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The Sky This Week, 2010 September 28 - October 5

Planets going, planets coming, and autumn skies at their best.

Jupiter, cylindrical projection map of features
Based on observations made 2010 SEP 18 - 22
Map produced with WinJUPOS 8.3.3 from webcam imagery.

The Moon wanes in the morning sky this week, rising with the stars of the Great Winter Circle before wending her way toward the spring constellations by the week’s end. Last Quarter occurs on the 30th at 11:52 pm Eastern Daylight Time. On the morning of September 30th Luna stands high above the stars of the upraised "club" of Orion. Before dawn on the morning of October 2nd, look for her in the company of Castor and Pollux, the Twin Stars of Gemini.

The Moon’s disappearance from the evening sky means that it’s a great time to take advantage of prime fall observing weather. We still have summer’s bright star patterns and the magnificent star clouds of the Milky Way to enjoy in the early evening. Astronomical twilight now ends well before 8:30 pm, and as it fades the stars of the Summer Triangle are passing directly overhead. In the south and southwestern sky the densest star clouds of the Galaxy linger for binocular perusal, and you can trace the river of amorphous light overhead and down to the northeast horizon, panning the view for star clusters and bright knots of nebulosity. By 10:00 pm the fainter stars of autumn begin to invade the south and southeastern sky, while off to the east you’ll see one of the fall’s more distinctive star patterns climbing high. As the Summer Triangle passes west of the meridian, four second-magnitude stars rise to meet it. These stars form an asterism known as the "Great Square" and are part of the constellation of Pegasus, the flying horse of the Perseus and Andromeda myth. All of the characters of this celestial play are located in the northeastern sky at this hour, and as the night moves toward midnight Andromeda, Cassiopeia, Cepheus, and Perseus all climb higher to displace summer’s setting patterns. This part of the sky is a deep-sky observer’s dream, loaded with bright star clusters in the Milky Way and faint galaxies far beyond.

Early evening twilight still offers a chance to see the bright dazzle of Venus in the southwestern sky, but her time to shine in relative prominence is about to end in a big hurry. Look for her shortly after sunset a few degrees above the horizon. On a clear night she should be relatively easy to spot about 20 minutes after sunset, but you’ll only have about 20 minutes to find her before she slips too low into the horizon haze. The ruddy planet Mars finally catches up to her and passes by just over six degrees to the north on the evening of the 29th. You’ll definitely need binoculars to spot the red planet since he is over 100 times fainter than his brilliant companion.

Jupiter now dominates the evening and overnight hours, virtually unrivalled by any other object except the Moon. Old Jove is just a week past his closest opposition in nearly 40 years and stands out like a beacon in the star-poor reaches of the autumnal constellations. The giant planet is now the prime target of most backyard telescopes, and his ever-changing bright cloud belts and dark zones offer a different palate for viewing every night. If you tire of trying to eke out fine detail on the planet’s cream-colored disc, you can enjoy the antics of the four large moons discovered by Galileo in 1610. On the evening of October 1st, if you turn a modest telescope toward Jupiter at 8:00 pm EDT, you’ll see the moon Callisto passing just south of the planet’s disc, while the moon Io drags its shadow across the cloud tops. At 9:33 pm Io’s disc emerges from the planet’s limb, with the shadow following 17 minutes later. It seems as though there’s always something interesting going on out there!

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