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Sky Report -December 2009

Double Full Moon,  Earliest Sunset and Shortest Day

December starts with a full moon on the evening of the 1st and ends with a full moon on New Year’s Eve. This second full moon of the month is the first since 2007 when there were two full moons in June. 1999 was a unusual year as we had double full moons in January, no full moon at all in February and a double full moon in March. (The usual term for a second full moon in a month is ‘a blue moon’.)  The year’s earliest sunset is on December 9th, when the sun sets  locally about 4:50 p.m. The reason why we don’t have our earliest sunset on the first day of winter is due to our clock time being based on the ‘mean’ sun, an imaginary body that moves uniformly along the celestial equator. The actual sun seems to move fastest in early January when we are closest to the sun and slowest in early July when we are farthest from the sun. Of course, it is not the sun that is moving, but the Earth. The sun’s apparent motion is a reflection of the Earth’s motion about the sun. The difference in time between the smoothly moving ‘mean sun’ and the real sun is called the Equation of Time.
We add this correction to our clocks to find the actual sun’s position. In early December, the sunsets are quite early and the Equation of time correction combines to give us our earliest sunsets of the year. December 21st is the first day of winter, when our part of the world (Northern hemisphere) is tilted farthest  from the sun. On that day, the sun rises farthest South (of East) and also sets farthest South (of West). The sun has its lowest sky path, peaking about 27.5 degrees in the South at mid day and stays in view for about 9 hours and 20 minutes. In the following weeks, there are very slow changes in both the mid day height of the sun, the sunrise time and sunset time. Sunrises will continue to get later until the first week of January. The most rapid change in both the sunrise and sunset times and length of daylight are at the start of spring and fall (up to 2 minutes per day). At the start of summer, the sun peaks about 73.5 degrees in the South and stays in view for 15 hours.

When the moon is full on the evening of the 1st, the moon appears in Taurus and near Aldebaran, the star marking the Bull’s eye. On the next night, the moon is between the golden star Capella and the star group Orion. On December 4th, the moon is near Pollux and Castor, the bright stars of Gemini from which the Geminid meteor shower on December 14th can be traced back to. On December 6th, the waning gibbous moon appears near the yellowish planet Mars in the late evening. On the morning of December 9th, the moon appears half full high in the dawn southern sky. On December 10th the moon appears below Saturn in the 6 a.m. southern sky. The last time to see the crescent moon at dawn will be December 14th. The moon swings from the morning to the evening side of the sun on December 16th. On Thursday, December 17th at sunset, the Muslim New Year (1431 A.H.) begins with the first sighting of a very slender crescent moon from Mecca. On Friday, December 18th, the moon should easily visible (weather permitting) low in the West Southwest at 5:30 p.m. Just below the moon then will be the planet Mercury at its greatest angle to the East of the sun. On December 21st, the moon will appear to the right of the very bright planet Jupiter. On Christmas Eve, the evening moon will appear half full in the southwestern evening sky. On December 31st,  our ‘blue moon’ (full moon) will appear in the star group Gemini.

All through December, the very bright planet Jupiter shines in the early evening sky in the Southwest. The yellowish planet Mars becomes visible in  the middle hours of the evening as it’s rising time drops from 10 p.m. to 8 p.m. The planet Mercury sets more than an hour after sunset from December10th to the 27th and can be seen from sites with a very flat southwestern horizon. Venus is lost in the sun’s glare in December. The planet Saturn is best in the last few hours of the night when it is high enough above the horizon for telescopic views of its rings.

The December feature at the Frostburg State Planetarium is “Christmas and Seasonal Festivities” with free public programs at 4 p.m. and 7 p.m. on December 6th, 13th and 20th. Our programs feature a lively tour of the current evening sky for beginners, free December sky maps and a 2 page 2010 Celestial Highlights to our visitors. The Planetarium is in Tawes Hall, near the FSU Clock Tower and across the street from the Compton Science Center. There is convenient free parking, nearby parking for the less mobile and a ramp. Call (301) 687-7799 to request a Planetarium schedule (with small map showing the key buildings and parking lots around the Planetarium).   

By Dr. Bob Doyle

To contact Dr. Doyle, his mailing address is Planetarium, Frostburg State University, Frostburg, MD 21532 or by email at rdoyle@frostburg.edu.









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