NASA’s October 28 Astronomy Picture of the Day shows Ganymede moving in front of Io as viewed from Earth. The apparent collision is an alignment, not a physical encounter: one moon blocks the other along our line of sight. The same footage also shows Ganymede’s shadow on Jupiter, creating a solar-eclipse-like effect for a hypothetical observer there.
NASA explains that mutual occultations among the Galilean moons are unusual from Earth because the moons’ orbital plane must be seen nearly edge-on, a geometry that recurs around Jupiter’s equinox roughly every six years. The image is a good reminder that celestial mechanics can be visibly dynamic rather than a static collection of telescope photographs.
Open the NASA video rather than reading a secondary recap. It is also a useful short demonstration for the boys of the difference between an eclipse, an occultation and an ordinary conjunction.
Watch NASA’s original occultation clip and look for the moon shadow on Jupiter.