Interestingly, there was a Greek thinker, Aristarchus of Samos, who had proposed the heliocentric theory as far back as the third century BC. “Aristarchus has been celebrated for his anticipation of Copernicus.” historian David Lindberg writes. But were Aristarchus and his followers good scientists? To avoid the retroactive fallacy of using current knowledge to judge the merits of past scientific claims, we have to examine the data available at the time. As Lindberg puts it, “The question is not whether we have persuasive reasons for being heliocentrists, but whether they had any such reasons, and the answer is that they did not.”

The data right up to Galileo’s day favored Ptolemy. Kuhn notes that throughout the Middle Ages there were people who proposed the heliocentric alternative. “They were ridiculed and ignored,” Kuhn writes, adding, “the reasons for the rejection were excellent.” Consider some examples he gives. The earth does not appear to move, and we can all witness the sun rise in the morning and set in the evening. If the earth moves at high speeds around the sun, then birds and clouds and other objects not attached to the ground should be left behind. A stone hurled into the sky would land many miles away from the spot at which it was thrown, as the earth would have traveled a considerable distance while the object was in the air. Human beings standing on the ground would be flung about. As none of this was observed, the earth was held to be stationary.”

Galileo was a Florentine astronomer highly respected by the Catholic church. Once a supporter of Ptolemy’s geocentric theory, Galileo became persuaded that Copernicus was right that the earth really did revolve around the sun. Copernicus had advanced his theory in 1543 in a book dedicated to the pope. Copernicus admitted that he had no physical proof, but the power of the heliocentric hypothesis was that it produced vastlybetter predictions of planetary orbits. Copernicus’s new ideas unleashed a major debate within the religious and scientific community, which at that time overlapped greatly. The prevailing view half a century later, when Galileo took up the issue, was that Copernicus had advanced an interesting but unproven hypothesis, useful for calculating the motions of heavenly bodies but not persuasive enough to jettison the geocentric theory altogether.