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The Sky This Week, 2017 May 23 - 30

The scattered stars of spring.
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Messier 51, the "Whirlpool Galaxy" in Canes Venatici
imaged 2013 April 14 from near Philomont, Virginia

New Moon occurs on the 25th at 3:44 pm Eastern Daylight Time. This will afford you a great opportunity to spot the hairline lunar crescent on the evening of the 26th. Luna will be about five degrees above the west-northwest horizon half an hour after sunset. She will become much easier to spot over the next few evenings. On the 28th the Moon will lie between the Twin Stars of Gemini, Castor and Pollux, and the bright star Procyon in Canis Minor.

Castor, Pollux, Procyon, and Capella are the last of winter’s stars to exit the sky, briefly yielding the evening to the more subdued springtime constellations before the bright stars of summer begin to rise in the east during the late night hours. The "signature" constellations of spring are marked by the seven star asterism of the "Big Dipper", which you’ll find high in the northern sky at the end of evening twilight. This group of stars is a part of the much larger constellation of Ursa Major, the Great Bear. The fainter stars of this constellation sprawl over a large area of the sky and partially border on the constellation of Leo, the Lion, which may be found south of the meridian. Leo’s signature outline consists of two asterisms. The first is marked by the bright star Regulus and a semicircle of stars to its north which form a grouping commonly called The Sickle. The second is a right triangle of stars with the constellation’s second brightest star, Denebola, marking its eastern apex.

If you go back to the Big Dipper and follow the "arc" of its "handle", you’ll easily find your way to the northern hemisphere’s brightest star, Boötes, the Herdsman. The other stars of this constellation resemble a narrow kite, but at third- and fourth-magnitude they are difficult to identify from urban skies. Arcturus is a relatively nearby star as these things go, located just under 37 light-years from us, and it shows one of the largest proper motions across the sky of any first-magnitude star. It will appear to move a bit more than the apparent diameter of the Moon over the next 1000 years. Its pleasing rosy tint is due to its evolution into a late-stage giant star that is beginning to accumulate helium in its core, causing its outer layers to expand and grow cooler. Once you’ve followed the "arc to Arcturus", you can "speed on to Spica", the blue-tinted star to the southeast of bright Jupiter. The area of the sky bounded by these stars and asterisms surrounds the northern pole of the Milky Way galaxy, so our line-of-sight is unobscured by galactic dust clouds. Sweep a modest telescope around this seeming void from a dark sky site and you’re bound to run into a number of faint luminous smudges, each of which is a galaxy similar to our own.

Jupiter continues to be the showpiece of the evening sky. The giant planet is in prime viewing position as soon as the sky begins to darken, and he crosses the meridian during the mid-evening hours. After the Moon, Jupiter is the best target for the small telescope owner to examine. As soon as you get him in the eyepiece you’ll see some combination of the four bright moons first recorded by Galileo 407 years ago. Galileo’s telescope was very primitive, and yet he was able to deduce the periods that the moons revolved around their hulking master. Often you might only see two or of these planet-sized satellites. This happens when they either transit the planet’s disc or disappear behind it. This will be the case on the evening of the 27th, when the innermost moon Io crosses in front of Jupiter and Europa lies in its shadow. If you have a four-inch or larger telescope you can see the shadow of Io projected on the planet’s clouds. Io will move off the disc by 9:30 pm, and half an hour later Europa will emerge from its eclipse.

Saturn now rises at around 10:00 pm, and you should be able to spot the ringed planet low in the southeastern sky an hour later. He will remain relatively close to the southern horizon throughout the night, so the best time to view him is early in the morning at around 3:00 am.

Venus greets early morning dog walkers and commuters, blazing in the gathering morning twilight of the eastern sky. She is moving through the dim autumnal constellations, and will eventually pass through some of the bright winter star patterns as the year moves on.

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