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The Sky This Week, 2016 October 18 - 25

Sky geometry
Globular Cluster Messier 15 in Pegasus
Imaged from Morattico, Virginia on 2016 October 16
with an Explore Scientific AR102 4-inch f/6.6 refractor.

The Moon moves into the morning sky this week, waning to the Last Quarter phase which will occur on the 22nd at 3:14 pm Eastern Daylight Time. Luna spends much of the week among the rising stars of the winter sky. She begins the week with a spectacular occultation of the bright star Aldebaran in the constellation of Taurus, the Bull. In the DC area you can watch Aldebaran disappear behind Luna’s bright limb at 1:37 am EDT on the morning of the 19th. It will reappear from behind the Moon’s dark limb just over an hour later. On the morning of the 21st she passes just over a degree north of the second-magnitude star Alhena in the constellation of Gemini. By the week’s end she closes in on the bright star Regulus in the pre-dawn sky.

As the Moon retreats into the morning sky we once again have the chance to see the last stars of summer gracing the early evening. With astronomical twilight now ending before 8:00 pm, the bright star clouds of the Milky Way bisect the sky from northeast to southwest as seen from dark-sky locations. Urban astronomers can still see the bright stars of the Summer Triangle just west of the zenith in the early evenings. As the night unfolds, the triangle moves further toward the west and another geometric figure takes center stage. By 10:00 pm you should notice a square-shaped asterism of second-magnitude stars approaching the meridian from the east. The "Great Square" represents part of Pegasus, the mythical Flying Horse that figures prominently in the legend of Perseus and Andromeda. Andromeda herself is represented by two diverging "chains" of stars that trend northeastward from Alpheratz, the upper-left corner star of the Square. If you follow the brighter chain of stars you’ll run across a wishbone-shaped group of stars that represents Perseus, centered on the bright star Mirfak. In mythology Perseus rides Pegasus to free Andromeda from the chains that bound her to a rock where she was placed as a sacrifice for her mother’s vanity. For the amateur astronomer this area of the sky holds many treats for binoculars or the small telescope. One object that’s often overlooked is now close to the meridian. To find it, look for a lone second-magnitude star about 20 degrees west of the right side of the Great Square. This star, known as Enif, has a distinct golden hue. Four degrees northwest of Enif you’ll find a bright fuzzy knot that betrays the globular star cluster Messier 15. In a telescope of four inches or more aperture this knot resolves into a beautiful cluster of thousands of stars that’s brightly concentrated toward its center. Crisp October evenings offer ideal conditions to admire this object.

Venus appears in the southwestern sky, becoming visible as soon as the Sun sets. As evening twilight deepens she begins to dominate the view, drawing your attention as the brightest object in this part of the sky. As the sky darkens you may notice her in the company of the three second-magnitude stars that form the "head" of Scorpius early in the week. By week’s end she is moving closer to the bright star Antares and the planet Saturn.

Saturn is still hanging tough in the southwestern sky and can be picked out once the Sun has gone well below the horizon. You can still glimpse his rings in the deepening twilight, but don’t expect a crisp view against a dark background.

Mars continues his eastward trek through the sky and spends the week receding from the "Teapot" asterism of Sagittarius. He should be easy to spot in deep twilight thanks to his distinctive reddish tint.

Jupiter is climbing higher each morning in the pre-dawn sky. You’ll find him about 10 degrees above the eastern horizon half an hour before sunrise. If you follow him in binoculars you’ll see him close in on the second-magnitude star Porrima in the constellation Virgo.

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