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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Grab a Swan by the Tail
by Geoff Chester, USNO Public Affairs
31 August 2021
Saturn and Jupiter, imaged 2021 August 25
imaged with a Celestron 23.5-cm (9.25-inch) f/10 Schmidt Cassegrain telescope
and a ZWO ASI183MC CMOS imager from Alexandria, Virginia
The Moon is a waning crescent in the pre-dawn sky this week. If you’re up before the Sun, try to see Luna’s slender crescent just before sunrise on the 5th. New Moon occurs on the 6th at 8:52 pm Eastern Daylight Time.
The absence of the Moon in the evening sky means that it’s time for the September citizen-science observing campaign for the Globe at Night project. This week the featured constellation is Cygnus, the Swan, one of the principal constellations of the summer sky. Finding Cygnus should be fairly easy, even for suburban skywatchers. The Swan’s brightest star is Deneb, the northernmost and faintest of the stars that form the asterism known as the Summer Triangle. Deneb marks the Swan’s “tail”, while its “head” is marked by the third-magnitude star Albireo. A line of three second-magnitude stars trace out the Swan’s wings. Together these stars form another asterism sometimes called the Northern Cross. Another interpretation of this pattern, given to us by the Inuit people, is that of a man in a kayak paddling along the “Pebbly River”, the faint band of light that betrays the Milky Way. Cygnus is directly overhead for most residents of the “lower 48” at 10:30 pm local time, so it should be a simple matter to locate the constellation and compare your view with the star charts on the Globe at Night web app.
Cygnus is a wonderful constellation to explore with binoculars and small telescopes. One of my favorite sights is the aforementioned star Albireo. To the naked eye it looks like a single star located almost in the middle of the Summer Triangle. A small telescope at low power will reveal Albireo to be a stunning double star, whose components shine with Navy blue and gold tints. Located some 400 light-years away, the two stars may be a prime example of an “optical double”, created when two stars just happen to line up along our line-of-sight. Whether the stars are physically related or not, they provide an easy target for new telescope owners and can spark a life-long interest in observing other double stars. Between Deneb and Albireo lies the Cygnus star cloud, one of the most prominent features of the Milky Way. This is a great area to scan with binoculars from a dark sky site. The amorphous glow of the Milky Way begins to resolve into a myriad of faint stars interspersed with bright and dark knots. The bright knots are distant star clusters that resolve to varying degrees depending on the aperture of your telescope, while the dark patches are enormous clouds of cold gas and dust absorbing the light of the stars behind them. One bright knot just to the northeast of Deneb has a very distinctive shape that is best seen in binoculars. Known as the North America Nebula, this large, faint glowing cloud is the result of ionizing radiation causing hydrogen atoms to emit a subtle glow. Eventually this cloud of gas and dust will become a nursery for infant stars that will some day delight our distant descendants.
Venus is the bright object that you are undoubtedly seeing in the southwestern sky at dusk. This week the dazzling planet slides past the star Spica. The two objects will be closest together on the evenings of the 3rd and 4th. You will probably need binoculars to see the star, but you should have no trouble finding the bright planet.
Once Venus sets, turn your attention to the southeast, where two more bright planets command your view. The first of these is Saturn, whose soft yellow glow is only overshadowed by nearby Jupiter. This distant world, currently some 1.35 billion kilometers (842.5 million miles) away, is always a special treat for telescope owners. The planet’s rings can be seen with almost any instrument, but larger telescopes will reveal more detail in these icy appendages and subtle cloud belts in the planet’s atmosphere.
Jupiter shares the sky with Saturn and offers a larger disk for telescopic viewing. His most striking features are the alternating bright and dark cloud belts that define his upper atmosphere. Since Jupiter’s axial tilt is only about three degrees, these cloud belts are confined to specific latitudes on his surface. Small telescopes will easily show the dark equatorial belts that surround the planet’s bright equatorial zone, while larger instruments will show more alternating belts and zones. Subtle details in these belts and zones change daily, and since the planet rotates in less than 10 hours a new area of interest is constantly rotating into view.
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