Galilean moons close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. GIF animation of the resonance of Io, Europa, and Ganymede <br> Galilean moons The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit another planet. Their names derive from the lovers of Zeus. They are among the largest objects in the Solar System with
Ring Around the Moon (Space: 1999) from the sphere and enveloped the Moon. Tracking sensors reveal the sphere to be stationary but, from their perspective, it is still getting closer to the Moon's surface; John Koenig realises that the Moon has been trapped in an orbit around the sphere. An audio signal is received and a sibilant voice announces that they are prisoners of the planet Triton. The staff reviews the damage caused by the Moon's sudden deceleration. The fact that all but four are non-operational and the main generators show no sign of damage but are only putting out minimum power indicates that the sphere's