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The Sky This Week, 2019 June 25 - July 2

Wading into the Milky Way.
Globular cluster Messier 4, imaged 2014 July 5 from Morattico, Virginia
Globular cluster Messier 4 and Antares, imaged 2014 July 5 from Morattico, Virginia
with a Canon EOS Rebel T2i DSLR and an Antares Sentinel 80mm (3.1-inch) f/6 refractor.
Note the smaller globular cluster NCG 6144 to upper right of Antares.

The Moon wanes in the pre-dawn sky this week, becoming an ever more slender crescent as she moves toward the rising Sun.  New Moon occurs on July 2nd at 3:16 pm Eastern Daylight Time.  Since the Moon crosses the ascending node of her orbit as New Moon falls, observers over a narrow swath of the southern Pacific Ocean will see a total solar eclipse.  The path crosses mostly open ocean until it reaches the west coast of South America, bisecting Chile and Argentina in the late afternoon at those locations.  

The year’s latest sunsets occur this week, with Old Sol disappearing below the Washington horizon at 8:38 pm EDT on the 28th.  The time of sunset will slowly start to regress as June turns to July.  However, our earliest sunrises occurred two weeks ago and the Sun now rises four minutes later than he did on June 14th.  By July 1st the length of day has decreased by three minutes, so we’re now on the way toward the equinox in September.  

The gradually lengthening nights play host to the bright stars of summer, which flank the amorphous glow that delineates the Milky Way.  It’s well worth a trip to a good dark site to enjoy the full spectacle of this luminous band.  Unfortunately, most people today have never had a good view of its bright and dark patches that trace the plane of our home galaxy around the sky.  To the unaided eye the Milky Way is a softly glowing ribbon of light that defies resolution into discrete sources, but a simple pair of binoculars begins to show the true nature of the glowing clouds in the form of countless stars.  This is a part of the sky that can offer spectacular views through small telescopes at low magnifications.  By midnight the brightest part of the Milky Way rises up from the southern horizon, and as we gaze in that direction we are looking toward the Galaxy’s center.  This area is a wonderful area of the sky to explore with the small scope.  You’ll not only find clouds of innumerable stars, but brighter knots that betray the presence of star clusters and bright glowing gaseous “nebulae”.  You may also notice large dark voids that have virtually no stars within their bounds.  These “dark nebulae” are clouds of cold, opaque gas and dust that provide the raw materials for star formation.  This year the galactic center lies just below and to the left of the bright glow of Jupiter, but we can’t actually see it.  It is located over 30,000 light-years from our solar system, but intervening clouds of star-stuff prevent our direct view of the Milky Way’s heart.  However, we can peer through much of the gas and dust with infrared and radio telescopes, and these tell us that the center of the galaxy is a wild and violent place.  At the very heart of the Milky Way we have discovered the presence of a massive “black hole” whose mass is estimated to be more than 4 million times the mass of the Sun.  Although the Milky Way appears as a narrow band of light across the sky, it is actually a huge spiral disc of stars that slowly rotates about its center.  When we look at many of the constellations of the summer sky we’re looking stars that populate nearby spiral arms in our general vicinity.  At our distance from the galactic center the solar system and most of the stars visible to the naked eye wheel around the Milky Way’s heart in an orbit that takes some 250 million years to complete.

If you peruse the sky around the summertime Milky Way with your small telescope or binoculars you may run into several concentrated balls of hazy light.  Unlike many of the clusters and nebulae that you’ll find in the galactic plane, these objects defy resolution in telescopes of less than 6 inches of aperture.  Known as “globular clusters”, these fuzzy spheres are made up of hundreds of thousands of faint stars.  These clusters have orbits that carry them through the galactic plane and they form a sort of “halo” around the Milky Way’s core.  One of the easiest to locate is the cluster Messier 4, which lies just west of the bright star Antares in Scorpius.  It appears as a fuzzy spot in binoculars, and it begins to resolve in telescopes as small as four inches.

Mars and Mercury may still be glimpsed half an hour after sunset in the west, but you’ll need binoculars and a flat western horizon to track them down.  The two planets part company by the week’s end. 

Jupiter pops into view in the southeast shortly after sunset.  He is now the dominant object in the summer sky, sharing the limelight with the ruddy star Antares and the large star clouds surrounding the center of the Milky Way.  

Saturn trails Jupiter in the southeast, loitering just to the east of the “Teapot” asterism of Sagittarius.  You can spot his rings in binoculars, and while you’re in the area, scan the space between him and Jupiter and take in the star clouds and bright star clusters that populate this rich part of the sky.

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