For one October night, Ganymede slid in front of Io — a meeting that only lines up every six years

Jupiter is not a lonely planet: 115 moons have been found orbiting it so far. On 20 October, two of the largest — Ganymede, the biggest moon in the solar system, and volcanic Io — met in the sky, Ganymede briefly passing directly in front of Io as seen from Earth. NASA's Astronomy Picture of the Day for 28 October features Michael Wong's video of the mutual occultation, an event that can only be seen when Earth sits edge-on to the moons' orbits, around Jupiter's equinox — a geometry that comes round roughly once every six years. In the same frames, Ganymede's shadow falls on Jupiter itself: stand inside it, and you would watch a solar eclipse from the top of the clouds.