Galileo Galilei did not invent the telescope that made him famous. In 1609 he wrote to his brother-in-law, telling him how he discovered it: "You must know that it is nearly two months since news reached here that in Flanders there had been presented to the Count Maurice a spy-glass, made in such a way that very distant things are made to look quite close, so that a man two miles away can be distinctly seen. I undertook to think about its fabrication - and so perfected one which I made that it did far surpass the reputation of the Flemish one".
Galileo's model surpassed the prototype in that he had mounted the lenses in a tube. He first built a telescope with a threefold magnifying power, and then one with a magnifying power of 32. Almost immediately he made some very startling observations, which he recorded for posterity in his famous book Sidereus Nuncius, "The Starry Messenger":
"I have seen stars in myriads that have not been seen before, and which surpass the old previously known stars in number more than tenfold. But that which will excite the most astonishment by far, and which I especially called to the attention of all astronomers and philosophers is this; namely that I have discovered four planets, neither known nor observed by anyone before my time."
These were the four satellites of Jupiter, which he named Ganymede, Europa, Io and Callisto. He also discovered sunspots, and the mountains on the Moon, and saw that the Milky Way was not the enigmatic "inflammation of a drie exhalation" asserted long ago by Aristotle, but the fused light of countless stars too small and close together to be distinguished one by one.
Galileo's observations led him to advocate the Copernican system of astronomy, with the Earth and the other planets moving around the Sun rather than the Ptolemaic system which was the approved doctrine of the time, and it was this assertion which put the old astronomer in the dock.
Sick and ageing, he was called to Rome to appear before the Inquisition, and forced to renounce his controversial views.
His declining years were sad and poignant. In 1638 he wrote despondently: "Alas, poor Galileo your devoted friend and servant, has been for a month totally and incurably blind; so that this heaven, this earth, this universe, which by my remarkable observations and clear demonstrations I have enlarged a hundred - nay a thousand fold - beyond their previous limits, are now shrivelled up for me into such a narrow compass as is filled by my own bodily sensations."