The Sky This Week
U.S. Naval Observatory's Weekly Blog

Some Stories in the Stars

by Geoff Chester, USNO Public Affairs
05 October 2021

NGC891, the "other" Andromeda galaxy,
imaged 2021 October 2 from Great Meadow, Old Tavern, Virginia
with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a ZWO Optical ASI183 CMOS color imager

The Moon returns to the evening sky this week, waxing through her crescent phases as she dives to the southernmost reaches of the ecliptic.  Look for Luna close to the dazzling planet Venus low in the southwestern sky on the evening of the 9th.  If you have a clear view to the southwest look for the second-magnitude star Dschubba, one of three stars that mark the “head” of Scorpius, the Scorpion.  It will be less than one degree above Venus on the same evening.  First Quarter occurs on the 12th at 11:25 pm Eastern Daylight Time.
 
This is the time of year when we see the departure of one constellation in the west and the appearance of another one in the east.  What’s interesting about this event is that both constellations are linked to each other in ancient sky lore.  

As mentioned above, Scorpius is still visible in the southwest as evening twilight ends.  Its brightest star, the red supergiant Antares, sets at around 9:00 pm local time, following bright Venus below the horizon by half an hour.  By midnight a similar-colored star appears over the eastern horizon as the star Betelgeuse heralds the rise of Orion.  Antares and Betelgeuse are stars with very similar characteristics.  Both are highly-evolved massive stars that have swollen to gigantic proportions as nuclear fuel burns in expanding shells surrounding inert cores.  Place either one in the Sun’s place in our solar system and our Earth would be swallowed up in the stars’ outer layers.

In mythology Orion was seen as a proud, powerful, and boastful hunter, son of the Gorgon Euryale and the Sea-god Poseidon.  He claimed to have the ability to kill any animal on the planet, which raised the ire of Gaia, goddess of the Earth.  To teach Orion a little humility Gaia sent Scorpius to kill Orion, but the Hunter was saved thanks to the efforts of Ophiuchus, whom the Romans associated with Asclepius, the first medical doctor.  When it came time to place Orion in the heavens, Zeus place him in a spot opposite that of Scorpius so that the two would never meet each other again.

Between Scorpius and Orion another ancient mythological story plays out in the autumn evenings.  The season’s brightest stars are climbing in the northeast, marking the constellations of Andromeda, Cassiopeia, Pegasus, and Perseus.  Cassiopeia is identified by a “W”-shaped group of stars set in the faint band of the Milky Way.  Pegasus is identified by a large square.  Between Pegasus and Cassiopeia two diverging “chains” of stars mark Andromeda.  Perseus lies between Cassiopeia and the horizon and to my eye forms a nice “wish-bone” asterism.  These constellations were inspired by a story that dates back over 2500 years.  In a nutshell, Cassiopeia, the vain Queen of Ethiopia, boasted that her beauty was greater than that of the half-mortal Sea Nymphs, who naturally objected to this brash claim.  To exact a suitable penalty for her vanity, Poseidon forced Cassiopeia to chain her daughter Andromeda to a rock as a sacrifice to a sea monster.  Fortunately, the hero Perseus arrives in the nick of time on his flying horse Pegasus, dispatches the sea monster, and frees the hapless Andromeda, whom he later married. 

As mentioned earlier, Venus greets evening skywatchers during evening twilight.  When she appears in the fading twilight you should be able to find Jupiter rising in the southeast.  Old Jove is currently perched above a pair of stars, Deneb Algedi and Nashira, which mark the “tail” of Capricornus, the Sea-goat.  Jupiter is nearing the second stationary point in this year’s apparition, so he’ll be close to the stars for the next couple of weeks.

Saturn reaches his second stationary point on the 11th and will gradually resume eastward motion across the stars of Capricornus through the rest of the year.   You will find the ringed planet on the meridian at the end of evening twilight.  This is the optimal time to view him through a telescope, since he will be at his highest elevation above the southern horizon.  Any telescope will reveal the planet’s icy rings, so take a few moments to give him a look.  You’ll be the “rock star” of your neighborhood!
 
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