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The Sky This Week, 2019 August 20 - 27

Scoping out The Swan
Messier 71, globular star cluster in Sagitta, imaged 2015 August 15 at Fishers Island, New York.
Messier 71, globular star cluster in Sagitta
imaged 2015 August 15with an Antares 80mm (3-inch) f/6 refractor telescope
and a Canon EOS Rebel T2i DSLR from Fishers Island, New York.

The Moon moves from the autumnal stars to join the rising constellations of the winter this week.  You’ll find her gracing the early morning sky as she climbs northward along the ecliptic.  Last Quarter occurs on the 23rd at 10:56 am Eastern daylight Time.  Look for Luna’s fat crescent just over a degree north of the bright star Aldebaran in the constellation of Taurus, the Bull, before dawn on the 24th.  On the following morning she closes in on the star Zeta Tauri, the star that marks the tip of the Bull’s southern horn.  In ancient Chinese sky lore this star was known as Tien Kwan, the “Gate of Heaven”.

The August campaign for the Globe at Night citizen-science observing program begins on the 22nd and runs through the 31st.  This effort, now in its twelfth year, has compiled over 180,000 observations of the visibility of stars from people in 180 countries around the world.  It is sponsored by the National Optical Astronomy Observatory consortium and is intended to promote sky awareness and the impact of artificial light on our night skies.  The premise is simple: identify a prominent constellation, then compare your naked-eye view with charts on the Globe at Night website or phone app.  This month’s featured constellation is Cygnus, the Swan, also known as the Northern Cross.  Its brightest star, Deneb, is the northernmost star in the Summer Triangle asterism, located high in the east near the zenith at 10:00 pm.  From a relatively dark site you should be able to trace out the rest of the cross-shaped constellation.  The name Deneb means “the tail”, and it doesn’t take too large a stretch of imagination to see a stick-figure swan flying southward along the Milky Way.  The Swan’s head is marked by the third-magnitude star Albireo, which is located almost directly in the center of the Summer Triangle.  From rural skies the Swan flies in front of one of the brighter patches of the Milky Way, which splits into two distinct parts near Deneb.  Cygnus represents the swan guise that Zeus adopted to seduce Leda in Greek mythology, but other cultures see it a little differently.  To the Inuit people it depicts a man in a kayak paddling south along the “Pebbly River”, their name for the Milky Way.

Cygnus holds a multitude of treats for binoculars or small telescopes.  The bright star clouds begin to break up into uncountable numbers of stars punctuated by glowing knots of light from star clusters and glowing gas clouds.  For the telescope, the star Albireo reveals itself to be a lovely double star with yellow and blue colors.  Just northeast of Deneb a low-power eyepiece on a 4-inch telescope will show a faintly glowing patch of diffuse light that’s as large as the apparent size of the full Moon.  Known as NGC 7000, it is a huge cloud of glowing hydrogen gas that is thought to be stimulated to glow by the intense ultraviolet radiation of the nearby star Deneb.  Between Albireo and the bright star Altair are a pair of obscure constellations, Vulpecula (the Fox) and Sagitta (the Arrow).  The latter was known to Ptolemy and was included in his epic work “The Almagest”, while the Fox was “invented” by the astronomer Johannes Hevelius in the 17th Century.  These constellations don’t have many bright stars, but they do have some interesting telescope targets.  Your binoculars should easily reveal a nearly straight line of half a dozen stars with a small “hook” hanging below the middle stars.  Known as “The Coathanger”, it seems to always surprise me when I sweep my small telescope over it.  Just under seven degrees southeast of the Coathanger you may notice a fuzzy blob of light in binoculars.  A small telescope will resolve it into a swarm of stars set against a rich Milky Way background.  This is Messier 71, a very peculiar type of globular star cluster located some 12,000 light-years away.

Jupiter spends the early evening hours wheeling through the southwestern sky.  You should be able to spot him just west of the meridian shortly after sunset, and that should give you a couple of hours to view him in the telescope.  The giant planet reached the second stationary point of this year’s apparition last week, and he will now slowly begin to creep eastward against the stars.  

Saturn starts the evening east of the meridian, transiting the southern point on the horizon at around 10:00 pm.  Like Jupiter, Saturn never gets very high above the horizon for our viewing pleasure, but it is still worth a look through the telescope.  Almost any telescope should show the planet’s signature rings, and once you’ve seen them you’ll find yourself staring at them in rapt wonder.

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