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The Sky This Week, 2018 October 2 - 9

A tale of two galaxies
Messier 31, the Great Andromeda Galaxy, imaged 2017 October 21
from Old Tavern, Virginia with a 4-inch f/6.6 refractor telescope
and a Canon EOS Rebel T2i DSLR. Note the "satellite" galaxies Messier 31 (lower center)
and NGC 205 (upper right).

The Moon wanes in the pre-dawn sky this week, passing through the stars of early spring as her crescent diminishes.  New Moon occurs on the 8th at 11:47 pm Eastern Daylight Time.  Look for Luna just to the south of the binocular star cluster Messier 44, also known as the Praesepe or “Beehive” cluster as the first rays of twilight brighten the sky.

Take advantage of the Moon’s absence this week to do a little citizen science by participating in the Globe at Night observing program.  To do this you’ll need to find and identify the “Great Square” asterism formed by the brightest stars in the constellation of Pegasus, the mythical Flying Horse.  You’ll find the square high in the east at around 10:00 pm.  The square’s brightest stars are mostly second-magnitude luminaries, so they should be visible even from urban locations.  To participate in the star count, visit the Globe at Night website and follow the directions, comparing your view of the constellation with their star charts.  I use Pegasus as an indicator of the quality of the sky over my various observing sites; at the best of these I can spot about a dozen faint stars within the boundaries of the square.  This week, if you’re in the Washington DC area you have an excellent opportunity to see Pegasus from a good dark site, courtesy of the Northern Virginia Astronomy Club.  They will be holding their annual Star Gaze event at C.M. Crockett Park in Midland, Virginia.  This event will run from 3:00 to 11:00 pm and include safe viewing of the Sun and a number of telescopes for nighttime viewing.  There is a nominal park entry fee, but the event is free and offers a great opportunity to experience amateur astronomy.

These moonless nights offer a great mix of objects to enjoy from a dark site like Crockett Park.  During the early evening hours after twilight the panorama of the summer Milky Way plays out over the sky from the southwest to the northeast.  You’ll notice that most of the brighter stars in the sky tend to be close to the amorphous glow, and bright star patterns are relatively easy to pick out.  Low in the southwest you can still trace out the “teapot” asterism of Sagittarius, while overhead you’ll find the first-magnitude stars of the Summer Triangle.  Follow the Milky Way to the northeast and you’ll see the five stars that make up the compact “W” of Cassiopeia.  These stars, as well as the countless others that make up the Milky Way, are the Sun’s neighbors in a gigantic collection of hundreds of billions of stars that make up our home galaxy.  By midnight the Milky Way is well to the west, but now you can see another galaxy that marks the most distant object that we can see with the naked eye.  To find it, go back to the Great Square of Pegasus.  The star in the upper left corner is where you’ll see two “chains” of stars converge.  “Star hop” out to the second star in the brighter chain, then hop up past two fainter stars.  At a reasonably dark site you should see a faint streak of diffuse light that looks like a detached portion of the Milky Way.  This streak is the Great Andromeda Galaxy, a star system similar to but larger than our Milky Way.  It is located about 2.5 million light-years from us, so the light we see tonight began its journey toward us before we existed as a species on the Earth!

Even though she won’t pass between the Earth and the Sun for another two weeks, Venus is now virtually invisible in the evening sky.  We have enjoyed her presence in the evening sky for most of the year, but now we must wait a few weeks to see her emerge into the morning sky, where she will spend most of 2019.

Jupiter is still hanging on to visibility in the early evening sky, but your best chance to catch him will be during evening twilight.  He exits the sky by 8:30 pm.

Saturn remains visible in the early evening, becoming prominent in the southwest as twilight fades.  He sets at around 11:00 pm, so you have a couple of hours to enjoy a view of the planet’s magnificent rings against a background of a Milky Way star cloud.

Mars is now the lone planet that you can still see for most of the nighttime hours.  He dominates a part of the sky that’s devoid of bright stars, and his distinctive reddish tint makes him virtually impossible to miss.  Mars has faded considerably from his opposition brightness, and his disc is now about 60 percent of the size it subtended back in late July.  If you train a telescope in him you should see a distinct gibbous phase and the bright white carbon dioxide frost that covers his south pole.

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