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The Sky This Week, 2019 July 9 - 16

Exploring summer's brightest stars.
Moonlight on the Rappahannock River, imaged 2019 July 5 from Mollusk, Virginia
Moonlight on the Rappahannock River, imaged 2019 July 5
with a Canon EOS Rebel SL2 DSLR from Mollusk, Virginia.

The Moon waxes in the evening sky this week, diving southward along the ecliptic as her phase increases to Full Moon, which occurs on the 16th at 5:38 pm Eastern Daylight Time.  July’s Full Moon is popularly called the Hay Moon, Buck Moon, or Thunder Moon in various native traditions, and this year here in Washington the last one seems particularly appropriate.  Look for the Moon poised just above the “head” of Scorpius, the Scorpion on the evening of the 12th.  On the following night you’ll find her next to the bright planet Jupiter.  On the evening of the 15th Luna sidles up to the yellow-hued glow of Saturn.

July 16th marks the 50th anniversary of the launch of Apollo 11, on what has to be considered one of the greatest voyages of exploration in history.  With the benefit of hindsight it is still remarkable to me that the mission succeeded at all.  It had been less than 12 years from America’s first attempt to launch a satellite, Vanguard, which ended in a spectacular failure on the launch pad, to the flight of the world’s largest rocket, the Saturn V, hurling three men toward the Moon.  While the political motivations for going to the Moon have been debated over the past five decades, the fact remains that Apollo astronauts visited the lunar surface six times, and the scientific and technological advances that they produced have radically changed our lives.  Today we all depend on technology descended from the development of the Apollo program, routinely using computer systems and space-based communication and positioning systems that are all but taken for granted.  This week, when you look at the Moon, think about those pioneering boot prints in the fine lunar soil made in July, 1969.  If you remember the mission, pass your memories on to those who now read about it as “history”.

The light of the Moon washes out the Milky Way’s soft glow this week, but you can still follow the Galaxy’s course through the summer sky by looking for the brightest stars that lie along its path.  Most prominent of these are the stars that form the asterism known as the Summer Triangle, which is now high in the east as evening twilight ends.  The bright blue-tinted star Vega leads the Triangle across the sky accompanied by a small parallelogram of third-magnitude stars that make up the constellation of Lyra, the harp.  Vega is the fifth-brightest star in the sky, and for many years was used as a “standard” calibration star for astronomical observations.  Today we know that it is anything but “standard”.  New observation techniques have shown that Vega rotates very rapidly, squashing it into a flattened sphere that makes it quite unusual.  It is a beautiful sight in a small telescope.  Nearby in a low power telescope field you’ll see a close pair of stars; ramp up the magnification and each of these stars resolves into a close binary pair.  Known as the “Double Double”, it is a very popular target for amateur astronomers.  The southernmost star in the Triangle is Altair, a whitish-hued luminary just under 17 light-years from Earth.  Like Vega, Altair is a fast rotator, spinning once every 8.9 hours.  Compare that figure to the Sun’s figure of around 24 days!  The third star in the Triangle is Deneb, which marks the northern apex.  Deneb is one of the most luminous stars visible to the naked eye, shining with the radiance of nearly 200,000 Suns!  If you look in the middle of the Triangle you’ll see a second-magnitude star that’s well worth a look.  Known as Albireo, it is one of the best double stars to observe in a small telescope.  Easily resolved at low magnifications, what appears to the eye as a single star splits into a dazzling pair that show wonderful yellow and blue colors.  

Jupiter dominates the southern sky as he arcs across the meridian at around 11:00 pm local time.  The giant planet offers a fine view for the small telescope.  You can watch the changes in the configurations of his four bright Galilean moons from night to night, and if the air is steady you should be able to discern details on his cloud-streaked face.

Saturn is now visible all night long, rising in the southeast at sunset and crossing the meridian at around 1:00 am.  Saturn’s distinctive rings can be seen in a steadily held pair of binoculars, but the real treat is the view through a telescope.  On nights of steady air look for the gap in the rings known as Cassini’s Division.  This thread-like dark line near the rings’ outer edge is a region where ring particles are swept out of the ring system by gravitational resonances with the planet’s innermost moons.

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