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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. 

David Wootton's Galileo: Watcher of the Skies (Yale, £25) portrays a far more radical character. Contrary to the received scholarship of the last few centuries, Wootton claims that Galileo was not a Catholic, nor even a Christian, but some sort of deist who believed that a god might have created the universe but has since left it well alone. Wootton's grounds for this conjecture are quite shaky. Galileo's surviving private correspondence points to him being what he always claimed to be: an observant Catholic. Wootton must rely on an effective cover-up of all the contrary evidence. But it seems likely that Galileo was not especially devout, which explains why one of his more pious friends got so excited when the great man appeared temporarily to cultivate a more personal faith in his old age. 

Despite its unwarranted speculation about Galileo's religious unbelief, Wootton's biography has much to recommend it. It is engagingly written and offers fresh insights into Galileo's intellectual development. That Wootton passes over some of the mathematical aspects of Galileo's achievement must be accounted an advantage of this book if it is picked up by a non-specialist. 

Heilbron makes no such concessions to his readers. His book is packed with diagrams and seeks to understand Galileo's work as the great man saw it. Galileo was a philosopher, not a scientist. He sought to solve problems from first principles and used experiments only to demonstrate his solutions. In contrast, modern science deals with hypotheses that experiments can falsify but not prove.

Galileo's observations certainly reordered the universe. But did he also help to banish God from men's minds? The evidence would suggest not. To Johannes Kepler, whose spectacularly accurate astronomical tables based on elliptical orbits effectively proved that the Earth moves, science was a sacred duty. His scrawled notes contain spontaneous prayers and he always gave thanks to God for his achievements. Robert Boyle and Isaac Newton were both even more concerned with religion than they were with science. And both were convinced that there could be no conflict between faith and reason.

Even in the 19th century, it was the discovery of historical errors in the Old Testament, rather than science, which caused the Victorian crisis of faith. And the tragic death of his young daughter Annie destroyed the last vestiges of Charles Darwin's Christianity. It had already been severely weakened by the thought of his rationalist father burning in hell. The problem of suffering is, and always has been, the biggest challenge to faith in a loving God. Just ask Job.

Today, only a minority of scientists are religious believers and the evidence suggests that the more eminent the scientist, the graver his doubts. But modern science can discomfit the atheist just as much as the Christian. A century ago, it was scientific orthodoxy that the universe could have no beginning, just as Aristotle had always said. For Fred Hoyle, the avuncular Cambridge physicist, this was almost an article of faith. When a Belgian Jesuit by the name of Georges Lemaître suggested that time and space originated at a singularity, Hoyle disparaged the theory as postulating a "big bang". Later, it transpired that Lemaître and, incidentally, Christian doctrine were correct. The universe is not eternal. 

Nonetheless, Stephen Hawking's new book The Grand Design (Bantam Press, £18.99) says that even though the universe started with a bang, God was not required to light the blue touch paper. All that is needed is a working theory of gravity and the universe will make itself spontaneously. This is true enough, but Hawking has forgotten the lesson Galileo wanted to teach the Catholic Church: science cannot threaten religious truth. The origin of the universe requires that the laws of physics exist and these laws do not explain themselves. A divine legislator still has something to do. That the universe appears fine-tuned to allow for the appearance of intelligent life suggests He did it rather well.

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