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The Sky This Week, 2016 September 13 - 20

The "Coathanger" in the cosmic closet.
Collinder 399, "The Coathanger" in Vulpecula
imaged 2016 August 25 from Fishers Island, NY

The Moon brightens the overnight hours this week, with Full Moon occurring on the 16th at 3:05 pm Eastern Daylight Time. Since this is the Full Moon that occurs closest to the autumnal equinox it is widely recognized in the Northern Hemisphere as the "Harvest Moon". Most of our monthly Full Moon names have origins in Native American skylore, but the Harvest Moon is named for an actual phenomenon peculiar to this time of the year. At north temperate latitudes the angle that the Moon’s orbital plane makes with the horizon is very shallow at the point where the Moon rises. This results in the Moon appearing to rise at nearly the same time on successive nights around the time of Full Moon. Here in Washington that interval is about 30 minutes, but as you travel farther north the interval decreases. It’s 20 minutes in northern Scotland and just two minutes in Tromsø, Norway. Venture to Svalbard and you’ll find that the Moon actually rises a few minutes earlier on successive nights! All of this means that back before the use of artificial lighting the rising Moon added a little extra light at the end of the day that farmers could use to bring in a bit more of their crops. A similar phenomenon occurs in the Southern Hemisphere, but it happens in March, when we experience the vernal equinox.

The days continue to shorten as we approach the equinox. With the earlier times of sunset and twilight we can enjoy the pleasures of the sky at increasingly reasonable hours. This week we lose nearly 20 minutes of total daylight, and by this time next week we’ll see longer nights that will last for six months.

The Moon moves through the dim constellations of the autumn sky, thoroughly washing out the faint star patterns of the season, but we still have summer’s bright stars to enjoy. Last week we highlighted a pair of double stars that are easy to locate in small telescopes; this week we’ll help you find a very distinctive asterism that is popularly known as "The Coathanger". This group of 10 stars lies in the very obscure constellation Vulpecula, The Fox, a grouping invented by the Polish astronomer Johannes Hevelius in the 17th Century. While the constellation is almost impossible for even experienced amateur astronomers to trace out, The Coathanger is quite distinctive and comparatively easy to find in binoculars. It was first described by the Persian astronomer Al Sufi in the year 964 CE and was cataloged as a widely scattered galactic star cluster by the Swedish astronomer Per Collinder in 1931. We now know that it is a chance alignment of stars that form its distinctive shape. You can find it about one-third of the way along a line from the star Altair to the star Vega along the western side of the Summer Triangle. It is easy to find even from urban skies in binoculars, and on moonless nights from dark sky sites you might be able to spot it with the unaided eye.

You should now be able to spot dazzling Venus in deepening evening twilight. As the third-brightest object in the sky she should become visible at or just before sunset in the west. As the sky darkens on the evening of the 18th look at Venus with binoculars as she passes two degrees above the bright star Spica.

Saturn may be found in the southwestern sky as twilight fades. The ringed planet is about six degrees north of the bright star Antares in the constellation Scorpius. You can still get a very nice view of the planet and its enigmatic rings during the early evening hours. As the hazes of summer give way to the clearer skies of autumn you can follow Saturn a bit longer as he settles toward the horizon.

Mars continues to race eastward through the signature constellations of summer. This week he closes in on the "teapot" asterism in Sagittarius. He forms an elongated triangle with Saturn and Antares, and you can watch that triangle become a bit flatter on each successive night.

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