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The Sky This Week, 2012 February 5 - 12

Mercury returns to the evening sky
LithCloud_130129_2313UT_02small.jpg

The rocket's red glare finds Orion still there!
A sounding rocket launched from NASA's
Wallops Flight Facility on Virginia's eastern shore
released a pair of lithium cannisters ~75 miles up
on 2012 January 29, providing this unusual view
from the roof of the Naval Observatory 


The Moon is a thin crescent for most of the week, playing hide-and-seek with skywatchers during the twilight hours. New Moon occurs on the 10th at 2:20 am Eastern Standard Time. You should be able to catch a glimpse of her in the glow of dawn well east of the ruddy star Antares on the morning of the 6th. If you have a clear view to the southeast, try to spot her on the next few mornings. She will return to the evening sky on the evening of the 11th. Look for a pencil-thin sliver of a waxing crescent over the west-southwest horizon about 45 minutes after sunset. If you have a pair of binoculars and a clear view to the horizon, look for the bright glimmer of fleet Mercury about five degrees below the Moon at this time. If it’s very clear you may also spot the dimmer glow of Mars just over two degrees below Mercury.

The Moon’s absence from the hours of darkness mean that this is another good week to contribute an observation to the year’s second "Globe At Night" observing campaign. This is a great activity to introduce friends, family, and neighbors to the stars while at the same time contributing meaningful data to science. I managed to gather some observations last week on a couple of exceptionally clear and cold nights when the sky was too turbulent for planetary imaging but otherwise very transparent. For most of us under urban or suburban skies the observation should only take a few minutes since city street lighting robs us of our night vision, but if you’re out away from the city let your eyes adapt to the dark for about 20 minutes before you count stars in Orion. You’ll be rewarded for your patience by a view of one of the most colorful areas of the entire sky.

The evening twilight sky is well worth watching this week as the elusive planet Mercury begins his best evening apparition for the year. Since Mercury, the smallest of the major planets, orbits closest to the Sun, he is only visible for a couple of weeks around times of morning and evening "elongations" from Old Sol. These elongations are quite varied due to Mercury’s orbital parameters, but they never occur more than about 28 degrees from the Sun’s disc, and these "extreme" ones heavily favor southern hemisphere observers. This elongation peaks on the 16th, when Mercury is 18 degrees from the Sun, but his orbit is nearly perpendicular to the horizon, enabling us to get a decent view. To find him, start panning the west-southwest horizon with a pair of binoculars between 30 to 45 minutes after sunset. By the evening of the 8th Mercury should be about five degrees high, shining at magnitude minus-1, almost as bright as the star Sirius. As an added bonus on this evening the fleet planet will be less than half a degree from fainter Mars, who is still lingering in the twilight glow. Mercury and Mars will rapidly part company over the next several evenings as Mercury climbs higher into the sky. By the evening of the 11th he will be about 10 degrees above the horizon and about five degrees below the slender crescent Moon. Mercury will stay in this part of the sky for another week or so, but he will start to fade dramatically as he starts his turn back toward the Sun.

Jupiter now stands on the meridian as evening twilight ends. The early evening hours present the best time to look at the giant planet if you have a small telescope. You don’t need much optical aid to see the four bright moons discovered by Galileo. These objects, which attend the planet like four obedient stars, are a diverse set of worlds in their own right. Two of them, Ganymede and Callisto, are comparable to the planet Mercury in size, while Io and Europa are similar to our Moon. The moons undergo a variety of interesting phenomena with the giant planet, sometimes tracking across his face dragging a shadow in their wake, or passing behind the planet and disappearing into his massive shadow. They are a never-ending source of entertainment to watch.

Saturn straddles the meridian as morning twilight begins to brighten the eastern horizon. The planet’s signature rings have been steadily opening to our line of sight for the past few years, and they are now tipped a generous 20 degrees in our direction. This offers a splendid view in the telescope which only gets better with increasing aperture. A good six-inch instrument should show the famous Cassini Division in the rings if the air is steady, and the planet should be surrounded by up to a half-dozen of his small icy moons.

Those of you who are longing for spring have probably already noticed the increasing length of day. As we approach the middle of the month we’re not only seeing more of Old Sol in the afternoon, but he’s making rapid progress towards earlier sunrises as well. Here in Washington sunset will occur an hour later than it did in the dark days of December by the time the 15th rolls around. The groundhog may be right after all.