The Sky This Week, September 12 – 19, 2017 !

The Sky This Week, 2017 September 12 – 19

The swan in the Milky Way
The Summer Milky Way, 2017 August 21,
imaged from Smith’s Ferry, Idaho by Geoff Chester
with a Canon EOS Rebel T2i DSLR, 30-second unguided exposure.

The Moon climbs northward along the ecliptic this week, passing through the rising stars of the Great Winter Circle as she wanes to a slender crescent. You’ll have some excellent photo opportunities as Luna joins the planets Venus, Mercury, and Mars in the pre-dawn twilight on the mornings of the 17th and 18th. New Moon occurs on the 20th at 1:30 am Eastern Daylight Time.

With Luna waning in the morning sky it’s time once again to go out and count stars for the Globe at Night citizen-science program. From now until the 21st skywatchers are encouraged to look for the constellation of Cygnus, the Swan, and compare the number of stars you see with the viewing charts on the program’s website. Cygnus is one of the easiest constellations to locate in the summer sky. Its brightest star, Deneb, is the third-brightest star in the Summer Triangle asterism and passes almost directly overhead at 10:00 pm local time. “Deneb” is a loose translation of the Arabic word for “tail”, and it marks the tail of the celestial swan that represents a number of legends in Greek mythology. The most popular of these has Zeus transforming himself into the bird to seduce the beautiful Leda, who ultimately bore him the Gemini twins Castor and Pollux. In the sky the swan’s body stretches from Deneb to the middle of the Summer Triangle, where its head is represented by the beautiful double star Albireo. The swan’s wings cross the “body” about one third of the way from Deneb to Albireo, and under a dark sky the constellation bears a striking resemblance to its namesake. Cygnus, like most Northern Hemisphere birds, appears to fly southward along the Milky Way. One of my favorite alternative interpretations of this constellation comes from Inuit skylore, where the pattern represents a man in a kayak paddling along the “Pebbly River” of the Milky Way. If you can get away from city lights to do your star count, take your binoculars with you. This area of the sky contains some of the densest star clouds of the Milky Way as well as the head of the “Great Rift” that begins to bisect the Milky Way, splitting it into two apparent streams as it wends its way to the southern horizon.

Jupiter appears briefly in the evening twilight, popping out of the dimming sky above the southwest horizon shortly after sunset. He doesn’t spend much time in view, though, setting by the end of evening twilight. Use your binoculars to see if you can spot the bright star Spica; Old Jove will slowly pass just north of the star over the course of the week.

Saturn continues to shine down in the evening hours in the southwestern sky. The ringed planet will be the brightest object in this part of the sky as evening twilight ends. On the morning of the 15th, the long-lived Cassini spacecraft will plunge into Saturn’s atmosphere after orbiting the planet for more than 13 years. Entry is scheduled for 7:55 am EDT, and NASA engineers hope to receive data on the composition of Saturn’s cloud tops before the spacecraft succumbs to its fiery destruction.

Pre-dawn skywatchers with flat eastern horizons and binoculars have an opportunity to see three planets and the Moon gather together shortly before sunrise. The best time to look is the morning of the 18th. First look for dazzling Venus, which will be about 15 degrees above the horizon at 6:00 am. Look about five degrees below Venus for the slender arc of the thin crescent Moon. Five degrees below the Moon you’ll find Mercury, and with luck you might be able to spot ruddy Mars just one degree above Mercury. If we get a nice crisp September morning, this should be quite a show!


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