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	SEEING IS BELIEVING - cont.


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	Ptolemy’s model stood unchallenged for 1400 years. And because its proposition that Earth was the center of the universe dovetailed so nicely with the Church’s teaching that Earth and its inhabitants were the center of God’s creation, the two became inextricably intertwined. The Church endorsed Ptolemy’s model precisely because it fit biblical accounts of the creation of the universe by God. The geocentric system had by then become an article of faith, closed to any scientific scrutiny.

It fell to a Polish astronomer to raise a different point of view. In 1543, Nicolas Copernicus reintroduced the idea that the Sun was the center of the universe and Earth just one of several planets circling it. His heliocentric model did not gain immediate acceptance, however. Despite its elegant simplicity, it, too, had problems. Copernicus believed, as Ptolemy had before him, that the planets move in perfect circles at constant speeds. That's not the way the solar system works, and Copernicus also had to add circles within circles to make his model reproduce the planets’ motions.

Two men helped make Copernicus’ basic idea work, one an observer, the other a theorist. The Danish observer Tycho Brahe took accurate measurements of the positions of the planets over a 20-year period at the end of the 16th century.



		Johannes Kepler


	Those observations helped the German theorist Johannes Kepler learn the secrets of planetary motion. He found that instead of traveling in circular orbits, the planets moved on slightly elliptical paths. This modest change eliminated the need for any double motions and, more importantly, made it easy to predict accurately the future positions of the planets. Ptolemy’s system began to lose favor in 1610 when Galileo made his first observations with the recently invented telescope. He saw mountains and craters on the Moon and dark “blemishes” on the Sun (what we call sunspots), in contrast to the perfect, smooth spheres these heavenly bodies were supposed to be.


	He discovered four moons orbiting Jupiter—we now call them the Galilean moons in his honor—proving that Earth was not the only center of motion in the universe. And he observed phases on Venus that mimicked all those seen on the Moon, while Ptolemy’s system would allow the planet to show only crescent phases. Although it took Isaac Newton to develop a theory that explained why the planets moved as they did, the heliocentric picture of the universe had won, at least in most of northern Europe. In the strongholds of the Catholic Church in southern Europe, Ptolemy’s system still reigned, and Galileo was forced to recant his support of the Copernican system.


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