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The Sky This Week, 2019 August 6 - 13

The waxing Moon dances with the bright planets
Saturn, imaged 2019 July 27, 03:18 UT from Washington, DC.
Saturn, imaged 2019 July 27, 03:18 UT
with the USNO's 30.5-cm (12-inch) Clark/Saegmüller f/15 refractor
and an Antares 1.6X 2-inch Barlow lens from Washington, DC.

The Moon courses through the evening skies this week, skimming over the southern horizon as she passes through the heart of the summer Milky Way and the signature constellations of the season.  First Quarter occurs on August 7th at 1:31 pm Eastern daylight Time.   Luna cozies up to bright Jupiter on the evening of the 9th, then visits Saturn two evenings later.

The annual Perseid meteor shower reaches its peak on the night of August 12-13.  This is normally considered to be the year’s best meteor display, consistently producing up to 100 bright, fast meteors per hour for observers in dark locations.  Unfortunately this year the nearly-full Moon will wash out all but the brightest shower members, but if the weather is clear it still may be worth your while to find an open patch of sky to look for bright Perseid “fireballs”.  The best time to look is during the early morning hours when the shower “radiant” in the constellation of Perseus is rising in the northeastern sky.  First recorded by Chinese astronomers in the First Century CE, we now know that they are caused by the trail of dusty debris shed by Periodic Comet 109/P Swift-Tuttle, which was discovered in 1862.  In 1866 the Italian astronomer Giovanni Schiaparelli found a link between the Perseids and the comet; since then many comets have been identified with annual meteor showers.

Early August is the time that many people refer to as the “Dog Days”, marked by hot and humid conditions.  This expression has its origins in an interesting astronomical phenomenon that was first described in ancient Egyptian texts dating back over 5000 years.  In these ancient times early skywatchers noted that, shortly after the summer solstice, the bright star Sirius could be seen rising just before the Sun.  This event coincided with the annual life-giving flood of the Nile River and was seen as a harbinger of this vital occurrence.  This “heliacal rising” of Sirius served as a foundation for the Egyptians’ agricultural calendar for the next three millennia.  Over time the effects of precession of the equinoxes shifted the heliacal rise of Sirius later into the summer, so by Roman times it occurred in late July and then early August.  This was the hottest and most uncomfortable time of the year to the citizens of Rome, and the scorching days became known as the “Dies Caniculares” since Sirius was the lead star in the constellation of Canis Major, the Larger Dog.  It was believed that the light of this, the brightest star in the sky, combined with the light of the Sun to produce the sultry climate.  If you find yourself at a location with a flat eastern horizon, try to spot Sirius in the southeast as morning twilight gathers.  You’ll be looking at one of the first astronomical phenomena to be recorded in all of history.

The waxing Moon takes away more and more of the faint glow of the summer Milky Way, but in return offers us a great target for exploration through the telescope.  Each night the advancing terminator reveals a different landscape of mountains and smooth plains pockmarked with innumerable impact craters.  The Moon’s surface reveals the ancient violence that built the planets of the solar system from thousands of small components known as “planetessimals”.  Most of the lunar surface that we see today was formed nearly four billion years ago by the successive impacts of these bodies.  Today the largest of these objects have been swept out of the solar system, but the Moon’s battered face tells us that collisions can still happen over geological time-scales.

Jupiter shines brightly in the southern sky as evening twilight fades.  The giant planet lies just below the Moon on the evening of the 9th, and it is interesting to compare the telescopic view of both objects when they are close to each other in the sky.  Jupiter’s apparent disc is about the same size as an average lunar crater, but his true diameter is over 40 times that of the Moon.  He appears small in the telescope due to his vast distance of 700 million kilometers (437 million miles)!

Saturn stalks the southeastern sky as Jupiter dominates the early evening.  By 11:00 pm he, too is on the meridian and is in the best position for telescopic observation.  His real diameter is almost as large as that of Jupiter, but in the telescope he appears much smaller than his large cohort.  Saturn’s smaller disc is bracketed by his extensive ring system, easily visible in any telescope.  

If you’re up before the Sun, look to the eastern horizon for a glimpse of the fleet planet Mercury.  This innermost planet never strays very far from the Sun, so seeing him with the unaided eye is a feather in any skywatcher’s cap.  The best time to look for him is about half an hour before sunrise, when he will be about 10 degrees above the horizon.  This week the twin stars of Gemini, Castor and Pollux, serve as handy “pointers” to Mercury’s elusive glow.

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