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The Sky This Week, 2012 February 21 - 28

The forge of Orion's sword.
Messier 42, the Great Nebula in Orion
Imaged from Shoestring Observatory, Alexandria, VA, USA
on 2012 FEB 21 with an 80mm (3.1-inch) f/6 Antares Sentinel refractor
and a Canon EOS Rebel T2i DSLR camera.

The Moon returns to the evening sky this week, waxing through her crescent phases while she visits the evening’s two brightest planets. First Quarter occurs on the 29th at 8:21 pm Eastern Standard Time. Look for Luna near dazzling Venus after dusk on the evenings of the 24th and 25th. The following night finds her just four degrees from bright Jupiter. By the end of the week she’ll be parked just a few degrees south of the Pleiades star cluster.

The first few nights of the week still offer dark skies for viewing some of the winter’s celestial showpieces. Top on the list of these sights is the Great Nebula in Orion. This glowing cloud of gas and dust is the home to one of the most active star-forming regions of the Milky Way galaxy, and it can be seen quite easily with only slight optical aid from all but the most severely light-polluted locations. To locate it start at the easternmost star in Orion’s famous belt, then look a few degrees to the southwest. You’ll see three small clumps of stars which form an asterism known as The Sword. The middle clump has a distinct "fuzzy" appearance to the naked eye when seen from a dark-sky site, but from the city you’ll need binoculars to see that this spot looks different from its companions. A small telescope will show a small knot of four stars with three outliers, all surrounded by a wispy glow. This is the Great Nebula’s heart. The four stars in the middle are very young, energetic stars that put out most of their radiation in the ultraviolet portion of the spectrum. This ultraviolet energy causes the gas surrounding the stars to glow like the gas in a neon tube except that here the fluorescing gases are neutral hydrogen and doubly-ionized oxygen. The visible part of the nebula is just the tip of the iceberg in terms of its true size. From the city we can see some hints of detail covering perhaps half a degree of the sky. Larger telescopes in dark-sky sites show many more faint eddies and swirls stretching out of view in low-power eyepiece fields, while deep wide-angle photos show nebulosity and dust clouds pervading the background of the entire constellation. The nebula is a star factory; most of the stars in Orion originated there, and it is estimated that the brightest parts of the nebula hold enough raw material to form another 10,000 stars of a solar mass or greater! The nebula lies on the meridian at around 8:00 pm, so enjoy it while you still can. There is nothing else quite like it in the sky!

By the end of the week be on the lookout for the appearance of fleet Mercury in the evening sky. This elusive world is beginning a brief stint in the evening sky for the next two weeks. You’ll find him about five degrees above the western horizon about 45 minutes after sunset, where he should be an easy sight in binoculars for those with a flat view to the skyline.

Despite the approach and passage of the waxing Moon, Venus and Jupiter are the two sights to see in the early evening hours. Venus shaves another six degrees off the gap separating the two bright planets as she continues to pull farther ahead of the Sun, and by the end of the week she’s only 13 degrees away from the giant planet. This is an interesting time to compare the two worlds through the telescope. Venus shows a dazzling white gibbous disc that is almost entirely featureless thanks to her global, stagnant mantle of clouds. Jupiter, by contrast, shows a larger disc with a distinctive yellowish cast and a banded cloud structure, the result of his rapid axial rotation. Jupiter spins in just under 10 hours, while Venus turns once (in the "wrong" direction) every 243 days!

Mars is now just over a week away from opposition, and you can easily spot him in the east as Venus and Jupiter slip toward the western horizon. The red planet is now well-placed for viewing by 10:00 pm, appearing as the brightest object in the eastern sky at that time. His warm rosy glow beckons for a peek through the telescope, and despite his rather small apparent disc the patient observer will be rewarded with a glimpse of his north polar ice cap and some of the more prominent dark features on his distant dusty surface.

Rounding out the night’s planet parade is Saturn, entering the sky just after 10:00 pm. The ringed planet will reach opposition in mid-April, but you can see him now if you stay up until midnight. He’ll be close to the bright star Spica throughout this year’s opposition.

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