November 4, 2004: At first glance, Jupiter looks like it has a mild case of the measles. Five spots – one colored white, one blue, and three black – are scattered across the upper half of the planet. Closer inspection by NASA's Hubble Space Telescope reveals that these spots are actually a rare alignment of three of Jupiter's largest moons – Io, Ganymede, and Callisto – across the planet's face. In this image, the telltale signatures of this alignment are the shadows [the three black circles] cast by the moons. Io's shadow is located just above center and to the left; Ganymede's on the planet's left edge; and Callisto's near the right edge. Only two of the moons, however, are visible in this image. Io is the white circle in the center of the image, and Ganymede is the blue circle at upper right. Callisto is out of the image and to the right.See the rest:
Io, Ganymede, and Callisto whip around Jupiter at different rates. So it takes time for them to catch up with each other. One reason why the moons orbit at different rates is that they are not all at the same distance from Jupiter. Their orbits are nested like Russian dolls. Io is the closest, followed by Europa, Ganymede, and Callisto. These are the largest of Jupiter's moons. The moons closest to Jupiter travel faster than those farther away. For example, during the time it takes Io to complete four orbits, Europa completes two orbits, and Ganymede one orbit. The ratio between the orbital periods of these three moons means that they can never be on the same side of Jupiter. Only Io, Ganymede, and Callisto can line up on the same side of Jupiter.
The moons orbit in nearly the same plane around Jupiter because they were formed from the same disk of debris that created the solar system. These moons were the first spotted by Galileo in the early 1600s. It was the first observational evidence that a body could be gravitationally bound to a system other than that of the Earth.