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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Falling Stars and Rising Planets
by Geoff Chester, USNO Public Affairs
10 August 2021
Saturn and its moons, imaged 2015 July 12 at the U.S. Naval Observatory
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor.
We’ll be taking a brief break from the Washington heat next week, so this edition won’t be updated on the 17th.
The Moon returns to the evening sky, starting the week in the company of bright Venus before gliding southward along the ecliptic. First Quarter occurs on the 15th at 11:20 am Eastern Daylight Time. Full Moon will occur on the 22nd at 8:02 am EDT. August’s Full Moon is popularly called the Corn Moon, Barley Moon, and Fruit Moon, but the name that most folks recognize is the Sturgeon Moon. This name comes from Native American lore as the indigenous people living along the shores of the Great Lakes noticed that these large fish were most easily caught when this particular Full Moon was in the sky. Luna passes south of Saturn on the evening of the 20th; on the following night she rises with Jupiter.
This week’s highlight is the peak of the annual Perseid Meteor Shower, which occurs during the overnight hours of the 11th and 12th. This display is the most consistent and productive of the annual meteor showers, and this year it occurs well after the Moon has set. These meteors originate from debris sputtered off Periodic Comet 109P Swift-Tuttle, which circles the Sun on an eccentric path every 133 years. The comet was co-discovered by Lewis Swift and Horace P. Tuttle in July, 1862 and was linked to the Perseid meteor stream by the Italian astronomer Giovanni Schiaparelli in 1866. Records of the Perseids date back over 1000 years, when early Christians identified them with the “Tears of St. Lawrence”, who was martyred on August 10, 258 CE. Under clear skies at a dark location a single observer can expect to see up to 75 to 100 fast-moving meteors per hour. If you follow the meteor trails backwards you will find that they seem to originate from a point in the sky, known as the radiant, in the constellation of Perseus. The radiant rises in the northeast in the late evening and climbs higher in the east during the morning hours. The best way to observe them is comfortably seated in a beach chair with a clear view of the sky. No telescope required! Although the shower is expected to peak on the night of August 11-12, you can expect to see Perseids for several nights afterward.
If you are out in the wee hours watching for Perseids, take a few minutes to look for one of the most unusual stars in the sky. Sprawling over the southeast horizon, the constellation of Cetus, the Whale sports few bright stars. Its brightest star, Diphda, marks the whale’s head, and its second-brightest star, Menkar, forms part of its tail. About one-third of the way from Menkar to Diphda you should see a star of comparable brightness that is normally not there. This star, Mira, is the prototype of a type of star called a long-period variable. Roughly every 330 days the star reaches a peak in its brightness, then fades to invisibility some 1700 times fainter than its peak. The star is near its peak brightness, which should occur on the 18th, and recent reports indicate that it is now the second-brightest star in the constellation.
The evening sky sports a nice selection of bright stars for the casual skywatcher. As evening twilight ends the bright star Arcturus beams down from the western sky, and Spica twinkles furiously in the southwest. The northwestern sky holds the asterism known as the Big Dipper, which slowly moves to skim the northern horizon in the early morning hours. The late night hosts the three bright stars of the Summer Triangle, Vega, Altair, and Deneb, while the ruddy star Antares leads the stars of Scorpius across the southern skyline.
Venus is the bright object that you are seeing in the western sky as evening twilight gathers. The planet’s dazzling glow is due to its cloud-laced atmosphere, which reflects nearly 70 percent of the sunlight that strikes it. As beautiful as these clouds appear, though, you wouldn’t want to fly through them; they are composed of sulfuric acid!
Saturn is now high enough by 9:00 pm local time to enjoy a long look through the telescope. The view of the planet’s cream-colored disc floating inside its system of rings is one of the most enthralling sights in all of Nature, and I never get tired of looking at it. There is something almost magical about seeing it for the first time “live” in the eyepiece.
Jupiter reaches opposition on the evening of the 19th. At this time the giant planet rises at sunset and sets at sunrise. He is ready to view in the telescope by 10:00 pm, and offers his own interesting sights to see. A casual glance will reveal the bright moons first described by Galileo in 1610. Depending on the time of your observation, you can see two, three, or all four of these near planet-sized bodies in the gravitational grip of their huge master. Sometimes they pass into the planet’s shadow, while at others the moons drag their tiny shadows across Old Jove’s vast face. Close scrutiny will show the planet’s striped atmosphere, which is under a state of continuous change.
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