December 2009 astronomy events

December evenings start with Jupiter high in the sky just a little west of the meridian. Jupiter is just a half degree south of Neptune on the 20th. If you have a telescope that can pick up the fainter Neptune and an eyepiece with a wide enough field of view, you may be able to catch both in one view. A nearby crescent moon could throw a little lightwash into it though.

Mercury barely inserts itself into the evening sky for northern hemisphere viewers before reaching greatest elongation on the 18th at 20 degrees east of the sun. It's not a great view for northern viewers this time, because it's so far south. If you can spot a very thin crescent moon on that date, look for Mercury below it. Mercury is so far south, it's even in the southwest for southern hemisphere viewers.

Mars rises late in the evening looking as though it might be caught in the sickle of Leo rising just under it. The red planet is slowing down in direct motion, about to reverse to retrograde motion. Many observers like to describe a planet as being stationary at such times, although careful watchers will notice the motion from night to night.

Saturn comes up a little later deeper into Virgo.

Venus is out of sight this month, heading toward superior conjunction next month.

Viewers in much of Asia and the western Pacific Ocean will be treated to an occulation of Alcyone by the moon on the 1st. Another showing will occur on the 29th for Central America, the Caribbean, far southern United States, part of the Atlantic Ocean, and western Africa.

The Geminid meteor shower should peak around the 14th.

Io and Callisto will cast their shadows on Jupiter at the same time on the 21st. This lasts for about an hour and a half. Io's shadow is on the big planet first and is also the first to leave. The dual shadow event will be visible in the western hemisphere, west of about 60 degrees west where Jupiter is up but the sun isn't.

The southern solstice happens on the 21st at 17:47.

There are two full moons in December. The second occurrence is sometimes called a "blue moon". The use of this term as it relates to the calendar is apparently the result of an accident. Sky & Telescope magazine says one of its writers, in 1946, misinterpretted another source's definition (numerous almanacs over the years have named the full moons by month or season). Few people used the mistaken definition until it suddenly became a popular trivia game "fact" in 1988. It became fashionable to "know" that the second full moon in a month had been long called a blue moon, kind of like it's fashionable to "know" that humans use only 10% of their brains. It's just not so.

The year ends with a partial lunar eclipse. Africa, Europe, and Asia will be the viewing locations for the entire event. Australia will get the show too, at moonset. The partial phase lasts just over an hour.