Galileo and Jupiter
On January 7, 1610 in Padua, Italy, Galileo Galilei happened to aim his newly developed spyglass at the planet Jupiter and discovered previously unknown worlds. Galileo thought he was simply observing fixed stars, but their position aroused his curiosity; they lay in a straight line with Jupiter, two to the east and one to the west. For Galileo, that night was the beginning of what would be a long series of observations of these curious objects.
The next night, "led by what, I do not know," Galileo looked at the stars again. They were still visible, but to his amazement, they had changed position. Now all three stars were in a straight line on the west side of Jupiter. Galileo could not understand how the planet could have moved east of these stars when, the night before, it had been west of two of them. He feared that astronomers had miscalculated Jupiter's movement.
Galileo could hardly wait for the next night, but he was destined for disappointment,
". . . for the sky was covered with clouds in every direction." He had to wait another night. On January 10, Galileo could find only two of the stars, both on the east side of Jupiter. The third, he assumed, was hidden behind Jupiter. Galileo realized that the planet's movement could not possibly be so erratic. He decided at last that, contrary to everything he knew, the stars themselves must be moving.
On January 11, Galileo again saw only two stars, both on the east side of the planet. But the next night a new, extremely small star which Galileo was certain he had not seen before joined the others; two were on the east and one was on the west. Then on January 13, the fourth star, missing since January 8, reappeared; one star was on the east side of Jupiter and three were on the west. Two nights later, all four stars were on the west. Galileo now realized that these stars were actually tiny planets, later called satellites, revolving around Jupiter. For the first time in history, man had seen the moons of a planet other than our Earth.
Galileo continued his observations through February. On March 10, he announced his discovery in the "Starry Messenger". It created a sensation, spurring interest in astronomy as a science and providing support to the Copernican theory that the planets orbit the Sun. The "stars" that Galileo saw are the four largest moons of Jupiter, now known as the Galilean satellites in honor of their discoverer. The satellites are named Io, Europa, Ganymede, and Callisto after four of Jupiter's lovers in Greek mythology. These are the satellites that will be closely investigated during the Galileo mission a fitting tribute to the man.
"Galileo to Jupiter," NASA, JPL 400-15 7/79, GPO: 1979-691-547, p. 1.