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On March 13 1610, an obscure professor at the University of Padua published a short pamphlet containing his observations of the heavens made with the newly-invented telescope. That same day, the English ambassador to Venice enclosed a copy in the diplomatic packet, noting that the author ran "the risk of either being extremely famous or exceedingly ridiculous". The pamphlet was called The Starry Messenger and its author was Galileo Galilei.

Taking a risk: the title of Galileo's "The Starry Messenger" (1610) 

Four hundred years of hindsight allow us to see just how significant his discoveries were. He noted that the moon was not a smooth sphere but had mountains and ravines. He could count many more stars in the sky with his telescope than were visible with the naked eye. Most significantly, he found four new "planets" closely associated with Jupiter. These planets, which we know as Jupiter's moons, proved that not everything in the universe had to orbit the Earth. Surely, said Galileo, this answered one of the central objections to the astronomical theory of Copernicus. If Jupiter had moons like the Earth, did it not follow that the Earth could be a planet like Jupiter?

That the Earth was the stationary centre of the universe was not just the position of the Catholic Church. Scientific authorities agreed, not least Aristotle, whose system of philosophy still predominated. If the Earth is moving, it was said, we should be able to measure this directly by noting how the relative positions of the stars change as we travel through space (something that was not observed until 1838). And besides, to assert the Earth was moving, necessarily at very high speed, defied common sense.

Galileo interpreted his findings as support for Copernicus's theory because, as we know from his private letters, he already believed it. Others were less easily convinced. But what really mattered was the attitude of Cardinal Robert Bellarmine, the most powerful man in Rome after the Pope. Bellarmine determined that a moving Earth was contrary to the literalistic reading of the Bible that he favoured and he engineered a ban in 1616. Galileo could no longer openly support Copernicus and it was for flouting this prohibition that he was eventually brought to trial in 1633.

Banning Copernicus was a catastrophic error by the Church. It didn't matter that, at the time, it looked very unlikely that the earth really did move. It should never have been a religious question in the first place. During the Middle Ages, an arrangement had been reached whereby philosophers could get on with philosophy as long as they didn't meddle in theology. In return, theologians avoided circumscribing science unless it touched on the essentials of the faith. In cases of conflict, the Bible could be interpreted figuratively. After all, the Old Testament implies that the Earth is flat and almost no Christian has ever believed that.

Galileo continues to polarise historians and two new books paint very different pictures of him. John Heilbron's Galileo (OUP, £20), shows us a good Catholic struggling to help the Church correct its mistake over Copernicus. This Galileo keeps science and religion in separate boxes. The Bible teaches us how to go to heaven and not how the heavens go, as he wrote in his letter to Grand Duchess Christina of Tuscany. 

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