FIGURE 1.1 The heliocentric system of Copernicus, depicted in this mid-17th-century copper-plate engraving, included the recently discovered moons of Jupiter. Galileo's "Medicean stars" encircling Jupiter are today called the Galilean satellites, Io, Europa, Ganymede, and Callisto. (From Atlas Coelestis seu Harmonia Macrocosmica of Andreas Cellarius, 1660-1661 edition, Amsterdam; 25" × 28 5/8". Courtesy of the Mendillo Collection.)