Illustration by A.Fazekas, SkySafari
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Jupiter’s moons Io and Ganymede (not shown) simultaneously cast shadows on top of the gas giant’s clouds in this orbital simulation of Wednesday night’s eclipse.  

Illustration by A.Fazekas, SkySafari

This Week's Night Sky: Jupiter’s Moons in a Double Eclipse

A backyard telescope will reveal the shadows of Io and Ganymede merge on the surface of the gas giant.

The moon acts as a guide to stunning celestial sights throughout the week, and Jupiter’s largest moons converge in a special eclipse.

Worlds align.  After nightfall on Monday, May 25, check out a beautiful alignment of  bright celestial objects in the western sky.

The first-quarter moon pins down the grouping, with the planet Jupiter appearing to the lower right and Venus closest to the horizon. After darkness falls completely, Regulus, the brightest star in the constellation Leo, joins the party between the moon and Jupiter. The Gemini twins Pollux and Castor, perched above Venus, complete this not-to-be-missed  cosmic get-together.

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On Monday evening the moon will pin down a pretty line-up of planets and stars in the western sky.

Jovian shadows.  Late night on Wednesday, May 27, train a small backyard telescope on Jupiter to see two of its largest moons cast their shadows on the gas giant.  

Starting at 8:58 p.m. EDT, icy Ganymede’s shadow makes its first appearance on Jupiter’s disk. The shadow of the volcanic moon Io begins its own transit at 10:01 p.m. EDT.

What makes this Jovian eclipse even more special is that both black dot-like shadows will merge at 11:38 p.m. EDT when the shadow of the innermost and faster-orbiting Io catches up to that of Ganymede. Io’s shadow leaves the planet at 12:18 a.m. EDT, and the show ends when Ganymede’s shadow does the same at 12:34 a.m. EDT.

Moon and Spica.  As the moon glides across the sky this week, it will park itself by Spica, the brightest star of the constellation Virgo, the maiden, on Friday, May 29. The waxing gibbous moon will appear less than 4 degrees from the 250 light-year-distant star.  

Often represented as the ear of wheat held by the goddess, the blue-white giant star Spica shines 2,000 times brighter than and has 10 times the mass of our own sun.

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On Friday night look for the moon nestled within the constellation Virgo, the maiden, next to the bright blue-white star Spica and nearby Saturn.

Lunar arc.  By the night of Sunday, May 31, the near full moon will be in the low southeast sky wedged between the white star Spica and cream-colored Saturn to its lower right. The ringed world will be less than 10 degrees from the moon, about equal to the width of your fist held at arm’s length.

If you have a small telescope, train it on Saturn. Only a week past its official opposition, when we get the brightest and best views of the planet all year, its magnificent rings and moons will still be great sights.

Happy hunting!

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