Jupiter is one of the five objects that has been called a planet since antiquity (ΓÇ£planetΓÇ¥ is Greek for ΓÇ£wandererΓÇ¥). Long before the telescope, Jupiter was known to be different from stars because it changed position, or ΓÇ£wanderedΓÇ¥ across the sky. As far back as the second century A.D., ancient astronomers like Ptolemy under- stood that Jupiter followed a fixed course through the constellations, returning to its starting point in the sky after a number of years.
The modern study of the planet began with Galileo Galilei, who turned the newly invented telescope on it in the early seventeenth century. Galileo discovered that Jupiter had its own moons; the four he discovered he named Io, Europa, Ganymede, and
