The telescope, mankind's first extension of one of its senses, was the prototype of modern scientific instruments. It showed that ordinary observers could see things even Aristotle had not imagined. But the telescope was not invented by scientists -- it was the product of craftsmen. Since craftsmen of that time were usually illiterate and therefore invisible to historians, much of the story about the origin of the telescope is unknown. But there are many clues.
Lenses as we know them were introduced in the West at the end of the thirteenth century. Glass of reasonable quality had become relatively cheap, and in the major glass-making centers of Venice and Florence, techniques for grinding and polishing glass had been well developed.
The earliest illustrations of spectacles date from about 1350. These spectacles were reading glasses. A person who had trouble reading went to a spectacle-maker's shop and found a suitable pair by trial and error. Spectacles that correct the refractive error called myopia were first made in Italy in the middle of the fifteenth century. So by about 1450 the tools for making a telescope were available.
Why was the telescope not invented in the fifteenth century? There is no good answer to this question, except perhaps because suitable lenses were not made until later.
When the telescope finally appeared, it was a simple device unveiled in the Netherlands. In October 1608, the Dutch government in The Hague discussed a patent application on a device for "seeing faraway things as though nearby." It consisted of a tube with a convex and a concave lens, a combination that magnified three or four times, similar to modern opera glasses. Word of this new invention spread rapidly through Europe, soon followed by the device itself. By April 1609 three-powered spyglasses could be bought in spectacle-makers' shops in Paris, and four months later there were several in Italy. The Englishman Thomas Harriot observed the moon with a six-powered telescope in July 1609.
But it was Galileo Galilei who made the telescope famous. Though he had not seen the new invention, he figured out how it worked and constructed his first three-powered spyglass in June or July 1609. In August he presented an eight-powered instrument to the Senate of Venice, and he turned a twenty-powered instrument to the heavens by October or November. With this telescope he observed the Moon and discovered four satellites of Jupiter.
It took time for others to confirm Galileo's observations. In the spring of 1610 no one else had telescopes of sufficient quality and power, though many had weaker instruments. About six months passed before other observers could obtain instruments good enough to see Jupiter's "Galilean" moons.
Galileo knew that Venetian interest in the telescope was commercial rather than scientific. The city's wealth and power were based on overseas trade, and its merchant vessels were being attacked at sea. To demonstrate the potential of his telescope for spotting distant enemies, Galileo took a group of senators up into a tower in Venice from which ships could be seen far away in the harbor. The senators were so impressed with his telescope that they doubled his salary!
Like the earlier Dutch versions, Galileo's telescopes used lenses to bend or refract light. Though they were simple to make, he had difficulty finding clear and homogeneous glass for his lenses. It was also hard to shape the lenses perfectly. The images of stars were blurry and surrounded by color halos. But the most limiting factor of these early telescopes was their small field of view. For example, only part of the full Moon could be seen at one time. Galileo continued to improve his devices until they were over four feet long and could magnify up to thirty times.
In 1609 the Roman Catholic Church's account of creation placed Earth at the center of the universe. Galileo was treated as a celebrity when he first went to Florence and Rome to show off his new instruments, and his discoveries with the telescope after 1610 were applauded by bishops and cardinals.
But it was only a matter of time before Galileo's discoveries would contradict the Church's doctrine; enabling observers to see Moon-like phases in the appearance of the planet Venus, which could only be explained if the solar system had the Sun at its center. Galileo's discovery that Jupiter was orbited by its own moons established beyond a doubt that Earth was not the only center of motion in the universe, and in 1632 Galileo was summoned before the Inquisition.