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The Sky This Week, 2011 May 17 - 24

A tale of two stars, and the traffic jam eases in the pre-dawn sky.
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Saturn and its large moon Titan, 2011 May 12, 02:27 UT


The Moon wanes from Full to First Quarter this week, scudding low above the southern horizon during the early morning hours as she moves from the summertime to autumnal constellations. Last Quarter occurs on the 24th at 2:52 pm Eastern Daylight Time. Look for Luna’s Full disc just northwest of the ruddy bright star Antares in the constellation of Scorpius on the night of the 17th/18th. In the early morning hours of the 20th she is parked just above the "Teapot" asterism in the constellation of Sagittarius. From there she spends the rest of the week drifting through the star-starved constellations of Capricornus and Aquarius.

As the Moon drifts into the morning sky we find last of winter’s stars setting in the west at the end of evening twilight. Sunset now occurs at around 8:15 pm, which places the end of astronomical twilight at around 10:00 pm. At this hour the only bright stars left from the once-dominant Great Winter Circle are Procyon, Castor and Pollux, and the third-brightest star in the northern hemisphere sky, golden-hued Capella. This star is closely associated with our celebration of Thanksgiving some six month hence, when it transits the meridian at around midnight. The Capella system actually consists of four stars in two binary pairs. The first of these is a pair of nearly identical evolved yellow giant stars that orbit each other with a period of 104 days. The second pair, only discovered in 1936, is a pair of very fait red dwarf stars that orbit the larger pair with a period of some 400 years. The Capella system lies just over 42 light-years away from us.

The spring sky holds only one first-magnitude star in the form of Arcturus, which is high in the east as twilight ends. Arcturus is the brightest star in the northern sky and fourth brightest in the heavens. It is a single star with a rosy tint that indicates that it, too, is an evolved star beginning its long evolutionary decline to a "white dwarf". Arcturus lies just under 37 light-years from us, and it has a very high proper motion through space. This motion has led some astronomers to believe that Arcturus was once part of a small dwarf companion galaxy to our Milky Way which was destroyed by gravitational interaction with our home Galaxy.

Much closer to home, and forming the western apex of a triangle that includes Arcturus and blue-tinted Spica, is the golden glimmer of the planet Saturn. The ringed planet has been putting on a good show so far this year, at least on the nights when we can see him. Our cloudy and turbulent weather of the past several weeks has limited me to one or two good views each week, but in those times Saturn never fails to delight. There is something about seeing the planet’s tan disc, faintly streaked with cloud bands, surrounded by the nestled arcs of the ring system that gives the observer the sense that Saturn is really "out of this world". While we now know that all of the solar system’s outer giant planets have ring systems, only Saturn’s are easily observed with earthbound telescopes. They have delighted and mystified astronomers for nearly 400 years!

The pre-dawn sky is still the scene of much planetary jockeying as Venus and Mercury overtake and pass ruddy Mars. They have already left bright Jupiter in their wake as they race their way toward conjunction with the Sun. You can spot them about 20 minutes before sunrise a few degrees above the eastern horizon. Use binoculars to first track down Venus. At a dazzling -4.0 magnitude she should be easy to find. Now look just over a degree below Venus for the dimmer glow of Mercury. Toward the end of the week, look a degree above Venus to see if you can detect the faint ruddy glimmer of Mars. All of this action occurs within a few degrees of the horizon. Don’t be confused by the bright glow of Jupiter, who will be some 10 degrees above the horizon at this time.

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