from night to night, but they never strayed far from the planet. Galileo deduced that they were moons orbiting Jupiter, and indeed they were. Today we know them as Io, Europa , Ganymede , and Callisto --the Galilean satellites. He also studied mountains on the Moon and the changing appearance of Venus, which waxes and wanes from crescent to full and back again, just like the Moon. Galileo calculated that if the Sun and venus both circled Earth, as in Ptolemy's scheme, Venus could never appear as more than a crescent. Thus, its waxing and waning could occur only if Venus circled the Sun rather than Earth.

Galileo's astronomical discoveries , and his stubborn persistence in promoting them, so infuriated the Roman Catholic Church that near the end of his life he was forced to recant his findings . Legend asserts that after his trial, where he was pronounced guilty of heresy , Galileo proclaimed about the planet beneath his feet, "Eppur si muove"--"Nevertheless, it moves." Less disturbing to the church were his investigations into how things move on Earth itself, work that effectively laid the foundation for the science of physics.

Before Galileo's studies, people believed that not only did Earth stand still but that objects upon it preferred to stay at rest. All motion seemed to stop of its own accord, and rather quickly most of the time. Galileo disproved that notion with careful experiments involving metal balls, cylinders, and smooth inclined tracks. His measurements revealed the tendency of a moving object to keep moving until some external force, such as friction, makes it change. We call this concept Inertia . An air hockey table illustrates the idea perfectly. Before you plunk quarters into the game, the plastic puck skids just a few inches across the table as friction grinds it to a halt. But when that cushion of air is turned on, almost all friction vanishes. The slightest tap sends the puck gliding toward your opponent's goal without slowing down.

Galileo died in 1642. The English physicist Sir Isaac Newton was born the same year. The world has never witnessed a more symbolic passing of the scientific baton. Newton used his extraordinary intellect to synthesize and extend many of the studies of motion that had preceded him. In his historic book The Mathematical Principles of Natural Philosophy, published in 1687, Newton laid out the three laws of motion (page 21 ) that students still learn today in physics classes.