<P> All four Galilean moons are bright enough to be viewed from Earth without a telescope, if they appear farther away from Jupiter . (They are, however, easily visible with even low - powered binoculars .) They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun, and are about one unit of magnitude dimmer when Jupiter is in conjunction . The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness . The maximum angular separations of the moons are between 2 and 10 arcminutes from Jupiter, which is close to the limit of human visual acuity . Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked - eye observation . </P> <P> GIF animation of the resonance of Io, Europa, and Ganymede </P>

Io and europa the inner two of jupiter's four largest moons