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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Sailing the Milky Way with The Swan
by Geoff Chester, USNO Public Affairs
13 September 2022
The Cygnus Milky Way and Summer Triangle,imaged 2022 August 22 from Thoroughfare Mountain Overlook, Mile 39, Skyline Drive, Shenandoah National Park
with a Canon EOS Rebel SL2 DSLR, Omegon Mini LX-2 mechanical star tracker, 18mm @ f/4, 90 seconds, ISO 3200.
Deneb is the bright star to the upper left, Albireo is the star in the center of the Triangle.
The Moon wanes as she continues her northward climb along the ecliptic. Last Quarter occurs on the 17th at 5:52 pm Eastern Daylight Time. Over the course of the week Luna makes her way through the rising winter constellations. She forms an attractive triangle with the bright star Aldebaran and the planet Mars as she rises late on the evening of the 15th. On the following night she rises with the red planet, which will be just three degrees south of the Moon’s gibbous disc.
As the Moon wanes she seems to linger in the late evening sky, rising before midnight for most evenings of the week. This is due to the effects of the Harvest Moon phenomenon that we highlighted last week. Those of us who like viewing from dark-sky sites have to wait a bit longer than usual for several hours of viewing before Luna’s bright glare lights up the sky. However, the time of sunset now occurs about a minute and a half earlier each night as we near the date of the equinox. By the evening of the 17th we’ll have just under three hours of darkness between the end of evening twilight and moonrise, so it’s will be a good night to start the September observing campaign for the
Globe at Night citizen-science program
. This month’s featured constellation is one of my favorites, Cygnus, the Swan, which may be found directly overhead at 10:00 pm local time over most of the U.S. mainland. The brightest star, Deneb, which marks the Swan’s “tail”, is the faintest star in the Summer Triangle asterism, while the bird’s “head” is the beautiful double star Albireo, located in the middle of the Triangle. The Swan’s “wings” are perpendicular to the line between Deneb and Albireo, giving rise to an asterism called the “Northern Cross”. Under dark skies you can see some of the denser star clouds of the Milky Way spanning the constellation’s length. In the clear dark skies of the arctic north, the Inuit saw the constellation as a man in a kayak paddling along the “Pebbly River” of the Milky Way. Once you have located Cygnus, compare your view of the constellation to the charts on the
Globe at Night web app
. Your observations will help scientists map the trends in long-term sky brightness over your chosen location.
The Cygnus Milky Way is one of the best places in the sky to explore with binoculars and small telescopes. If you’re using binoculars, the best way to view the area is to set up a lawn chair so you can lie back and look straight up. The misty glow of the Milky Way will begin to resolve into countless fainter stars and knots of light that betray star clusters and glowing gaseous nebulae. One of my favorite binocular sights in this area may be found about one-third of the distance from Albireo to the bright star Altair, southernmost star in the Summer Triangle. Known as Collinder 399, its more popular name is The Coathanger. If you sweep the area around the Coathanger with a small low-power telescope you will find a number of fine star clusters embedded in dense Milky Way star clouds.
Often overlooked when touring the Milky Way are the dark areas that seem to split the glowing band in two. The “Great Rift” begins near Deneb in Cygnus and runs southward to the horizon. While it appears to be empty space to the eye, in reality the dark band consists of cold interstellar gas and dust that blocks the light of more distant stars in the Galaxy. This is the stuff that stars, planets, moons, and life are made of. Interestingly, the dark areas were widely observed by the ancient Inca and their forebears as constellations in their own right. They are fascinating areas to explore with a telescope.
By 10:00 pm Saturn is well up in the southeastern sky. The ringed planet is gradually drifting across the dim stars of the constellation of Capricornus, the Sea-Goat. A small telescope will easily show the planet’s signature rings, and each increase in aperture brings out more detail. A six-inch telescope should easily show a dark division in the rings near the outer edges. Known as Cassini’s Division, this is a true gap in the ring system some 4800 kilometers (3000 miles) wide. It is the result of gravitational resonances between ring particles and the planet’s inner moons. The planet itself has dusky belts of ammonia clouds in a hydrogen-rich atmosphere.
Jupiter is now just two weeks away from opposition. By 10:00 pm he is high in the eastern sky, and beckons for telescopic observation. Old Jove will experience a close opposition this year, and his apparent disc will be the largest it has been for the past 12 years. Unlike Saturn, Jupiter’s atmosphere is in a constant state of turmoil, so we never see the same features from night to night, except for the planet’s famous Great Red Spot. This feature of the jovian atmosphere has persisted for hundreds of years and is thought to be a very persistent storm that’s as large as the entire Earth! It will be well-placed for viewing on the evening of the 16th.
Mars now rises before midnight, but you will still get your best view of him in the pre-dawn sky. You will find him drifting eastward through the stars of Taurus, joining the bright stars of the Great Winter Circle.
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