilight, only to reappear toward the end of December in the early morning sky.

Binoculars will be needed to pick out Mars just above the southwestern horizon at dusk. Mars sets roughly one hour after sunset at the beginning of December, but that interval gets cut nearly in half by month’s end. On the evening of the 13th, Mercury and Mars are very close together very low in the southwest, but buildings and trees may obstruct the view.

Jupiter continues to reign supreme all evening, mimicking a brilliant cream-colored star well below the Great Square of Pegasus. Jupiter is well up in the south in the early evening throughout December, and sets at around midnight at midmonth. Saturn, which currently lies on the opposite side of the sky from Jupiter, rises above the eastern horizon during the early morning hours of December, and resembles a bright cream colored star in Virgo well above the true star Spica. Saturn rises at about 2:30 am on the 1st and at 12:30 am on the 31st.

Early morning risers with a window facing east will be greeted by a beautiful beacon in the sky this December – the planet Venus. There is no mistaking this magnificent light in the sky, as it outshines all other nighttime objects except for the Moon. At midmonth, Venus rises around 3:30 am, which is a full two hours before the onset of twilight. About an hour later Venus should be high enough to clear most obstructions on the horizon, and its brilliance will be all the more apparent against the still dark pre-dawn sky.

The Geminid Shower originates from asteroid 3200 Phaethon. The shower is expected to reach its peak on the nights of December 13 and 14.
 

  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/. 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/