Galileo Galilei

Galileo did not invent the telescope. He heard of the Dutch device in 1609, built better ones than anyone else within months, and did the thing nobody had thought worth doing: pointed it up and published immediately. The Starry Messenger appeared in March 1610 and is a short book with enormous consequences. The moon is not a polished sphere but has mountains and valleys, with shadows that shorten as the sun rises on them. The Milky Way is not a vapour but a mass of individual stars. And Jupiter has four small bodies going round it, which he named for the Medici.

Later work added the phases of Venus and the spots on the sun. Both mattered. Venus showing a full set of phases, from crescent through gibbous to full, is impossible in the Ptolemaic arrangement, in which Venus never gets to the far side of the sun. Sunspots meant that the heavens change, against the doctrine of an incorruptible celestial region.

The honest qualification, which is usually left out, is that none of this proved Copernicus. It disproved Ptolemy, which is not the same thing, because Tycho Brahe had proposed a system in which the planets circle the sun and the sun circles a stationary earth, and every one of Galileo's observations fits it perfectly. His opponents could and did make that point, and there was no stellar parallax to answer them with. He was right, and the case he could actually demonstrate in 1616 was weaker than he claimed.

The affair has two acts. In 1616 a theological commission judged the proposition that the sun is the centre of the world to be foolish and formally heretical, and that the earth moves to be at least erroneous in faith. Copernicus's book was suspended pending correction, and Cardinal Bellarmine admonished Galileo personally. Bellarmine's own stated position was reasonable: while there is no demonstration, treat the motion of the earth as a calculating device; if a demonstration appears, the scriptural passages will have to be re-read. Galileo had argued the same in his Letter to the Grand Duchess Christina, adding that scripture teaches how to go to heaven and not how the heavens go.

In 1632 he published the Dialogue Concerning the Two Chief World Systems, with an imprimatur obtained in Florence and Rome, formally presenting both sides. It does not read as balanced, and the argument Urban VIII had personally asked him to include, that God could produce the appearances by means beyond our understanding, is placed in the mouth of the character named Simplicio and dismissed. Urban had been an admirer and a patron.

The 1633 trial turned on a disputed document from 1616 recording an absolute injunction never to hold or teach the doctrine in any way. Galileo produced Bellarmine's milder certificate. He was found vehemently suspect of heresy, abjured on his knees, and spent his remaining years under house arrest at Arcetri, where he went blind and wrote his best book on mechanics. He did not mutter that it moves anyway; the phrase first appears more than a century later. In 1992 a papal commission concluded that his judges had erred.