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

Looking For The Herdsman

by Geoff Chester, USNO Public Affairs
17 May 2022

Messier 3, globular star cluster in Canes Venatici, imaged 2022 April 30 from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a ZWO ASI183MC CMOS color imager.

The Moon skims the southern horizon this week, passing through the southern summer constellations as she wanes in the early morning sky.  Last Quarter occurs on the 22nd at 2:43 pm Eastern Daylight Time.  Luna begins the week among the stars of the constellation of Sagittarius, whose distinctive “Teapot” asterism marks one of the densest parts of the Milky Way.  The Moon lies just south of the Teapot’s brightest star, Nunki, on the morning of the 19th.  On the 22nd she passes five degrees south of Saturn.  Luna passes south of ruddy Mars and bright Jupiter in the gathering morning twilight on the 24th and 25th.

The May observing campaign for the citizen-science Globe at Night program will begin on the 21st and run through the end of the month.  The program’s aim is to enhance awareness of the night sky by encouraging people to locate familiar constellations and reporting the number of stars that they see from their particular location.  May’s target constellation is Boötes, the Herdsman.  This star pattern is marked by the bright star Arcturus, which is now rising high in the eastern sky as evening twilight ends.  Arcturus is hard to miss.  It is the brightest star in the northern hemisphere sky and has a distinctive rose-hued tint.  The rest of the constellation is a bit harder to pick out from urban skies, but it occupies much of the space between the “handle” of the Big Dipper and Arcturus.  To the ancients it represented a “Herdsman” or “Plowman”, and the stars of the Big Dipper represented his plow.  In some traditions Boötes is the inventor of the plow, while in others he holds two dogs on a leash and herds the Great and Little Bears in their never-ending journey around the celestial pole.  However, to my eyes the constellation looks more like a kite or an ice cream cone, with Arcturus marking the come’s tip.  The fainter stars of the constellation are difficult to sight from the city, but more stars become visible as you travel to darker sites.  Use the handy charts on the Globe at Night website to locate Boötes and to report your findings.

The second-brightest star in Boötes is Izar, which lies about 10 degrees northeast of Arcturus.  Izar is one of the prettiest double stars in the sky, but it’s a challenge for the small telescope.  The stars sport yellow and blue colors, but they are very closely spaced.  They are a challenge in my 3.1-inch refractor, but on a night of steady air the sparkle like a closely set pair of jewels.  

About 12 degrees northwest of Arcturus is one of the true telescopic showpieces of the night sky.  Messier 3 is a globular star cluster, a dense congregation of hundreds of thousands of stars that may be the remnant nucleus of a dwarf galaxy consumed by its passage through the Milky Way.  These clusters are very old; spectrographic analysis indicate that their component stars are some of the most ancient stars in the universe.  Globular clusters are characterized by a very concentrated core of stars that become more scattered with distance from the core.  Small telescopes will show a bright glowing ball that may show a few resolved stars around the edges, but the view through a six-inch or larger telescope will show many of them in their full glory.  These clusters dominate the summer sky, and Messier 3 is one of the first to be well-placed as late spring turns to summer.

The evening sky remains bereft of bright planets, so if you want to see them you will need to rise before the Sun.  Morning astronomical twilight now begins at around 4:00 am, when you will find Saturn well up in the southeastern sky with Mars and Bright Jupiter just clearing the horizon.  An hour later the dazzling planet Venus should be easily visible for viewers with a clear view of the horizon.  During the week Mars inches closer to Jupiter, and on the mornings of the 24th and 25th the two planets are joined by the waning crescent Moon.  Photo opportunity, folks!
 
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