December 2, 2005
December Astronomy Note
Mid-December has a lot of things going on...
Minor to modest meteor showers are peaking all month long, with none being standout. This means that general night watching can pay off with frequent falling stars, but no big shows.
Venus is so bright now as of 1 Dec that, in rural Arizona, it casts visible shadows. At magnitude -4.7, it is at peak brightness, even though not a "full" disk. It will rise less high, set sooner, fade in brightness, and the disk will "wane" significantly, by 1 January.
The earliest sunset of the year is on Wednesday, 7 December, but the Winter Solstice is not until Wednesday, 21 December. Why are they not on the same day?
Well, to an interplanetary inertial navigator like Dr Who, they are. Here on your puny planet, clock time attempts to match the period from high noon to high noon by averaging the length of the day. But every day, the earth moves one 365th of the way around the sun, moving high noon to the west by an amount that varies during the year, constantly but predictably, back and forth around the average. The difference in minutes is called "The Equation of Time" by astronomers, geodesists, and space, naval, and aircraft navigators, and most James Bond villains. By true natural solar time, the sun will set 3 minutes later on Dec 7th than 21 Dec. But after we substract "The Equation of Time" (about 7 minutes on 7 Dec, but only 2 minutes on 21 Dec) the sun will set 3 minutes sooner on 7 Dec than 21 Dec, according the the clock on your wall, at 4:35pm.
On 10 Dec, a still-bright Mars becomes stationary at the end of its retrograde motion, and then resumes direct eastward motion.
December is a good month to view Mercury, if you are an insomniac. Its greatest Elongation (apparent distance from the sun,) is on 12 Dec. That means that on mornings around that date, Mercury appears more than an hour before sunrise. On the 12th, it appears at about 5:20 am, almost an hour and 45 minutes before sunrise, and it is two thirds "full" and at magnitude -0.4, pretty bright.
Posted by hargers at December 2, 2005 8:10 PM