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The Sky This Week, 2017 July 25 - August 1

Interesting sights on moonlit nights
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Epsilon Lyrae, the "Double-Double" Star
Imaged on 2015 November 16 from Alexandria, VA

The Moon returns to the evening sky this week, waxing through her crescent phases as she moves into the southern summer constellations. First Quarter occurs on the 30th at 11:23 am Eastern Daylight Time. Luna passes just two degrees north of Jupiter on the evening of the 28th. She ends the week among the stars of Scorpius and will pass near Saturn as next week opens.

The waxing Moon gradually brightens the evening sky and erodes the subtle glow of the Milky Way as the week progresses. Late night skywatchers will have to compete with the increasing sky glow to make out the more subtle features of our galaxy’s best portions. Fortunately there are many other sights for the washed-out sky of bright moonlight and suburban skies. If you look high in the east at the end of evening twilight you’ll find the three bright stars of the Summer Triangle, Vega, Deneb, and Altair. The brightest and highest of these is Vega, lead star in the small constellation of Lyra, the Harp. If you locate Vega in a pair of binoculars you’ll notice a close pair of stars just to the northeast of the bright blue star. This wide pair, known as Epsilon Lyrae, share the same proper motion and parallax measurements, indicating that they are probably gravitationally bound to each other. If you now turn a telescope on them, you’ll find that each one is itself a close double star that can be split in a good three-inch aperture telescope. If you have a small telescope, this "double-double" star offers a good observing challenge. A much easier double star, and one of the best for a good color contrast, may be found smack in the middle of the triangle in the form of the star Albireo, the third-magnitude luminary that marks the head of Cygnus, the Swan. This star is easily resolved in any telescope, and in my view is best presented in smaller instruments. The brighter component has a golden tint, while the fainter companion is a striking blue color. For this reason I like to show it off as the "Navy Double" at star parties, where it is always a crowd pleaser. Unlike Epsilon Lyrae, the components of Albireo are probably not physically associated. They have very different proper motions, parallaxes, and distances from us and are separated by about 30 light-years. This makes Albireo a classical case of an "optical double" star, where the components just happen to lie along the same line of sight.

Bright Jupiter is still quite prominent in the southwestern sky during evening twilight, and you still have some time to enjoy him and his four Galilean moons in the fading light of dusk. By the end of the week the giant planet sets shortly after 11:00 pm, so you should be ready to look at him as soon as the Sun goes down. If the sky is clear and steady, you might be able to glimpse the planet’s famed Great Red Spot as it crosses the planet’s disc on the evening of the 29th. This mysterious feature, which has persisted for at least 150 years, was recently glimpsed by NASA’s Juno space probe from very close range, producing some of the most compelling images aver made of a solar system feature.

Saturn crosses the meridian at around 10:30 pm, placing him in prime observing position for viewing in the telescope. Unfortunately the planet is wallowing along the most southerly reaches of his path around the sky, so you’ll need a night of very steady air to see him at his best. Keep watching him when he’s visible and hope for spells of good "seeing".

Venus continues to greet early risers from the northeastern sky as morning twilight gathers. The dazzling planet is moving through a familiar part of the sky as she drifts through the heart of the rising stars of the Great Winter Circle.

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