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The Sky This Week, 2018 September 11 - 18

Trace the Milky Way despite the moonlight.
Mars,
Mars, 2018 September 6, 01:27 UT
imaged with a 9.25-inch Schmidt-Cassegrain telescope
from Alexandria, Virginia.

A waxing Moon moves along the southern reaches of the ecliptic this week, passing the four planets visible to the naked eye along the way.  First Quarter occurs on the 16th at 7:15 pm Eastern Daylight Time.  Look for her crescent nine degrees north of bright Venus in the gathering evening twilight on the evening of the 12th.  On the following evening she will be just northeast of Jupiter.  The Moon will be just east of Saturn on the evening of the 17th, then close in on ruddy Mars, passing four degrees north of the red planet on the night of the 19th.

The Moon gradually washes out the star clouds of the Milky Way as she works her way through the week.  As Luna’s glare erodes the view of the fainter stars, you can still trace out the path of the Milky Way by noting the distribution of the bright stars that the Moon can’t overcome.  You can start with the red-tinted star Antares, which may be found low in the southwest at the end of evening twilight.  Shifting your gaze to the northeast you’ll pass the “Teapot” asterism of Sagittarius.  This year the bright planet Saturn is perched just above the teapot’s “spout”; the asterism is made up of mostly second-magnitude stars, so it should be visible despite the presence of the Moon.  Looking higher in the south you should see the southernmost star of the “Summer Triangle” asterism.  This bright star is Altair, one of the closest stars visible to the naked eye.  It lies at a distance of just under 17 light-years.  It has an unusually fast rotation, which causes its equatorial diameter to be 25 percent larger than its polar diameter!  The brightest star in the Triangle, Vega, is overhead at around 9:00 pm.  It is also relatively nearby at a distance of 25 light years, and like Altair it is also a fast rotator; its equatorial rotation speed is some 100 times faster than that of the Sun.  The third star in the Triangle is Deneb.  Although it isn’t quite as bright as its companions, it is much farther away, some 100 times the distance to Vega.  This means that Deneb is intrinsically very bright, shining some 150,000 times brighter than the Sun.  If we could somehow throw a giant lasso around Deneb and drag it to the distance of Altair it would have the equivalent brightness of the first quarter Moon.  Our last stop along the Milky Way trail takes us to the compact constellation of Cassiopeia.  The five second-magnitude stars form a pattern that looks like the letter “W”.  In Greek Mythology the pattern is supposed to represent the figure of a queen sitting on her throne looking at herself in a mirror.  Personally I have trouble seeing that pattern, but I find that the Inuit interpretation is much better.  They see five reindeer drinking from the “Pebbly River”, which will be revealed as a bright patch of Milky Way once darker nights of the waning Moon return.

Venus continues to settle toward the southwest horizon during evening twilight.  Despite the fact that her elongation from the Sun is near its maximum, her orbital plane intercepts our local horizon at a very shallow angle, which keeps her close to the horizon when she appears in the twilight glow.  Fortunately she is reaching her maximum brightness for the year, so keep an eye out for her just after sunset.

Jupiter can also be found in the southwestern sky at dusk.  He’s higher in the sky than Venus, and you have about an hour to follow him after the end of evening twilight.  He’s joined by the crescent Moon on the evening of the 13th. 

Saturn is well placed for viewing as darkness settles over the landscape.  You’ll find the ringed planet in the south, about 25 degrees above the horizon.  Despite his low altitude, he is always worth a good look through the telescope, and his rings are as wide open as they can be to our line of sight.  If you have an evening with steady air look for the Cassini Division near the outer edge of the rings.  This gap spans a distance equivalent to about half the diameter of the Earth.

Mars has faded by a full magnitude since his close approach at opposition in late July.  This is an indication of how quickly the distance between Earth and the red planet is increasing.  Despite that Mars still dominates the southeastern sky in the mid-evening.  If you own a modest telescope, you’ve only got a few more weeks to get a good view of his enigmatic surface features.

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