Galileo’s contribution to the Copernican theory was significant but not decisive. This is a crucial point to keep in mind because of the elaborate mythology surrounding Galileo, mostly based on incidents that never occurred. Kuhn takes up the story we all learned in school about how Galileo went to the top of the leaning Tower of Pisa and dropped light and heavy objects to the ground. He supposedly discovered that, contrary to intuition, the objects all hit the ground at the same time. One simple experiment, the story goes, had refuted a millennium of medieval theorizing.

In reality, Galileo didn’t perform the experiment in Pisa or anywhere else; the experiment was done by one of his students. Moreover, the heavier bodies did actually hit the ground first. Today we understand why this was the case. Only when such experiments are conducted in the absence of air resistance do all bodies fall at the same speed. “In the everyday world,” Kuhn writes, “heavy bodies do fall faster than light ones…. Galileo’s law is more useful to science … not because it represents experience more perfectly, but because it goes behind the superficial regularity disclosed by the senses to a more essential, but hidden, aspect of motion. To verify Galileo’s law by observation demands special equipment. Galileo himself got the law not by observation … but by a chain of logical arguments.”

Having developed a more powerful telescope than others of his day, Galileo made important new observations about the moons of Jupiter, the phases of Venus, and spots on the sun that undermined Ptolemy and were consistent with Copernican theory. Galileo took these observations to the Jesuits, who were among the leading astronomers of the day, and they agreed with him that his sightings had strengthened the case for heliocentrism. The Jesuits told Galileo that the church was divided, with many clergy supporting Ptolemy but others holding that Copernicus was right. Even so, the Jesuits concluded that the question was still open and they did not think that Galileo had clinched the case. Tyco Brahe, the greatest astronomer of the period, agreed that Galileo’s proofs were insufficient and continued to support the geocentric theory. So great was Brahe’s reputation that it prevented the conversion of many astronomers to Copernicanism until after his death.

It may surprise some readers to find out that the pope was an admirer of Galileo and a supporter of scientific research that at the time was conducted mostly in church- sponsored observatories and universities. So was the head of the Inquisition, the learned theologian Cardinal Robert Bellarmine. When Galileo’s lectures supporting the heliocentric theory were reported to the Inquisition, most likely by one of Galileo’s academic rivals in Florence, Cardinal Bellarmine met with Galileo. This was not normal Inquisition procedure, but Galileo was a celebrity. In 1616 he came to Rome with great fanfare, where he stayed at the grand Medici villa, met with the pope more than once, and attended receptions given by various bishops and cardinals.