MINIATURE SOLAR SYSTEM
When Galileo discovered Jupiter's four large moons— Io, Europa, Ganymede, and Callisto—he was excited to see that Jupiter mimicked the Copernican model of the Solar System.
Being 5th to 6th magnitude in brightness, the four Galilean moons are conspicuous in any telescope. You can even duplicate Galileo's discovery with a pair of binoculars. Observe Jupiter at the same time each night, and plot the positions of the planet and the minute points of light lying beside it. After a week or two, a pattern will emerge, and you may experience some of the emotion that gripped Galileo.
From time to time, one or another of the Galilean moons will transit Jupiter's disk, where the dark shadow it casts will be more conspicuous than the moon itself. For observers with scopes of
10 inches (250 mm) or more, 3 transits are a good time to look for the color of the moon because Jupiter provides a light background. lo has a yellowish cast, Europa looks grayish white, Ganymede tan-gray, and Callisto bluish gray.
These moons orbit in the plane of Jupiter's equator, which is presented nearly edge-on to us. Every six years, when Earth passes through this plane, we witness a series of mutual eclipses and occultations among the moons. Consult astronomy magazines and almanacs for dates of these events.
Jupiter has at least 12 other moons. Unfortunately, all of them are small, with the brightest, Himalia, reaching only 15th magnitude.