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The Sky This Week, 2018 September 25 - October 2

Sights in triangles and squares.
Collinder 399, the "Coathanger" cluster, imaged 2016 August 25
imaged at Fishers Island, NY with a 4-inch f/6.6 refractor telescope
and a Canon EOS Rebel T2i DSLR. Note the more distant cluster NGC 6802
to the east (right) of the "Coathanger".

The Moon drifts into the morning sky this week, waning to the Last Quarter phase which will occur on October 2nd at 5:45 am Eastern Daylight Time.  Luna spends the first few evenings among the faint stars of the autumnal constellations, then slides into the rising winter stars by the week’s end.  Watch the Moon pass north of the bright star Aldebaran during the overnight hours of the 29th-30th.  The pair will be closest at around 2:00 am EDT on the 30th.  As October begins you’ll find Luna passing through the middle of the bright stars of the Great Winter Circle if you’re up before sunrise.

As the Moon retreats into the morning sky the window for darkness opens in the early evenings.  This affords an opportunity to catch a last glimpse of the summer Milky Way, which stretches across the sky from northeast to southwest.  At 9:00 pm the three bright stars of the Summer Triangle asterism are located overhead, and it is in this part of the sky that you’ll notice that the Galaxy appears to split into two distinct parts.  The so-called “Great Rift” in the Milky Way is formed by vast clouds of non-luminous dust and gas that is distributed along the Galaxy’s equator.  These clouds block the light of more distant stars, but they also provide the building blocks of future stellar systems.  If you sweep the Milky Way with a pair of binoculars or a low-power “rich-field” telescope, you’ll note areas where stars are forming in luminous knots of glowing gas.  In these regions the powerful ultraviolet radiation of these young stars cause the gas in the surrounding dark clouds to glow.  You’ll also find more evolved star clusters throughout the Milky Way.  Some of these are tightly-packed groups of thousands of stars, while others are widely scattered associations.  One of the more striking examples of the latter may be found between the bright star Altair, southernmost star in the Summer Triangle, and Albireo, the beautiful double star located in the center of the Triangle asterism.  Commonly called “The Coathanger”, the cluster is best seen in binoculars and consists of about a dozen stars in a very distinctive array.  If you have a telescope, look for the more distant and densely packed star cluster NGC 6802 just to the east of the Coathanger’s easternmost star.

By midnight the Milky Way heels over to the western sky, and with it the bright stars of summer.  The scene shifts to more scattered, fainter constellations that make up the constellations of autumn.  None of them are particularly bright, but one large grouping approaches the meridian high in the south.  From light-polluted skies you should still be able to make out a large square made up of second-magnitude stars in an otherwise near-empty sky.  This square is part of the constellation of Pegasus, the mythical Flying Horse from the Perseus legend.  The “Great Square” provides a good indicator of the quality of the sky.  Urban skywatchers can see little more than the stars of the square itself, but observers at a good dark-sky site should be able to see about a dozen faint luminaries within the asterism’s boundaries.  Pegasus will be the focus of the upcoming October campaign for the citizen-science Globe at Night program.  We’ll have more to say on this next week.

Venus is on the verge of dropping out of the evening sky.  Even though she’s at her brightest for the year and still at a considerable elongation from the Sun, she is only 10 degrees above the horizon when Old Sol sets.  Venus herself goes below the horizon shortly before 8:00 pm, so you will only have a brief time to find her in the southwest.

Jupiter is also headed for a speedy exit from the evening sky.  Old Jove now sets at around 9:00 pm, so your best views of the giant planet will be restricted to the twilight hour.

Saturn lingers in the evening sky along with the departing stars of summer.  The ringed planet sets at around 11:30 pm, so you have a couple of hours to get a good view of him with the telescope.  Saturn lies in a very interesting part of the sky that’s dominated by a dense Milky Way star cloud.  Using binoculars, look just to the right of Saturn and you’ll see two of the most popular star clusters and nebulae in the sky, the Lagoon Nebula and the Trifid Nebula.

Mars brings up the rear of the bright planet parade.  Although he is fading rapidly from his opposition glory of late July, he is still a striking object in the southern sky.  His distinctive ruddy tint stands out in a part of the sky that is notable for its faint star patterns.  You’ll be able to enjoy him until around 2:00 am, when he sets and leaves the sky to the rising bright stars of winter.

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