GALILEO SAW the four big moons of Jupiter first, but his rival astronomer, Simon Marius, named them. Lacking a Voyager spaceship, Simon couldn't see the colors of the moons, but he could see how they spun; so he called them Io, Europa, Ganymede and Callisto, for their relationships with Jupiter. From Jupiter, said the Romans came the light, the dawn, the weather, the harvest, justice and truth, laws, victories in battle, the past and the future. The four moons were named for women who, in several ways, were close to Jupiter, or to the Greeks' Zeus, with whom he was identical -- each woman quite different from the others, as the moons themselves are different, yet all revoling around a god who was nothing if not busy.

These women led extraordinary lives, which is not extraordinary for those who consorted with Zeus. Ganymede was merely Zeus's daughter. But Io and Callisto, who fooled around with the chief of gods, were changed into a white heifer and a shebear, respectively, by Hera, Zeus's wife, who celearly did not fool around. For Europa Zeus changed himself into a bull, which could not have taken much changing, and carried her off to Crete. All of which is slightly nutty, as well as beautifully mysterious, yet not half as mysterious or beautiful as the moons themselves.

Eventually, scientists will make practical use of the Voyager photos. Jupiter, like its nameake, has a strong magnetic field, and it can probably teach us much about our own climatic system. For the moment, however, the benefit of these pictures is purely poetic: A most modern machine sailing past those red and orange worlds named for the ancients, past Jupiter itself by now, with its "great Red Spot" like a bloodshot eye, peering back.