The Sky This Week, 2012 July 17 - 24

Waiting for darkness at the Naval Observatory

The Moon returns to the evening sky this week, waxing through her growing crescent phases as she climbs toward Mars and Saturn in the southwestern sky. First Quarter occurs on the 26th at 4:56 am Eastern Daylight Time. You should be able to spot her slender form in the twilight of the evening of the 21st; by the 24th she will be parked about five degrees south of ruddy Mars.

You still have the better part of the week to enjoy the view of the summertime sky before moonlight washes out the later evening hours. As I Mentioned last week, this part of the sky is chock full of bright stars and the more subtle features of the Milky Way. Viewing the latter requires a clear dark sky, but many of us take vacations to the mountains or the shore well away from city lights. If this is the case, throw a pair of binoculars in your bag with your bathing suit or hiking boots so you can enjoy some evening exploration.

Start your tour with the bright star Antares, which is on the meridian as evening twilight ends. Look just below and to the right of the star and you’ll see a misty patch of light. This is Messier 4, one of about 150 so-called "globular clusters" that orbit the galactic center. These clusters are characterized by populations of very ancient stars and may be among the oldest objects in the universe. Sweeping up and to the left of Antares your binoculars will show dozens of other luminous smudges. Several of these are other globular clusters, while others are softly glowing gaseous nebulae in which very young, energetic stars cause swirling clouds of hydrogen gas to glow. Still other knots correspond to "galactic clusters", the end-products of star formation in nebulae. Much more subtle are the "dark nebulae" which appear like "holes" among the swarming starfields. Although they are seemingly empty spots in space, in reality they are huge clouds of non-luminous gas and dust that’s obscuring the light of more distant stars. Such dark clouds contain the raw materials of star formation, and millions of years from now may be the bright nebulae that will be the targets of our very remote descendants.

A large feature of the Milky Way doesn’t even require binoculars to observe. This is the so-called "Great Rift" that seems to split the Galaxy into two branches from the area above Antares through the heart of the Summer Triangle. This rift is yet more vast clouds of opaque gas and dust and is a characteristic seen in many edge-on external spiral galaxies. These dark patterns are so prominent in the summer Milky Way that ancient Meso-American cultures built much of their skylore around them, creating dark "constellations" to pass the stories on to future generations.

Closer to home, two planets still chase through the evening sky as twilight deepens. Ruddy Mars continues to bear down on Saturn as he motors through the stars of the constellation Virgo. This week he passes south of the second-magnitude star Porrima, which is one of the sky’s more interesting double stars. Porrima’s components have the rather fast orbital period of about 169 years, and it is one of the few double stars which will show an appreciable change in its appearance from year to year. When the components were at their closest in 2005 I couldn’t resolve them in the Observatory’s 12-inch telescope; now they split easily in my 8-inch scope at home.

Saturn seems to watch as Mars hurtles in his direction. The ringed planet has resumed eastward motion against the stars, but his motion is almost imperceptible compared to his ruddy rival. Through the telescope Saturn beats Mars hands-down. He stays in the evening sky just long enough to get a good look and amaze your friends and neighbors.

In the pre-dawn sky Venus and Jupiter part company after last week’s spectacular gathering with the Moon and the star Aldebaran. Jupiter will rise earlier each morning while Venus will seem to stay in nearly the same place relative to the Sun. Early reports from amateur Jupiter observers indicate that major changes are underway in Jupiter’s northern cloud belts. As the planet climbs higher over the next several weeks you might want to follow the progress of these events with your own telescope.