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The Sky This Week, 2016 September 20 - 27

Things that go "Boom!" in the night.

NGC 6979, 6992, & 6995, The "Veil Nebula" supernova remnant in Cygnus
(and lots of Milky Way background stars!)
imaged 2016 August 25 from Fishers Island, NY

The Moon wanes in the early morning sky this week, wending her way through the bright stars of the Great Winter Circle that are now prominent as morning twilight breaks. Last Quarter occurs on the 23rd at 5:56 am Eastern Daylight Time. Look for Luna high above the bright figure of Orion on the mornings of the 22nd and 23rd. The Moon ends the week as a thinning crescent near the bright star Regulus in the constellation of Leo, the Lion.

The autumnal equinox falls on the 22nd at 10:21 am EDT. This is the moment when the center of the Sun’s disc appears to cross the celestial equator as seen from a hypothetical point in the Earth’s center. At this instant the Sun will be directly over a point in the Atlantic Ocean a few hundred kilometers north of the northeaster tip of Brazil. Although the term "equinox" literally means "equal night", the date when we actually experience 12 hours of daylight and darkness falls on the 25th here in Washington. From then until March 17th next year our nights will be longer than our days.

As the Moon moves deeper into the morning sky we once again have the opportunity to see the best of the late summer sky during the evening hours. From dark locations the brightest star clouds of the Milky Way bisect the sky from northeast to southwest at 9:00 pm. The summer constellations Scorpius and Sagittarius are still visible in the southwest along with the Milky Way’s densest star clouds, which lie along our line of sight toward the galactic center. The sheer scale of our home galaxy is something that’s very hard to fathom. Even measuring the distances in terms of light-years begins to boggle the mind when we look at just the bright stars in the Summer Triangle. Altair, the southernmost, is just under 17 light-years distant. Vega, the brightest, is another 10 light-years more distant. Deneb, the northernmost, is well over 100 times farther away than Altair, making it one of the most luminous stars known in the heavens. The thousands of stars that are revealed when you sweep the Milky Way in the area of the Summer Triangle are even more distant. Deneb marks the "tail" of Cygnus, the Swan. Not far from Deneb, at the tip of the Swan’s southeastern wing, is a "deep-sky" target that may be a preview of what Deneb will look like in the distant future. Since it is so luminous and massive, Deneb will go out with a bang as a supernova, spewing energy and pieces of itself far and wide. Between 5000 to 10,000 years ago a similar star exploded where we now see an object known as the Veil Nebula. Today the remnant is an expanding circle of wispy gas tendrils that covers about three degrees of the sky. My favorite views of it are with my small 80mm low-power refractor equipped with a filter to block natural sky glow. Larger telescopes will show details in the faint tendrils, but the wide view gives mute testament to the power of the force that created it.

The dazzling planet Venus is gradually making progress into the evening sky. You can see her low in the west in deepening evening twilight. Her gradual pace will increase over the next few weeks, and in another month she will seem to vault into the darker evening sky.

Saturn is slowly progressing eastward above the stars of Scorpius. You can mark his slow progress relative to the bright star Antares, about six degrees to the ringed planet’s south.

Mars spends the week traversing one of the Milky Way’s biggest star clouds, passing very close to the direction of the galactic center. By the end of the week you’ll find him placed over the "spout" of the "Teapot" asterism in Sagittarius.


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