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The Sky This Week, 2017 August 8 - 15

Perseids and rising pups.
CapitolRainbow_02small.jpg
Post-storm rainbow over the U.S. Capitol,
as seen from the U.S. Naval Observatory

The Moon moves into the barren star fields of the autumn sky this week, waning to the Last Quarter phase on the 14th at 9:15 pm Eastern Daylight Time. By the end of the week Luna finds herself entering the rising stars of the winter sky, closing in on Aldebaran, westernmost star of the Great Winter Circle.

The highlight for this week in August is usually the annual Perseid meteor shower, which is the most consistent and productive of the annual meteor displays. The Perseids are the result of dusty material that sputters off the nucleus of Periodic Comet 109P Swift-Tuttle, which orbits the Sun in a long looping path that brings it to the inner solar system every 133 years. One of its co-discoverers was Horace P. Tuttle, a prolific comet discoverer who spent some time working at the U.S. Naval Observatory. The orbit of the comet passes near the orbit of the Earth every year around August 12th or 13th, and when this occurs we run headlong into the stream of dusty debris. The Perseid meteors are generally quite fast and bright, and there are frequently quite a few of them. Under dark skies a single observer can see between 60 to 120 meteors per hour at the shower’s peak. This year, though the peak occurs on the night of August 12/13th, under the light of the waning gibbous Moon. This will drastically cut down on the numbers of meteors that we’ll see. At best we might see 20 or so per hour. The shower’s radiant point is located in the constellation Perseus, and the best time to watch for them is during the early morning hours as the radiant climbs in the northeastern sky. The Perseid meteoroid stream is quite wide, so it’s possible to see at least some of them between mid-July and the end of August.

Another phenomenon to look for is the heliacal rising of the star Sirius. This is the time when the brightest star in the sky is seen to rise just before the Sun. It’s best seen if you have an ocean horizon to the east, and it occurs for most of us in the eastern U.S. this week. This event was one of the first astronomical events to be recorded by ancient people. Pre-dynastic Egyptians noted the coincidence of the star’s rise with the annual flooding of the Nile valley, and they observed the event for the next three millennia, using it as a benchmark in their calendar system. The Romans noted the heliacal rise as well. Since Sirius is the brightest star in the constellation of Canis Major, the Greater Dog, the times of the heliacal rising became known as the Dies Caniculares, which today we call the "Dog Days" of August.

Jupiter continues his slow exit from the evening sky. The giant planet beams brightly in the southwestern sky during evening twilight, but he sets just an hour and a half after twilight ends. Your best views of him will now be through smaller telescopes, which are less affected by turbulence in Earth’s atmosphere.

Saturn crosses the meridian just before 9:30 pm. By this time the sky is quite dark, so you shouldn’t have any trouble locating the ringed planet in the southern sky. By late in the week skywatchers at dark sky locations will find Saturn embedded in one of the dense star clouds of the Milky Way. This may make identifying some of Saturn’s fainter moons problematic, but it also enhances the context of the planet as a distant point in space.

Venus continues to steadily move through the rising stars of Gemini in the early morning sky. As morning twilight begins to brighten the eastern horizon, Venus commands your attention. Look for the bright stars of the Great Winter Circle, most of which will have risen with the dazzling planet.

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