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The Sky This Week
U.S. Naval Observatory's Weekly Blog
The Veil in the Milky Way
by Geoff Chester, USNO Public Affairs
21 September 2021
NGC 6992, the eastern arc of the Veil Nebula in Cygnus,
imaged 2021 September 11 at Great Meadow, Old Tavern, Virginia
with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a ZWO ASI183MC CMOS color imager
The Moon climbs into the northern hemisphere of the sky this week, waning through her gibbous phases as she climbs into the rising stars of winter. Last Quarter occurs on the 28th at 9:57 pm Eastern Daylight Time. Look for the gibbous Moon perched between the bright star Aldebaran and the Pleiades star cluster as they rise in the late evening of the 25th.
The autumnal equinox arrives on the 22nd at 3:21 pm EDT. At this time the center of the Sun’s disc reaches an ecliptic longitude of 180 degrees and crosses the celestial equator into the sky’s southern hemisphere. Although the term “equinox” translates from Latin as “equal night”, the actual date when we experience exactly 12 hours between sunrise and sunset won’t occur until the 25th. This is due to the combined effects of the Sun having a disc that subtends about half a degree and atmospheric refraction. Sunrise occurs when the first discernable sliver of the Sun’s upper limb appears on the horizon, while sunset is when the last sliver of the disc disappears in the west. From September 26th until March 17th next year the nights will be longer than the days. The equinoxes are also the times when the length of daylight changes at its most rapid daily rate, about 2.5 minutes per day.
Early autumn evenings are a great time to catch up on exploring the splendors of the summer sky. The crisp evenings are generally free of the haze and humidity that’s inherent in the warmer months of July and August, and earlier sunsets mean that you don’t have to wait up for the sky to get dark. By 8:30 pm we reach the end of astronomical twilight, at which time the darkness of the sky is only limited by moonlight and artificial lights. At this time the three bright stars of the Summer Triangle may be found overhead, and the luminous band of the summer Milky Way bisects the sky from the southwest to the northeast. Under dark skies the Milky Way offers a treasure trove of star clusters and gaseous nebulae for owners of binoculars and small telescopes. Skirting the southern horizon, the “Teapot” asterism of Sagittarius offers a great place to start panning along the Milky Way. Looking just north of the teapot’s “spout” you will see a pair of bright knots of diffuse light. Binoculars will reveal these to be tight star clusters, while a telescope will show the stars enveloped in softly glowing clouds of gas. These so-called “nebulae” are stellar nurseries, and you can spot several more as you pan up to the bright star Altair. Within the bounds of the Summer Triangle you’ll find one of the denser star clouds of the Milky Way, along with a prominent dark “rift” that extends southward to the horizon. The prominent constellation of Cygnus, the Swan occupies most of the Triangle, and one can spend an entire evening just poking around the Swan’s environs. One of my favorite targets in Cygnus lies just off the eastern “wing” of the Swan. Wide-angle images show what appears to be a giant “bubble” nearby. This is the shattered remnants of a star that exploded some 20,000 years ago that is popularly known as the Veil Nebula. Ghostly wisps of light may be glimpsed in a low-power 4-inch telescope under dark skies, but long-exposure images reveal the delicate structure of expanding tendrils of the doomed star’s debris cloud.
Venus is still diving southward along the ecliptic, and this week the dazzling planet begins to close in on the “head” of the constellation of Scorpius, the Scorpion. On the evening of the 23rd binoculars will reveal the star Zubenelgenubi just two degrees north of the dazzling planet. In ancient times this star represented the southern “claw” of Scorpius, but today it marks the southern balance pan of the constellation of Libra, the scales, the only inanimate sign of the Zodiac.
Golden Saturn crosses the meridian at around 9:30 pm local time. This is the best time to check the ringed planet out with a telescope as he is as high in the sky as he will get for this apparition. The planet’s signature rings are still widely opened to our line of sight; they should show easily in just about any kind of telescope.
Giant Jupiter is the bright object that you’ve been seeing in the southeastern sky during the evening twilight hours. Old Jove spends the week close to the star Deneb Algeidi, the “tail” of the dim constellation of Capricornus, the Sea Goat. He follows Saturn to the meridian by just over an hour. His cream-white disc resolves in small telescopes, and instruments of three inches aperture will show his dark equatorial cloud belts. A six-inch instrument is sufficient to identify many more dark belts, and on occasion the Great Red Spot, an Earth-size storm that has persisted for some 300 years.
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