The telescope: how two lenses opened up the sky

For thousands of years, man looked at the sky with his naked eyes alone. The Moon was a shining ball, the planets bright dots, and the Milky Way a faint white band. Then, in the early 17th century, a simple tube with two lenses changed all this. Within a few months, mountains appeared on the Moon, moons were seen circling Jupiter, and it became clear that the Milky Way was a cluster of countless stars. The tube was the telescope.

A spectacle-maker’s shop in Middelburg

Writing in BBC Sky at Night Magazine, Dutch science writer Govert Schilling says the oldest known document to mention the telescope is an application dated 2 October 1608. It was submitted to the Dutch government by Hans Lipperhey, a spectacle-maker of Middelburg in the Netherlands, who sought a patent for his new device.

According to Schilling, Lipperhey’s device was a simple cardboard tube. It had a convex (outward-curving) lens at one end and a concave (inward-curving) lens at the other, near the eye. It made objects look about three times larger. Its demonstration before officials in The Hague was so successful that Lipperhey was ordered to build three larger instruments and a binocular telescope. He was not granted a patent, because the design was already known to several people.

Opinion is divided on who was the first inventor. A few weeks later, Jacob Metius claimed a similar invention. In 1656 a French writer named Zacharias Janssen, a claim for which, according to Schilling, no proof has ever been found.

The professor of Padua

Galileo Galilei, born in Pisa, Italy, in 1564, was then teaching mathematics at the University of Padua. According to the MacTutor archive of the University of St Andrews, in 1609 he heard through a friend that a Dutchman had arrived in Venice with a device that made distant objects look near. Galileo worked out the principle of the device and set about building one himself.

His first telescope magnified about four times. MacTutor writes that he ground and polished his own lenses, and by August 1609 had made a telescope magnifying eight or nine times. When it was demonstrated before the Venetian government, officials were greatly impressed.

A telescope made by Galileo is still preserved at the Museo Galileo in Florence. According to the museum’s catalogue, it dates from about 1610, is made of wood, paper and copper, has an objective lens 51mm wide and magnifies 14 times. The same catalogue says the name “telescope” was suggested in 1611 by Federico Cesi, founder of an Italian learned society.

A new picture of the sky

When Galileo turned this telescope towards the sky, something new appeared every night. According to MacTutor, he saw that the Moon was not smooth but had mountains on it, and that the Milky Way was made up of tiny stars.

The most important discovery came near Jupiter. According to NOVA, the science programme of the American broadcaster PBS, in 1610 Galileo noticed a few tiny bright dots near Jupiter. At first he took them for stars, but their positions kept changing from night to night. He finally concluded that they were moons circling Jupiter. Today these four moons are called Io, Europa, Ganymede and Callisto.

Galileo published these observations in 1610 in a short book called “Sidereus Nuncius”, or “Starry Messenger”. According to MacTutor, in the same year he saw that Venus waxes and wanes like the Moon. On 25 July 1610 Saturn appeared to him as three bodies, because his telescope could not show its rings clearly. He later wrote about sunspots as well.

Kepler’s improvement

The telescopes of Lipperhey and Galileo had a narrow field of view: only a very small part of the sky could be seen at a time. According to Schilling, in 1611 the German astronomer Johannes Kepler proposed that the lens at the eye end should also be convex. This gives an inverted image, but a much wider view, and in astronomy an inverted image makes no difference. The first working telescope of this kind was built in 1630 by Christoph Scheiner. The large astronomical telescopes of later centuries developed from the same idea.

Four centuries on

The cardboard tube with which this story began has today taken modern forms, from observatories on mountain tops to space. But the principle is the one that emerged in a spectacle-maker’s shop in Middelburg: two lenses placed at the right distance, gathering distant light to the eye. On a clear night in Quetta, point a small telescope at Jupiter, and you may see what Galileo saw in 1610: tiny bright dots beside Jupiter.