and Sunset Times for
December 21 officially launches the winter season. At 10:47am the sun reaches its southernmost point in the sky for this year, which marks the winter solstice. The time from sunrise to sunset is only 9hours 28 minutes on this day. Notice that the period of daylight does not vary much around the solstice. The sun continues to rise a little later each day throughout the month. However, the time of sunset reverses itself more than a week before the Winter Solstice, adding a minute here and there to otherwise shortening days. (The time of sunrise and sunset assumes a flat horizon. Actual time may vary depending upon the landscape.)
December begins with a full moon on December 2 at 12:30am. On the evening of December 1 it rises at 4:19pm and on December 2 at 5:18pm. On December 5 the waning gibbous moon rises soon after 9:00pm with Mars trailing behind. The last quarter moon precedes Saturn into the southeastern sky soon after midnight on December 8. On December 9 Saturn rises just ahead of the moon in the southeastern sky. Two nights later a waning crescent moon rises around 3:00am with Spica (Virgo). The new moon arrives on December 16. On December 20 a waxing crescent moon appears in the southwestern evening sky near Jupiter. With a telescope you could see Neptune above and to the right of Jupiter. The first quarter moon occurs December 24. On December 28 the waxing gibbous moon passes through the Pleiades. The Pleiades star cluster spans an area of three degrees in the sky. The diameter of a full moon is about 0.5 degrees. A second full moon occurs on December 31 at 12:13pm. It rises at 5:14pm.
MYSTERY OF THE BLUE MOON
Two full moons occur in December. The second full moon in a month is “traditionally” called a blue moon. Once in a blue moon is about once in two and one-half years. That’s how long it has been since the last blue moon of June 2007. Designating the second full moon of a month the blue moon is a good example of how folklore develops. In this case a mistake followed by failure to check original sources is involved. Typically, there is one full moon in a month which means three full moons per season. In the 19th century the Maine Farmers’ Almanac referred to a blue moon as the third of four full moons to occur in a season. The names of the full moons followed tight rules about traditional activities for each calendar month and the relationship of the dates for Easter and Lent with the full moons of winter and spring. When a fourth full moon occurred, there was no assigned name for it until the Maine almanac applied the term blue moon. They used it for the third full moon because the last moon of the season was likely to have an important connotation.
The mistake happened during the 1940s in two separate issues of Sky and Telescope magazine. In one, the author refers to a blue moon as occurring when there are 13 full moons in a year. He used the Maine almanac as his reference but gave no details. A few years later, another author, referring to the earlier article, surmised that the blue moon was the second full moon to occur in a calendar month. During the 1980s a radio astronomy program broadcast this misinformation. A few years later a children’s almanac listed the misinformation in a new book, then the game Trivial Pursuits used this definition as the answer to “What is a blue moon?” The rest, as they say, is...folklore.
A major meteor shower graces the middle of the month and viewing should be excellent since the moon will be near new phase. Look for the Geminids December 6-19 after 11:00pm when Gemini is high in the eastern sky. At its peak on the night of December 13/14 expect to see 50-80 meteors per hour. Geminid meteors appear white and seldom leave a trail. A second promising meteor event occurs December 17-25. This is the Ursids Meteor Shower. Its radiant is the center of the Little Dipper. These meteors are known for their flashy trails. The radiant is visible all night. Best viewing will be at its peak on the night of December 22/23 after 11:00pm when the moon has set.
NASA SIGHTING OPPORTUNITIES
To find out when the Space Shuttle and International Space Station are visible from your location, go to the following website and click on Sighting Opportunities: http://spaceflight.nasa.gov/realdata/sightings/index.html
Note: Hold your hand at arm’s length to measure apparent distances in the sky. Adjust for the size of your hand. The width of the little finger approximates 1.5 degrees. Middle, ring, and little finger touching represent about 5 degrees. The width of a fist is about 10 degrees. The hand stretched from thumb to little finger equals 20 degrees. The diameter of both the full moon and the sun spans only 0.5 degree.
Jupiter - The brightest of the evening planets and stars is easy to spot low in the southwestern sky. Jupiter sets around 10:00pm during the first half of December. By the end of the month, it sets by 9:00pm. Distant Neptune appears within one degree of Jupiter from December 15 to December 21, although it won’t be visible with the unaided eye. Both are in Capricorn. (Magnitude -2.0)
Mars - At the beginning of the month Mars rises before 10:00pm, about one-half hour before Jupiter sets. Later in the month look for it in the eastern sky before 9:00pm. Mars is west of Leo’s bright star Regulus throughout December. On December 21 it begins its retrograde motion when it drifts back towards Cancer. In the morning twilight Mars lingers in the western sky. Its reddish disk will brighten and increase in size over the next two months. (Magnitude -0.4)
Saturn - Look overhead in the morning twilight for Saturn’s yellow light. At the beginning of December it rises with Virgo in the southeastern sky around 2:00am. The plane of its rings are increasing their tilt to over 4 degrees from our perspective which reduces its brightness somewhat. By the end of the month it rises around midnight. (Magnitude +1.0)
Venus - Enjoy the brilliant “morning star” in the southeastern sky during the first week of December. Look for it as civil twilight brightens the sky about one-half hour before the sun appears. Venus thenmoves into conjunction with the sun and reappears in the evening sky in January. (Magnitude -3.8)
Note: Apparent magnitude values range from -4 to +6 for most planets and visible stars. The lower the value the brighter the object. A decrease of 1.0 magnitude is 2.5 times brighter.
Primary Sources: USGS, U.S. Naval Observatory,
CONSTELLATIONS this MONTH
The Great Square of Pegasus appears overhead, marking the midpoint of Autumn. The Summer Triangle sinks into the western horizon as the Pleiades leads the winter constellations into the eastern sky—Taurus, Orion, Gemini, Canis Minor, Canis Major.
Hold the star chart high above
your head and match the compass directions to the direction
you are facing.
Adjust the star chart by orienting Ursa Major (Big Dipper) to match its position
in the sky.
The star chart approximates the
night sky from astronomical twilight to midnight. As the night
and the month progress, the constellations will shift toward