The night of 7 January, 1610, in Padua, Venetian Republic, a 45-year-old mathematics professor opened his window and peered at the sky through an optical instrument he had built some six months earlier. This professor´s name was Galileo Galilei and the instrument was the telescope, known at the time as a "spyglass."
By the time Galileo came across the spyglass in 1609 the instrument had been known for some time as a peculiar toy sold in marketplaces in the Netherlands, France and England. Nobody knows who invented it. It is most likely one of those inventions that "are in the air" and which several people hit upon serendipitously and almost simultaneously. One Hans Lippershey, spectacle-maker from Middelburg, had applied for a patent to the States General of the Netherlands and tried to sell the instrument to the government by pointing out its possible military applications.
Galileo was a skillful constructor of scientific instruments, and once he had seen a spyglass he was quick to grasp the principles involved. He immediately set to work on an instrument which made distant objects appear to be three times closer than they really were. Some time later, he pushed contemporary technology to the limit by building a telescope ten times as powerful as the first one.
Realizing, as Lippershey had, the strategic potential of the spyglass, Galileo subsequently arranged a demonstration for the Venetian authorities. Later he wrote:
"Very many were the patricians and senators who, although aged, have more than once climbed the stairs of the highest campanili of Venice, to detect sails and vessels on the sea, so far away that coming under full sail toward the harbor, two hours or more passed before they could be seen without my spyglass."
As a consequence of this demonstration, Galileo´s salary was doubled and he was granted a lifelong professorial appointment at Padua.
Six months later, Galileo took his best telescope and trained it on the night sky. It was a stroke of genius. Today, one can hardly think of anything more natural to do with a telescope than to point it at the sky, but we must remember that at the time, although Nicolaus Copernicus had published his views about a sun-centered cosmos more than 60 years before, the sky was still held to be divine and pristine, a place of purity as opposed to earthly corruption. Many centuries before, Aristotle claimed that the stars and planets, including the sun and moon, were celestial emmanations that moved around the earth attached to crystal spheres and surrounded by a transparent substance called aether. Looking at the sky, therefore, meant peering into the abode of God. Galileo was treading unchartered and dangerous territory.
What he saw with his telescope during the Winter of 1610 confirmed what he had known for a long time, but had been cautious not to maintain openly --that Aristotle had been dead wrong about the nature of the stars and planets. The moon did not have a polished surface, as the Greek philosopher had said, but was instead deeply scarred with craters, crevasses and mountains, like the earth. The telescope revealed countless stars that were not visible to the unaided eye, and showed that Venus has phases like the moon, which is only possible if it orbits the sun and not the earth. But what stunned him the most was the discovery of four previously undetected "stars" in the vicinity of Jupiter. It gradually became clear that they were circling the planet. This demolished an old objection to the Copernican system in which the earth moved around the sun like the wandering stars, namely, that the moon could not possibly revolve around the earth if the earth itself revolved around the sun.
Galileo´s telescopic findings, which he wasted no time in publishing, changed humankind´s picture of the universe.