Hubble Captures Rare Triple-Moon Conjunction

Three moons and their shadows parade across Jupiter near the end of the event at 07:10 UT on January 24, 2015. Europa has entered the frame at lower left. Slower-moving Callisto is above and to the right of Europa. Fastest-moving Io is approaching the eastern limb of the planet. Europa's shadow is toward the left side of the image and Callisto's shadow to the right. (The moons' orbital velocities are proportionally slower with increasing distance from the planet.)

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

More info: Firing off a string of snapshots like a sports photographer at a NASCAR race, NASA's Hubble Space Telescope captured a rare look at three of Jupiter's largest moons zipping across the banded face of the gas-giant planet: Europa, Callisto, and Io. Jupiter's four largest moons can commonly be seen transiting the face of the giant planet and casting shadows onto its cloud tops. However, seeing three moons transiting the face of Jupiter at the same time is rare, occurring only once or twice a decade. Missing from the sequence, taken on January 24, 2015, is the moon Ganymede that was too far from Jupiter in angular separation to be part of the conjunction.

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