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The Sky This Week, 2016 April 26 - May 3

Lions and Planets and Bears, oh, my!
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Jupiter, with Ganymede on the planet's limb and Europa,
2016 April 25 @ 01:45 UT

Ganymede is just entering occultation behind Jupiter's disc

The Moon begins the week coursing through the summer Milky Way, skirting the southern reaches of the ecliptic in the morning sky. Last Quarter occurs on the 29th at 11:29 pm Eastern Daylight Time. As Luna wanes through her crescent phases she transits the star-poor constellations of the autumn sky.

With the Moon’s entry into the early morning sky the next citizen-science observing campaign for the "Globe at Night" program begins on the 29th and lasts through May 8th. This month’s target constellation is Leo, the Lion, which is easy to find thanks to the bright beacon of Jupiter, which lies just below the main stars of the Lion’s form. Leo consists of two distinct asterisms, one of which is commonly known as "The Sickle" and the other forming a right triangle. The Sickle begins with Leo’s brightest star Regulus, a first-magnitude star with a pale blue light. Above Regulus is the yellow-hued star Algeiba, a fine double star for the small telescope and the constellation’s second-brightest star. From a dark sky you’ll see a series of 3rd- and 4th-magnitude stars that make up the rest of the Sickle shape, which can also be seen as a backwards question mark. The triangle, which marks the Lion’s hindquarters, sits directly above Jupiter. Extend a line from Jupiter to the north and you’ll pass 3rd-magnitude Chort and 2nd-magnitude Zosma. To the east of Chort is the Lion’s third-brightest star Denebola. From my suburban yard I can see four or five of the constellation’s main stars, but from a dark site you may see several dozen fainter ones. Either way, if you report your results to the Globe at Night web page you’ll help scientists map out the adverse effects of poor outdoor lighting.

As Leo crosses the meridian, look on the opposite side of the zenith to see one of the sky’s most familiar star patters at its best. The seven stars popularly known as the "Big Dipper" wheel nearly overhead in the northern sky. Despite not having a single first-magnitude star, this pattern is probably the most familiar asterism after the figure of Orion, the Hunter. The Dipper is part of the much larger constellation of Ursa Major, the Great Bear, but the asterism has roots in many ancient civilizations. We’ll take a closer look at this constellation next week.

Mercury may still be glimpsed in the west after sunset, but you’ll probably need binoculars to see him in the twilight glare. The fleet planet is getting ready to pass between the Earth and the Sun, and as he turns more of his sunlit side away from us he fades dramatically. He will also drop from the western sky like a stone, and in just under two weeks he’ll cross the disc of the Sun. The last transit of Mercury occurred in 2006, but we’ll only have to wait until 2019 for the next one.

Jupiter is now almost two months past opposition but he’s still the dominant object in the evening sky. He may be found high in the east as twilight gathers, and by the time the sky becomes fully dark he’s near the meridian, perfectly placed for telescopic observation. He is moving very slowly against the background stars as he approaches the second stationary point in his current apparition. Despite the increasing gulf of space between us and the giant planet, he still offers a fine view to owners of small telescopes.

Mars and Saturn now rise in the southeast during the late evening hours, and by midnight they should be easy to find. Mars should be easy to pick out due to his brightness and distinctive ruddy hue. Saturn has a more subdued golden hue, but both planets are brighter than the nearby first-magnitude star Antares, brightest star in the summer constellation of Scorpius. Mars’ disc is now large enough to show many surface features to patient observers with modest telescopes. It is the only place in the solar system besides the Moon and Mercury that shows us a solid surface rather than a dense atmosphere.

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