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The Sky This Week, 2017 September 5 - 12

What's hanging in the cosmic closet?
Collinder 399, aka "Brocchi's Cluster" or "The Coat Hanger",
imaged on 2016 August 25 from Mollusk, Virginia by Geoff Chester
with an 80mm f/6 Antares Sentinel refractor and a
Canon EOS Rebel T2i DSLR.
Image is inverted, with south at the top.

The Moon starts the week off as the bright Full Corn Moon, then wanes as she climbs northward along the ecliptic through the dim autumnal constellations. Last Quarter occurs on the 13th at 2: 35 am Eastern Daylight Time. Luna won’t encounter much in the way of bright companionship until the morning of the 12th, when you’ll find her just over a degree west of the bright star Aldebaran in the gathering light of dawn. Luna occults Aldebaran for most residents of the U.S. during daylight on the 12th. Here in Washington the event begins at 9:00 am EDT when the star passes behind the Moon’s bright limb. It will reappear from Luna’s dark limb at 10:07 am. You’ll need very clear skies and a telescope to observe this lunar "eclipse" of a much more distant star than the Sun.

As the Moon moves into the morning sky we once again have the opportunity to enjoy some of the celestial wonders of our home galaxy, the Milky Way. Early in the week you can trace out the general path of the galactic plane by looking at the distribution of the brighter stars that arc from the northeast to the southwest at the end of evening twilight. Look overhead for the first-magnitude stars that make up the asterism known as the Summer Triangle. Now turn your gaze to the southwest where you’ll find the ruddy star Antares, brightest star in the constellation of Scorpius, the Scorpion. Antares owes its brightness and its pinkish tint to its evolutionary stage in its lifetime. It has exhausted the hydrogen "fuel" in its core and is now fusing hydrogen into helium inn an ever-expanding shell around its core. This causes the star’s diameter to bloat to huge proportions, which in turn makes its surface area much larger and cooler, making it bright and red. Now look to the northeastern sky for a small constellation that resembles the letter "W". This is Cassiopeia, a mythical queen of Ethiopia who plays a central role in the legend of Perseus and Andromeda. Each of these star fields will gradually reveal the soft glow of the Milky Way as the Moon works her way eastward through the sky. These are wonderful areas to explore with binoculars from dark locations as Luna slinks from the scene.

Swing back to the southernmost star in the Summer Triangle, Altair, to look for a binocular treat that can be seen from suburban skies. Altair is flanked by two fainter stars; follow the line made by these stars and Altair to the north. You’ll soon run into a straight line of faint stars that seem to have a "hook" dangling from the mid-point of the line. It doesn’t take much imagination to imagine an upside-down coat hanger in this group of stars, which was first described by the Persian astronomer Al-Sufi in the 10th Century AD. Also known as Brocchi’s Cluster, today we know that "The Coat Hanger" is a chance alignment of stars.

Jupiter is still visible in the evening twilight, but the giant planet now sets when dusk turns to full darkness. He’s trying to keep pace with the advancing Sun, but that’s a race that he will soon lose. He’ll pass behind the Sun in late October.

Saturn is now our sole easily observable evening planet, appearing in the southwest as twilight deepens. Take advantage of the planet while you can and give him a good look in the telescope. The planet’s famous rings are tipped at their greatest angle to our line of sight, allowing glimpses of their subtle colors and gaps.

Three planets gather in the glow of morning twilight this week. Venus is easy to spot soon after she rises and is well up by 6:00 am. Closer to the horizon you’ll find Mercury and Mars about five degrees above the horizon at 6:00 am. Use binoculars to locate them, and watch them dance with the first-magnitude star Regulus.

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