Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Every six years, Jupiter and Earth align such that the plane of the Jovian moons points straight at us, allowing viewers to watch Jupiter’s satellites eclipse and occult one another. Three of these events happen on three consecutive Tuesdays in September. A modest telescope or steadily held pair of binoculars will allow you to see the moons as they pass in front of and block the light from each other.
During each of these events, the volcanic moon Io will first pass in front of the icy moon Europa over the course of ten minutes, followed more than an hour later by Io casting its shadow on Europa and dimming its light. The timing of the first event favors those in the eastern United States and the second and third events are better suited to observers in the west.
Jupiter is easy to find, shining brilliantly at magnitude -2.8 in the south, the brightest starlike object visible after sunset. On September 15, fiery Io occults part of frosty Europa starting at 8:42 p.m. EDT and passes to the other side of the satellite by 8:50 p.m. The partial eclipse of Io on Europa occurs about an hour and a half later, at 10:10 p.m. EDT, also lasting approximately 10 minutes. Europa will not completely disappear, but its light will dim noticeably, falling into the deepest shadow at 10:15 p.m.
On September 22, Io partially occults Europa from 8:00 p.m. to 8:08 p.m. PDT, with the partial eclipse from 9:39 p.m. to 9:48 p.m. Europa appears faintest at 9:44 p.m. On September 29, Io passes in front of Europa from 10:18 p.m. to 10:14 p.m. PDT. Io eclipses Europa just after midnight at 12:06 a.m. until 12:14 a.m. PDT, with the moment of darkest eclipse at 12:10 a.m.
(The image of Io and Europa shown here was taken in 2007 during the New Horizons mission. Europa is the moon on the right, and Io is at left. Three separate volcanic plumes were captured in this photo, with the largest plume revealing a red glow of hot lava at its center.)
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Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She is currently the Feature Writer for Astronomy and Space at Suite101.com. You can follow her on Twitter at twitter.com/Astronomommy.