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The Sky This Week, 2020 September 8 - 15

Follow that Swan!
Jupiter and three moons, imaged 2020 September 6 fron the U.S. Naval Observatory, Washington, DC.
Jupiter and three moons, imaged 2020 September 6 fron the U.S. Naval Observatory
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor and a ZWO ASI224MC color imager

The Moon passes through the rising winter constellations as she wanes through her crescent phases.  New Moon occurs on the 17th at 7:00 am Eastern Daylight Time.  Look for the Moon between the bright star Aldebaran and the Pleiades star cluster before dawn on the 9th.  On the morning of the 13th she forms a straight line with Castor and Pollux, the Twin Stars of Gemini.  She wraps up the week with a close visit to dazzling Venus in the gathering morning twilight of the 14th.

The September observing campaign for the Globe at Night citizen-science project gets underway on the evening of the 9th and runs through the evening of the 18th.  This international cooperative program invites stargazers of all persuasions to determine the relative brightness of their night sky by counting stars in particular seasonal constellations.  This month’s target is Cygnus, the Swan, which is located near the zenith at around 9:30 pm local time for much of the temperate regions of the Northern Hemisphere.  Deneb, the constellation’s brightest star, is also the northernmost and faintest of the three stars that comprise the Summer Triangle asterism.  Deneb means “tail” in ancient Persian, and the swan’s “head” may be found in the middle of the triangle in the third-magnitude star Albireo.  Three second-magnitude stars delineate the Swan’s wings, each of which seem to sweep back toward Deneb along trains of fainter stars.  The basic outline looks like a cross, and the constellation is sometimes called the “Northern Cross”.  From suburban locations on good transparent nights you should have no trouble spotting these brighter stars, but try to find some tome to view the group from a darker location.  Now you can see the resemblance to a swan in flight, and it looks as if that swan, like most savvy fowl in the fall, is headed along the Milky Way bound for more southerly climes.  Compare your views of Cygnus with the charts on the Globe at Night Web app and join well over 20,000 other observers who have contributed their measurements so far this year.

Cygnus contains a number of treats for the unaided eye and the small telescope.  The star Deneb, while comparable in brightness to the other first-magnitude luminaries in the triangle, is a very different kind of beast that either Vega or Altair.  These latter two stars are relatively close to the solar system at 25 and 16 light-years respectively.  Their brightness is due to their proximity and the fact that they are each more intrinsically bright than our Sun.  Deneb, however, is 100 times more distant than Altair, which means its luminosity approaches 200 thousand times that of the Sun! 

Albireo is one of my favorite small telescope targets, exhibiting a golden color cast in the finder of my 3-inch refractor.  The telescope itself splits Albireo into two components with a wonderful blue and gold color combination.  At star parties I often call it the “Navy Double” due to its striking colors.  Scanning the region between Albireo and Deneb with the small scope reveals streams and clumps of faint stars and the brighter fuzzy patches of star clusters scattered along the Milky Way.  From dark sky locations you can see that the Milky Way itself seems to split into two distinct sections near Albireo, and the dark “rift” between the star clouds continues down to the southern horizon.  A cursory glance reveals a distinct absence of stars in these dark areas, which are made up of cold opaque clouds of interstellar gas and dust, the stuff that stars, planets, and all that inhabit them are made from.

Jupiter greets us from near the meridian as evening twilight ends.  Old Jove reaches the second stationary point of this year’s apparition on the 13th; he will slowly begin to plod eastward toward Saturn over the next few months.  The giant planet is full of surprises for earthbound observers as violent upwellings erupt in the planet’s dense atmosphere.  A good six-inch telescope will show these evolving features, and you’ll never have the same view of the planet from night to night.

Saturn’s atmosphere is serene compared to Jupiter’s.  Well-defined dark and light bands circle the planet’s disc, and eruptive activity is rare.  The planet’s disc is less than half that of Old Jove, though, so features need to be much more pronounced to be seen from Earth.  The planet more than makes up for its blandness with its rings, which look spectacular in almost any telescope.

I finally had my first good look at Mars in the pre-dawn darkness while the red planet was not far from the Moon.  While the Moon shows an endless variety of other-worldly landforms, Mars’ surface is much more subtle.  In both cases we are looking at solid ground, but Mars is much more remote than the Moon.  Large-scale features like the south polar ice cap and dusky patches are the most one can hope for on a night of typical “seeing” so you must wait for those fleeting moments of calm to catch the best views of this diminutive world.

Venus spends the week crossing the faint constellation of Cancer, the Crab.  On the mornings of the 12th, 13th, and 14th she lies close to the constellation’s most famous object, the Praesepe star cluster.  Binoculars should provide a fine view for early risers.

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