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The Sky This Week, 2016 June 28 - July 5

Finding Hercules.
Messier 13, the Great Hercules Globular Cluster
imaged from Fishers Island, New York, 2013 August 5
with an 80mm (3.1-inch) f/6 Antares Sentinel refractor
and a Canon EOS T2i DSLR

The Moon wanes in the pre-dawn sky this week, wending her way through the dim starfields of the autumnal sky before encountering the first of winter’s stars in the glow of morning twilight. New Moon occurs on July 4th at 7:01 am Eastern daylight time. If you’re up before the Sun on the morning of the 2nd, look for the slender crescent Moon just three degrees east of the star Aldebaran, brightest star in Taurus, the Bull.

The end of June finds the nights gradually getting longer as we are now a week past the summer solstice. We have also experienced the year’s latest sunset, but you still need to wait until after 10:30 pm to experience full astronomical darkness. For many of us, the upcoming Independence Day holiday will find many of us escaping the haze and lights of the city to find respite in the mountains or at the beach. This getaway weekend coincides with the current observing campaign for the Globe At Night citizen-science project, which seeks to document the effects of artificial lighting on our night skies. This month’s target constellation is Hercules, a large grouping of stars that takes up much of the eastern sky between the bright stars Arcturus and Vega. Unfortunately most of the stars in this group are second-magnitude and fainter, so it is almost impossible to see from bright urban skies. However, from a dark location you should be able to spot the constellation’s "Keystone" asterism, which consists of four third-magnitude stars. The Keystone lies about two-thirds of the distance from bright Arcturus, high in the south, and slightly dimmer Vega, high in the east. The rest of Hercules resembles a backwards letter "K" to my eyes, and is a constellation that I associate with some of the sky’s best telescopic treats. One of the best of these is Messier 13, a so-called globular star cluster. M13 is located just to the south of the northwestern star in the Keystone and is easily visible in binoculars as a round fuzzy patch. Through a modest telescope it blooms into a tight grouping of thousands of faint stars looking like a dandelion in bloom. M13 is the best example of this type of object in the northern sky, but around 150 others are known. Globular clusters orbit the hub of the Milky Way galaxy in long looping paths that occasionally take them through the galactic plane. They are believed to be the cores of ancient dwarf galaxies captured by the Milky Way billions of years ago since their stars are some of the oldest known in the universe. M13 is estimated to be about 22,000 light years away!

Closer to home, and easily visible for us city dwellers, a nice parade of planets offers a fine backdrop for July 4th fireworks. Jupiter becomes visible shortly after sunset in the southwestern sky while Mars shines low in the southeast. As darkness becomes deeper you’ll see Saturn about 20 degrees east of Mars, forming a triangle with the red planet and the bright star Antares in Scorpius.

Jupiter still offers a good opportunity for viewing in the telescope, but the best time to see him is now during the twilight hours. Fortunately Jupiter is very bright, so background light doesn’t affect the view too much. Even though his disc is now about 75% of its opposition size there’s still a lot to see. On the evening of the 28th you can watch the moon Europa appear out of Jupiter’s shadow, and on the 30th the Great Red Spot transits the disc during "prime time". All eyes will be on the giant planet late on the evening of the 4th as the Juno probe will arrive to enter orbit around Old Jove after a five-year journey.

Mars reaches the second stationary point in the current apparition on June 30th. As July opens the red planet will resume eastward motion against the stars, gradually closing the gap with Saturn. The two planets will meet in early September. Mars is still a good target for the telescope, but you’ll need very steady air to make out fine detail on his dusty face.

Saturn offers a spectacular view in the telescope despite his low altitude. The planet’s famous rings are tipped at nearly their maximum angle to our line of sight and may be seen in just about any telescope. Saturn is perched on the edge of the dense summer Milky Way, so the planet will be attended by a retinue of faint stars. Some of these "stars" are several of the planet’s icy moons. Titan and Rhea are visible in a 3-inch telescope. More will pop out with increasing telescope aperture.

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