For nearly three centuries after their discovery in 1610, the only known moons of Jupiter were the four large Galilean satellites. In 1892 E. E. Barnard, an American astronomer, found a much smaller fifth satellite orbiting very close to the planet, and between 1904 and 1974 eight additional satellites were found far outside the orbits of the Galilean satellites. The outer satellites are quite faint and presumably no more than a few tens of kilometers in diameter, and all have orbits that are much less regular than those of the five inner satellites. Four of them revolve in a retrograde direction, opposite to that of the inner satellites and Jupiter itself. In 1975 the International Astronomical Union assumed the responsibility for assigning names to the non-Galilean satellites of Jupiter. Following tradition, they named the inner satellite Amalthea for the she-goat that suckled the young god Jupiter. The outer eight were named for lovers of Jupiter: Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and Sinope. For the non-Galilean satellites, the “e” ending is reserved for satellites with retrograde orbits; those with normal orbits have names that end in “a.” Because they are so large, the Galilean satellites have attracted the most attention from astronomers. More than fifty years ago large telescopes were used to estimate their sizes, and a careful series of measurements of their light variation showed that all four always keep the same face pointed toward Jupiter, just as our Moon always turns the same face toward Earth. Also, the subtle gravitational perturbations they exert on each other were used to determine the approximate mass of each. [Illustration: The pattern of the Galilean satellites changes from hour to hour, as seen from Earth. Viewed edge-on, the nearly circular orbits produce an apparent back and forth motion with respect to Jupiter. These images recreate the kinds of observations first made by Galileo in 1610.] Callisto, the outermost Galilean satellite, is larger than the planet Mercury. It also has the lowest reflectivity, or albedo, of the four, suggesting that its surface may be composed of some rather dark, colorless rock. Callisto takes just over two weeks to orbit once around Jupiter. Ganymede, which requires only seven days for one orbit, is the largest satellite in the Jovian system, being only slightly smaller than the planet Mars. Its albedo is much higher than that of Callisto, or of the rocky planets such as Mercury, Mars, or the Moon. In 1971 astronomers first measured the infrared spectrum of reflected sunlight from Ganymede and found the characteristic absorptions of water ice, indicating that this satellite is partially covered with highly reflective snow or ice.