Italian astronomer Galileo Galilei discovered Jupiter’s four brightest moons in 1610. Because of his find, you’ll often hear them called the Galilean satellites. Observers frequently see them passing in front of or disappearing behind the planet. But every six years near Jupiter’s equinox, the orbital planes of these four moons tilt almost edge-on to Earth, and less common phenomena occur: The satellites eclipse and occult each other. Astronomers call these occurrences mutual events.
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