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The Sky This Week, 2021 October 19 - 26

What's there to see under a bright fall Moon?
Collinder 399,
Collinder 399, "The Coathanger" in Vulpecula, 2016 August 25,
imaged with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor,
and a Canon EOS Rebel T2i DSLR from Mollusk, Virginia.
Note the galactic star cluster NGC 6802 to the right. South is at the top.

The Hunter’s Moon brightens the overnight hours for the first few evenings of the week.  Despite her waning phase she rises well before midnight through the week.  Last Quarter occurs on the 28th at 4:05 pm Eastern Daylight Time.  Look for the Moon to the southwest of the Pleiades star cluster on the evening of the 22nd.  On the following evening she passes north of the bright star Aldebaran in the constellation of Taurus, the Bull.  Luna ends the week drifting among the stars of Gemini.

Late October’s night sky sees the transition from the bright stars of late summer to those of early winter.  At the end of evening astronomical twilight, which occurs shortly before 8:00 pm local time, you will find the bright stars of the Summer Triangle asterism just past the meridian.  The Triangle’s faintest star, Deneb, lies directly overhead at this time.  With the light of the Full Moon washing out most of the fainter stars and deep-sky objects, but there are still a number of sights to enjoy.  One of my favorite urban and bright-sky targets lies in the center of the Summer Triangle.  This is the star Albireo, which can be seen with the unaided eye under clear suburban skies.  Point a small telescope at this star and you will be rewarded with a beautiful pair of stars with a striking color contrast.  The brighter component shines with a golden yellow hue, while the fainter one has a distinctive blue cast.  For this reason I like to call it “The Navy Double”.

Another interesting object in this part of the sky is located about one-third of the way along an imaginary line from Altair, southernmost of the Triangle’s stars, and Vega, the asterism’s brightest luminary.  If you scan this area with binoculars you will see a straight line of six stars with a hook-shaped group of four star that seem to hang below the line.  First described by the 10th Century Persian astronomer Abd al-Rahman al-Sufi, it is variously known as Brocci’s Cluster, Collinder 399, or simply “The Coathanger”.  For many years it was thought to be a true (albeit sparse) galactic star cluster, but new data from astrometric satellites now show it to be a chance alignment of stars whose distances range from 230 to over 2000 light-years.  Like many amateur astronomers, I “discovered” this asterism by accident in one of my first nights out under the stars.  It is a fun target for small, low-power telescopes at star parties.

As the Summer Triangle heels to the west, the first bright stars of the Great Winter Circle rise in the east.  At the forefront of winter’s constellations is a small knot of stars that are the subject of more myths and legends than just about any other group of stars in the sky.  We call this stellar knot “The Pleiades” or “The Seven Sisters” from their place in Greek mythology, where they represented the seven daughters of the god Atlas.  The group is described early in the history of ancient Sumerian, Egyptian, and Chinese astrology as well as in the sky lore of Australian Aboriginal and various Mesoamerican societies.  Stories of the seven stars have been recorded in the folklore of Native Americans, and they even appear in the fictional sky lore of J.R.R. Tolkien’s Middle Earth, where they were called “Remmirath”, the “Netted Stars” in Elvish.

As the Sun sets, look toward the southwest for the bright glow of Venus.  The dazzling planet is now moving eastward along the lowest reaches of the ecliptic and will reach her most southerly declination on the 25th.  Over the course of the next several weeks she will begin to turn northward; this will allow her to become more prominent in a dark sky after twilight ends.

Saturn becomes visible as he transits the meridian at the end of twilight.  The ringed planet is very gradually beginning to creep eastward among the stars of Capricornus.  Over the next several weeks you can watch his progress compared to the constellation’s brightest star, third-magnitude Dabih.

Jupiter seems to hang above the stars that mark the “tail” of Capricornus, Deneb Algedi and Nashira.  The three objects form a nice triangle in binoculars, and you should be able to see Old Jove’s four bright Galilean moons flanking their master in your binoculars on the evening of the 24th.  

Early risers can spot the elusive planet Mercury in the southeastern sky as morning twilight begins to brighten the sky.  On the morning of the 21st look for the fleet planet one degree to the south of the third-magnitude star Porrima.  Mercury reaches greatest elongation from the Sun on the morning of the 25th.