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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Signs of the Season in the Stars
by Geoff Chester, USNO Public Affairs
23 November 2021
Perseus Milky Way, 2019 September 28
imaged from Blackwater Falls State Park, Davis, West Virginia
with a Canon EOS Rebel T2i DSLR.
The Moon begins the week high among the stars of the winter constellations. The waning gibbous rises close the Twin Stars of Gemini, Castor and Pollux, in the late evening of the 23rd. On the following night she passes three degrees north of the “Beehive” star cluster in the dim constellation of Cancer, the Crab. Try to spot the cluster in binoculars if you are up late preparing your turkey for the next day’s feast. Last Quarter occurs on the 27th at 7:28 am Eastern Standard Time. Early risers can watch the waning Moon move through the rising stars of the springtime sky. On the mornings of the 26th and 27th you will find her close to Regulus, the heart of Leo, the Lion. She ends the week near the bright star Spica in Virgo.
The November campaign for the citizen-science
Globe at Night
program begins on the evening of the 25th. If you are out walking off your Thanksgiving meal, take some time to
find the constellation Perseus
in the northeastern sky, located between the “W”-shaped constellation of Cassiopeia and the rising stars of the Great Winter Circle. Compare your view of the constellation to the charts on the
Globe at Night web app
to help scientists map out the spread of star-stealing light pollution.
To my eye Perseus resembles an appropriate symbol of the season in the form of a wishbone. Its brightest star, Mirfak, sits at the juncture of the wishbone’s tines. From dark locations Mirfak is surrounded by a loose grouping of half a dozen faint stars that become a wonderful cluster of blue-tinted luminaries in a pair of binoculars. Perseus’ other notable feature is its second-brightest star, Algol, whose name is the Latinized version of its ancient Arabic name, Al Ra’s al Ghul, “the Demon’s Head”. In mythology, Perseus managed to lop off the head of the Gorgon Medusa, and to the Arabs Algol represented the Gorgon’s eye. Why did this particular star receive such notoriety? If you watch it from night to night you will notice that at times it is much fainter than usual. This “winking” occurs with a very regular period of 2 days 20.8 hours. Over a ten-hour span the star fades by 1.3 magnitudes, then returns to its usual brightness. Its next minimum occurs at 6:00 am EST on December 2nd when Perseus will be in the northwestern sky before dawn. Algol is an eclipsing binary star in which a fainter star passes in front of a brighter component. It was the first of its type to be described by the British amateur astronomer John Goodricke in 1783.
Between Perseus and the northeast horizon in the early evening sky is the bright golden-hued star Capella, the brightest star in the constellation of Auriga, the Charioteer. Capella’s name roughly translates as “the Little She-Goat”, and classical depictions of the constellation show the goat perched on the shoulder of Erichthonius, the mythological inventor of the four-horse chariot. To the Romans, it represented Amalthea, the goat that nursed the infant Jupiter. During one of his more playful moments, Jupiter broke one of Amalthea’s horns, which subsequently became the Cornucopia, or Horn of Plenty, which had the ability to produce any food that one might desire. One of the symbols of our modern celebration of Thanksgiving is the Cornucopia, once again demonstrating our ancient connections to the constellations.
Venus continues to shine brightly in the southeastern sky as evening twilight falls. She is now nearing her brightest glow for this year’s evening apparition, and if you know where to look for her you should be able to see her well before the Sun sets. If you look at her with a small telescope you will see her bright disc in a crescent phase. This phase will wane as her disc grows bigger as the year approaches its end.
Saturn may be found mid-way between Venus and the bright planet Jupiter. The ringed planet’s days in our evening sky are numbered; if you want to get a good view of him in the telescope your best bet is to start observing him in twilight. He sets at around 9:30 pm local time.
Jupiter is also best seen in deepening twilight, but he lingers in the sky a bit longer than his more distant companion Saturn. Old Jove still puts on a good show for owners of small telescopes. On the evening of the 28th you can see his famous Great Red Spot as well as the shadow of his innermost moon Io at around 7:00 pm EST.
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