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

Seeing the Strongman's Stars

by Geoff Chester, USNO Public Affairs
01 June 2021

Globular Clusters Messier 13 (top) and Messier 92 in Hercules
imaged 2020 July 27 from Mollusk, Virginia

with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a Canon EOS Rebel T2i DSLR

The Moon wanes in the pre-dawn sky this week, climbing northward along the ecliptic as she passes through the rising constellations of the autumn sky.  New Moon occurs on the 10th at 6:52 am Eastern Daylight Time.  At that time much of the north polar regions and Atlantic Ocean will experience the first of the year’s two solar eclipses.  We’ll discuss this event more next week.

The Moon’s absence from the evening sky means that it is time for the June observing campaign for the Globe at Night citizen-science project.  Regular readers will know that I tend to tout this program often during the course of the year, and I do so because it is a great way to demonstrate the problems astronomers face trying to find accessible dark skies to make their observations.  Over the course of my forty-plus years of living in the shadow of the nation’s capital I have seen first-hand how the rapid proliferation of nighttime lighting has affected my local skies and the biomes that depend on them.  The once-pristine dark skies of Northern Virginia’s Blue Ridge are becoming brighter, and searching for elusive deep-sky objects has become much more of a chore than a pleasure.

Globe at Night helps to chart the disappearance of dark skies as well as track places where the night can be seen in all of its glory.  Participation is simple; just find the month’s target constellation, observe it with the unaided eye, and report your findings to their website.  This month’s constellation is Hercules, a sprawling constellation that lies about two-thirds of the distance from the bright star Arcturus to the slightly dimmer star Vega, now rising in the east in the late evening.  Hercules consists of mostly second- and third-magnitude stars which might make it a difficult sight for urban and suburban viewers.  Its most prominent feature is a central trapezoid asterism known to amateur astronomers as “The Keystone” due to its resemblance to the main support stone in a Roman arch.  Several “chains” of stars emanate from the corners of the Keystone to delineate the rest of the constellation.  Even though Hercules figures more prominently in Greek and Roman mythology than Orion does, I have often wondered why he doesn’t have as prominent a star pattern named for him.  

Hercules offers a number of fine sights for owners of small telescopes.  There are a number of colorful double stars within his bounds, as well as two of the northern sky’s best globular star clusters.  The first of these is Messier 13, the “Great Hercules Cluster”, which lies just below the star that marks the northwest corner of the Keystone.  It is visible in binoculars as a small round patch of diffuse light between a pair of stars, and from a very dark site it can be glimpsed with the naked eye.  My four-inch telescope reveals its true nature as a concentrated ball of faint stars that crowd together to form a soft glow across the center, while my 8-inch reflector resolves it into a myriad of tiny stellar points.  The cluster contains several hundred thousand stars and is located some 24,000 light-years away.  About six degrees north of the northeastern star in the Keystone is another globular cluster, Messier 92.  If it wasn’t located as close as it is to M13 this object would be a “show stopper” in its own right.  It is slightly smaller than M13, but its stars are easier to resolve in small telescopes.  It is located about 2500 light years beyond M13.

Closer to home, Venus is gradually becoming more visible in evening twilight as she enters the constellation of Gemini.  Her companion for the past several weeks, Mercury, is moving rapidly toward conjunction with the Sun.  Venus is now hot on the trail of Mars, and will overtake the red planet next month. 

Mars spends the week passing south of the Twin Stars of Gemini, Castor and Pollux.  By the end of the week he forms a straight line with the two stars, providing an interesting photo opportunity before he slides into the smaller dim constellation of Cancer, the Crab.

Saturn and Jupiter continue to dominate the early morning southern sky.  By the week’s end Saturn rises at local midnight, while Jupiter follows an hour later.  Early risers should get some great telescopic views of the two giant planets since Earth’s atmosphere tends to be very still in the hours before sunrise.  While I am not a “morning person”, I may have to start getting up early to see them for myself!
 
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