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The Sky This Week, 2019 September 17 - 24

Chasing wild ducks in the Milky Way
Messier 11, the
Messier 11, the "Wild Duck" cluster, imaged 2015 August 15 at Fishers Island, New York
with an 80mm (3.1-inch) f/6 Antares Sentinel refractor and a Canon EOS Rebel T2i DSLR.

The Moon graces the early morning sky this week, moving into the rising bright constellations of winter.  Last Quarter occurs on the 21st at 10:41 pm Eastern Daylight Time.  Look for the Moon to the northwest of the bright star Aldebaran on the morning of the 20th.  She cruises over the familiar outline of Orion before dawn on the 21st and 22nd and ends the week among the stars of Gemini.

The autumnal equinox falls on the 23rd at 3:50 am EDT.  At this time the center of the Sun’s disc reaches an ecliptic longitude of 180 degrees and crosses from the northern to the southern hemisphere of the sky.  If you happen to be located on the equator in the western Indian Ocean at that moment, Old Sol will be directly overhead.  The term “equinox” literally means “equal night”, so one might naturally assume that this is the date on which the interval between sunrise and sunset is exactly 12 hours.  However, a glance at an almanac tells us that this phenomenon doesn’t occur in Washington until the 26th!  The reason for this is two-fold.  First, the Sun subtends a disc of about one-half a degree in diameter.  Sunrise is defined as the moment the upper limb of the Sun rises above a theoretical “ocean” horizon, and sunset is when the last vestige of the disc goes below that horizon.  Secondly, when we look at objects near the horizon the Earth’s atmosphere acts as a weak lens, causing objects to appear a bit higher above the horizon than they actually are.  The net effect of these circumstances gives us a couple of extra days when the length of daylight exceeds that of night.  Either way, starting on the 27th, nights will be longer than days until March 16, 2020.

Stargazers can appreciate the rapid lengthening of the autumnal nights.  Each day adds about three minutes of darkness to enjoy the last of summer’s best features and to welcome the more subdued star patterns of the fall.  Astronomical twilight ends at around 8:40 pm EDT, and if you are out at a dark-sky site you should be able to see the summer Milky Way bisecting the sky.  The bright stars of the Summer Triangle asterism are located high overhead, while the ruddy star Antares and the surrounding stars of Scorpius hang in the southwest.  To the northeast the Milky Way runs through the diminutive “W”-shaped constellation of Cassiopeia.  If you have a pair of binoculars, slowly sweep along the Milky Way’s path.  Sprinkled among the softly glowing star clouds of the Galaxy you’ll notice dozens of brighter knots of light.  Many of these are clusters of newly formed stars that form from the abundant stores of hydrogen gas and dust that make up the spiral arms of our cosmic home.  One of my favorite of these galactic clusters is Messier 11, which lies in a wonderfully rich Milky Way star cloud.  It is located southwest of the star Altair, southernmost star in the Summer Triangle.  From Altair, trace the Milky Way southward until you come across a small group of stars that resemble the lower half of a pointed shield.  Near the “point” of the shield your binoculars will reveal a hazy patch of light set against a field of thousands of faint stars.  A small telescope will reveal the patch to be a cluster of hundreds of stellar pinpoints, fanning out from a pair of bright stars on its southeast edge.  It is popularly known as the “Wild Duck” cluster thanks to a description recorded by the British amateur astronomer Admiral William H. Smyth in his classic 1844 observer’s guide to the sky, “A Cycle of Celestial Objects”.  Today it remains one of the most popular sights at autumnal star parties.

Jupiter appears shortly after sunset in the southeastern sky, and as evening twilight deepens he slowly sinks toward the western horizon.  The best time to train a telescope on Old Jove is during twilight when his altitude remains above 20 degrees.  As the night deepens he sinks lower in the sky, and the thickness of Earth’s atmosphere decreases your ability to discern fine detail in the planet’s turbulent cloud bands.  That said, you can follow the planet’s four bright moons as they obediently orbit their master well into the night.

Saturn is near the meridian as evening twilight ends, offering several hours viewing opportunity as he follows Jupiter into the southwestern sky.  The ringed planet reaches the second stationary point in this year’s apparition on the 18th.  Saturn’s path against the stars will gradually resume an eastward track along the ecliptic, but you’d be hard-pressed to notice much progress with the naked eye.  Saturn is currently located just below a small asterism of 3rd magnitude stars known as the “Teaspoon” that should be easily visible in binoculars.  Over the course of the next few weeks you can watch him inch eastward below the Teaspoon’s “bowl”.

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