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The Sky This Week, 2019 August 27 - September 3

A sky for three seasons.
The Old Moon in the arms of the New Moon, imaged 2018 January 20 at Alexandria, Virginia.
"The Old Moon in the arms of the New Moon"
HDR image made 2018 January 20 with an Explore Scientific AR102 102mm (4-inch) f/6.5 refractor telescope
and a Canon EOS Rebel SL2 DSLR from Alexandria.

The Moon begins the week as a slender waning crescent visible just before sunrise and returns to the early evening sky by the week’s end.  New Moon occurs on the 30th at 6:37 am Eastern Daylight Time.  This is the second New Moon for the month of August.  Having two primary lunar phases occur in the same calendar month is not particularly unusual, occurring once or twice each year.  In 2020 April will have two First Quarter phases, and October will have two Full Moons.  This week look for the waxing crescent Moon near the bright star Spica in the early evening hours of September 1st and 2nd.

The August campaign for the Globe at Night citizen-science continues through the end of August.  This month’s target constellation is Cygnus, the Swan, which we highlighted last week.  If you are away from the city for the upcoming holiday weekend take a few moments to let your eyes adapt to the darkness, then look for the constellation and contribute your observations to the program’s website.  You’ll find Cygnus near the zenith at around 10:00 pm local time.

As August transitions to September we start to see the stars of late spring heeling over to the west while the first stars of autumn ascend in the east.  At 10:00 pm the bright springtime beacon of the star Arcturus flickers over the western horizon.  This is the brightest star in the northern hemisphere and the fourth-brightest in the entire sky.  Its rosy tint is due to its relatively cool surface that has been inflated by its evolution into an older star.  It has exhausted the hydrogen in its core so the fusion reactions that sustain its light output now come from a slowly expanding shell that is causing its diameter to grow to about 25 times that of the Sun.  Watch Arcturus as it dips toward the horizon.  As its altitude decreases its light must shine through more and more of Earth’s atmosphere, causing it to “twinkle” with increasing ferocity.  It’s not unusual to see it cycle through all of the colors of the rainbow as it nears the horizon and sets at around midnight. 

At 10:00 pm the bright stars of the Summer Triangle pass directly overhead, highlighting the constellations of the summer sky.  Bisecting the Triangle is the soft glow of the Milky Way.  From a dark site you can see this faint river of diffuse light arcing from the southwest to the northeast, punctuated by the light of brighter stars.  The constellation of Scorpius dominates the southwestern sky, highlighted by the ruddy star Antares.  This star is even more evolved than Arcturus, and its diameter is over 700 times that of the Sun!  To the northeast look for a small group of second-magnitude stars that resemble the letter “W”.  This is Cassiopeia, the queen of Ethiopia whose vanity resulted in a classic tale from Greek mythology that we’ll highlight in a few more weeks.

Looking to the east, a large square asterism is now climbing higher as the nights pass.  This easily recognized asterism is part of the constellation of Pegasus, the mythical Flying Horse that also figures prominently in the Cassiopeia tale.  Seeing this constellation climbing into the evening sky is one of the signs that tells me that cool autumn nights are on the way with the promise of more clarity than the hazy nights of summer.  The “Great Square” is also a good indicator of the darkness of your local sky.  From most urban and suburban locations you probably won’t see any stars inside the square, while rural skies will reveal at least half a dozen between the four corners.

Jupiter beams down in the southwestern sky during the evening hours.  He is hard to miss since he is the brightest object in the sky after the Moon.  He is slowly beginning to resume direct motion eastward against the stars, and over the next few weeks you can gauge his progress as he edges away from the star Antares.  By this time next year he will be closing in on Saturn.  

Saturn lingers just east of the top of the “Teapot” asterism of Sagittarius.  The ringed planet crosses the meridian just before 10:00 pm, offering you many hours to enjoy a view of his famous appendages. 

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