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The Sky This Week, 2015 May 12 - 19

Something for everyone who enjoys the night sky.
Jupiter & Io, 2015 May 12, 01:17 UT

The Moon spends most of the week out of the limelight, waning in the pre-dawn sky as she passes through the dim constellations associated with the autumn al sky.  New Moon occurs on the 18th at 12:13 am Eastern Daylight Time.  She’ll return to grace the evening sky next week.

The week finds us in the midst of the May campaign for the citizen-science Globe at Night program, a world-wide effort to enhance the public’s awareness of the night sky and sensitivity to the problem of light pollution.  As in April, the program focuses on the springtime constellation of Leo, the Lion.  You can find Leo just west of the meridian at the end of evening twilight, which now occurs at around 10:00 pm.  The premise is simple: compare your view of the constellation with charts found on the program’s website

and estimate your locale’s “limiting magnitude”.  Your data will contribute to a global map of observations contributed by thousands of other observers and will help to identify the best places for people to find dark sky observing sites.

Dark skies are becoming harder to find these days, but this is a great time of year to take advantage of them if you can.  The late hour of evening twilight and the short duration of night entice us to spend the cooler evening hours exploring the darkness around us.  Before the midnight hour we are staring out toward the north pole of the Milky Way galaxy, with just a few thousand light-years worth of stars separating us from the vastness of intergalactic space.  Beyond this thin veil of stars small telescopes reveal the faint wispy glimmers of hundreds of far-flung external galaxies; as you increase aperture more of these wisps emerge from the dark.  The area of the sky bounded by Denebola, the star that marks the “tail” of Leo, and the bright stars Arcturus and Spica holds the center of the Virgo Cluster of galaxies, of which our Milky Way is a far-flung member.  After midnight look for the bright star clouds of the summertime Milky Way rising over the eastern horizon.  It is hard to believe that this huge band of ghostly light is the same as any one of those faint fuzzy dabs of light that make up the members of the Virgo Cluster!

For those of you who prefer an early bedtime, bright Venus will grab your attention as evening twilight deepens.  The dazzling planet is now coursing through the stars of Gemini and is currently just beginning to descend from her most northerly declination of the current evening apparition.  She is also beginning to lose ground on the encroaching Sun.  Over the next week she will set at her latest time for the year shortly before midnight, and by the beginning of June she will begin to set earlier on successive nights.

Jupiter still shines brightly in the western sky at the end of evening twilight.  You’ve still got a few hours to get a good look at him in the telescope.  Despite the fact that his apparent disc is now only 80% of what it was at opposition, the planet still offers a wealth of detail for small telescopes on nights when the air is steady.  You’ll get a very unusual view of the giant planet on the evening of the 12th at around 9:00 pm.  The moon Io will be obscured by the planet’s disc, while Callisto will be in the planet’s shadow.  Europa and Ganymede will appear very close to each other, and between 9:10 and 9:18 pm Ganymede will pass in front of Europa, giving Jupiter the appearance of having only one moon!  On the following night the Great Red Spot crosses the planet’s disc during the evening hours, and on the 17th you can watch Io’s shadow creep across Old Jove’s cloud tops.

Saturn now rises before 9:00 pm and may be found in the southeastern sky at the end of evening twilight.  You’ll need a clear view of the horizon to catch him before midnight, but by 1:00 am he’s about as high as he’ll get in our late spring sky.  After viewing Jupiter you’ll find that the disc of Saturn is rather bland, but the eye will be drawn to the planet’s amazing ring system, which are generously tipped in our direction.  See if you can spot the Cassini Division about two-thirds of the distance from the rings’ inner edge to their outer edge.  This thread-like feature is about half the diameter of the Earth!



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