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

The Horse in the Square

by Geoff Chester, USNO Public Affairs
28 September 2021

Messier 31, the Great Andromeda Galaxy,
imaged 2017 October 21 from Mollusk, Virginia
with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a Canon EOS Rebel T2i DSLR

The Moon begins the week high among the rising winter constellations before she heads southward along the ecliptic toward the first stars of spring.  She wanes from her Third Quarter phase to New Moon, which occurs on October 6th at 7:05 am Eastern Daylight Time.  Due to her high declination she is very prominent in the clear autumn daytime skies for several hours after sunrise.  If you’re up before the Sun on the 30th look for Luna near the bright stars Castor and Pollux, the Twin Stars of Gemini.  On the morning of the 3rd she passes between the stars Regulus and Algieba, the two brightest stars in the rising constellation of Leo, the Lion.
 
With the Moon waning in the morning sky it is time for the October observing campaign for the Globe at Night citizen-science program.  Sponsored by the National Science Foundation, the program aims to engage people to become aware of the night sky and the impediments to seeing it.  So far this year over 21,000 observations have been submitted from observers around the world.  The basic idea is simple: find a familiar constellation and see how many of its stars you can see.  This month’s featured constellation is Pegasus, the mythical Flying Horse who figures prominently in the legend of Perseus and Andromeda.  You will find Pegasus rising in the eastern sky at around 9:00 pm local time, but don’t expect to find a group of stars with a clear resemblance to a horse, flying or otherwise.  The main feature of the constellation is a near-perfect square composed of second-magnitude stars leading to its popular name of the Great Square.  The test of a dark sky lies in the number of fainter stars that you can identify within the Square’s bounds.  To participate in the Globe at Night program, just go to the activity’s website and compare your view of Pegasus with the charts on the site, then report your results on the web app.  From my bright suburban sky I can see the main stars in the square, but from rural sites I can spot about a dozen.

The Great Square of Pegasus follows the Summer Triangle asterism across the later evening skies.  From a dark site you’ll notice that Pegasus is well away from the plane of the Milky Way.  Behind the stars that form the constellation is an enormous gulf of empty space that is interspersed with scattered fuzzy blobs of light, the signatures of distant galaxies.  Many of these are visible in modest telescopes, but one is actually visible to the unaided eye.  To find it, locate the star Alpheratz, the brightest of the Square’s “corner” stars.  Trailing this star is a diverging pair of star “chains” that delineate the constellation of Andromeda.  “Star hop” to the second star in the brighter chain, and look a few degrees above it.  Under dark skies you will see what appears to be a small detached piece of the Milky Way which, in fact, is another galaxy altogether.  Known as the Great Andromeda Galaxy, that fuzzy blob is the combined light of some 200 billion stars located 2.5 million light-years away!

Venus is now moving along the southernmost reaches of the ecliptic.  You can spot the dazzling planet low in the southwestern sky shortly after sunset, and as the sky darkens look for the stars of Scorpius to gradually appear.  During the course of the week Venus closes in on the three stars that demark the Scorpion’s “head”.  She will pass through those stars next week.

Saturn appears in the southern sky as darkness falls, leading the much brighter Jupiter across the southern sky.  The ringed planet lies just under the third-magnitude star Dabih, which marks the “head” of the mythical Sea-goat Capricornus.
 
Jupiter finds himself on the opposite end of Capricornus, slowly drifting westward above the stars Deneb Algedi and Nashira, which mark the Sea-goat’s “tail”.  The trio form a nice triangle for binocular owners, and careful scrutiny with these glasses will show Old Jove’s bright Galilean moons.  However, Jupiter is also well worth viewing with a telescope.  No other planet shows such a large and variable apparent disc.  Smaller instruments will show the planet’s dark parallel equatorial cloud belts, while each increase in aperture will show more detail.  Look for the planet’s famous Great Red Spot on the evenings of September 29th, October 1st, and October 4th.
 
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