The Night Sky: December 2009

Contents

The Night Sky in December 2009

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

Moon’s Phases in Month
Full Moon on the 2nd and on the 31stNew Moon on the 16th                                

 The Full Moon in December is appropriately referred to as the "Cold" Moon. It has also been called the "Full Moon before Yule." This month, there are two Full Moons, and so the second one, which falls closest to the winter solstice (when days are shortest) is designated "The Long Night Moon." The second Full Moon in any month is also often referred to as the "Blue Moon."

Stars and Constellations

The Sun reaches the southernmost position on its apparent annual path, the ecliptic, through the zodiac constellations on December 21st at 12:47 pm, thus marking the beginning of winter in the northern hemisphere. Paradoxically, the trio of stars comprising the "summer" triangle – Vega, Deneb, and Altair – is still viewable low in the western sky shortly after it gets dark. The stars of autumn dominate the early evening sky, and one of the most familiar beacons of autumn nights, the white star Fomalhaut, is now getting low in the southwest. Fomalhaut and Vega lie at approximately the same distance: 25 light years. High above Fomalhaut, also toward the southwest, is the box of four stars comprising the Great Square of Pegasus. High in the north is the constellation Cassiopeia, now looking like the letter "M." Cassiopeia is followed in the northeast by the constellation Perseus.

Located nearly overhead during the early evening hours of December are two moderately faint constellations from antiquity. One is famous, the other not. Aries, the Ram, lies just to the east of Pisces and the Great Square. Aries originally represented the first constellation of the zodiac, because two millenia ago it contained the Vernal Equinox, the point through which the Sun passes annually, marking the first day of Spring in the northern hemisphere. Due to precession, a wobble of the Earth’s axis, the Vernal Equinox has since shifted westward into neighboring Pisces. Today, the Sun passes through the constellation Aries between April 19 and May 13. In ancient mythology, Aries was the source of the Golden Fleece that was sought by Jason and the Argonauts.

Just above Aries is Triangulum, a tiny constellation (size ranking 78th out of 88) consisting of three stars in a narrow triangle. One legend says that Triangulum represents a wedding gift to Proserpina, daughter of Ceres, upon her marriage to Pluto, God of the Underworld.  Triangulum is noteworthy to astronomers because it contains M33, the "Pinwheel Galaxy."  M33 lies at a distance of over 2 million light years, which makes it (along with the larger and more famous Great Galaxy in Andromeda) one of the nearest spiral galaxies to our own Milky Way.

 By about 9 pm, the brilliant star groups of winter can be seen rising in the east. The first star of winter to catch your eye will probably be Capella, the yellow-white star in the constellation Auriga, the Charioteer, as it ascends in the northeast. Looking high in the east, to the right of Auriga, you can find the stars of Taurus, the Bull, which contains the bright reddish star Aldebaran. Aldebaran appears to be part of a "V" shaped cluster of stars known as the Hyades. Also part of Taurus is the Pleiades, a compact star cluster shaped like a miniature dipper. Each of those apparently dim stars of the Pleiades is intrinsically several hundred times more luminous than our Sun, but appears so faint because of its immense distance of over 400 light years from our solar system. At that distance, the light from the Pleiades which you see tonight has been traveling since the days of Shakespeare and Galileo. The undisputed champion of winter constellations is Orion, which contains two very bright stars, reddish Betelgeuse and bluish-white Rigel. By midnight, Orion is high in the south, and is sure to catch your attention if you are going out for midnight service on Christmas Eve. Located in the east-northeast between Auriga and Orion are the twin stars Pollux and Castor in the constellation Gemini. 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, look toward the southeast, below Rigel, and you will see the brightest star in the night sky: bluish-white Sirius, the "dog star" in the constellation Canis Major.

 Naked-Eye Planets In the Evening and Morning Sky 

Mercury makes an especially good appearance in the evening sky for much of this month. It reaches its greatest elongation with the Sun on the 18th, and sets nearly an hour and a half after sunset on that date. To spot it, look for what appears to be a bright yellow star low in the southwest about a half-hour to an hour after sunset. Not far from Mercury, but higher up, is the much brighter Jupiter, which remains dominant in the evening sky during December. You cannot miss Jupiter: it resembles a brilliant cream-colored star in the southwest during the early evening hours. Jupiter lies nearly on the meridian (due south) at sunset at the beginning of the month, and sets after around 10 pm. By New Year’s Eve night, it sets at 8:30 pm.

As Jupiter is setting in the southwest, Mars is rising in the opposite part of the sky. Look for a bright orange-red object low in the northeast and getting higher as the night progresses. Mars moves from Cancer into Leo this month, and continues to get brighter as the distance between it and Earth decreases. Mars rises around 9:30 pm EST at the start of December and by a little after 7:30 pm at month's end. By the pre-dawn hours, Mars will be located high above the western horizon. Note that Mars reaches opposition with the Sun (and its closest approach to Earth) late next month, and its red glow will be a beautiful sight against the cold winter night sky.

Having spent the past several months in the early morning sky, Saturn has been slowly but surely working its way into the evening sky, and this month will begin to rise before midnight. At the beginning of December, Saturn rises at about 1:30 am and on the 31th by around 11:30 pm. Look above the eastern horizon to spot yellow Saturn with the blue-white star Spica not far away. A telescope will display the famous ring system, tilted nearly edge-on. By dawn, Saturn is nearly due south on the meridian.

Venus rises less than an hour before sunrise at the beginning of December, but that time interval shrinks rapidly, so that Venus is essentially lost in the Sun's glare for most of the month. Venus will be in conjunction with the Sun in January 2010, after which it will slowly reappear in the evening sky during the spring.

The Geminid Shower is expected to reach its peak on the nights of December 13th and 14th. Meteor streaks result as Earth passes through the debris from the asteroid 3200 Phaethon. As the mostly sand-grain sized particles enter Earth’s atmosphere at enormous speeds, several tens of kilometers per second, they are incinerated by friction with the surrounding air, resulting in bright trails across the sky.

 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

Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.