Galileo's Telescope and Sidereus Nuncius (1610): Moons of Jupiter

Galileo’s telescope and Sidereus Nuncius mark the moment in 1609–1610 when the Italian mathematician Galileo Galilei turned a much improved telescope towards the night sky and published what he saw. In a short book printed in Venice in March 1610, he reported mountains on the Moon, thousands of unseen stars and four moons circling Jupiter. The discoveries made Galileo famous across Europe, transformed the telescope into a scientific instrument and gave powerful new arguments to supporters of the Sun-centred system.

Background: a new optical device

Spectacle lenses had been made in Europe since the late thirteenth century, but no one had combined lenses into a practical telescope until the early 1600s. In October 1608 the Middelburg spectacle maker Hans Lipperhey asked the States General of the Dutch Republic for a patent on an instrument “for seeing things far away as if they were nearby”. Other craftsmen, including Jacob Metius, claimed the same idea, and the patent was refused.

News of the “Dutch perspective glass” spread fast. Small, weak telescopes were on sale in Paris and in Italian cities by the spring and summer of 1609. The English mathematician Thomas Harriot drew the Moon through a six-power instrument on 26 July 1609, months before Galileo, but he did not publish his observations.

Galileo, born in Pisa in 1564, was then professor of mathematics at the University of Padua in the Republic of Venice. He was already known for work on motion and mechanics and privately favoured the theory of Nicolaus Copernicus that the Earth moves around the Sun.

Building a better telescope

Galileo heard about the Dutch device in the summer of 1609, probably through his friend, the Venetian friar Paolo Sarpi. Without seeing one, he worked out that it combined a convex lens at the front with a concave lens at the eyepiece. He began grinding his own lenses and soon outperformed the instruments on sale.

His first telescope magnified about three times. By August 1609 he had one of eight or nine times, which he demonstrated to Venetian senators from the bell tower of St Mark’s. Its value for spotting ships at sea was obvious, and the Senate rewarded him with a lifetime appointment at Padua and a much higher salary.

By late 1609 Galileo had a telescope magnifying around 20 times. This “Galilean” design gave an upright image but a very narrow field of view, showing only a small part of the Moon at once. Two of his telescopes survive at the Museo Galileo in Florence; they magnify about 14 and 21 times.

Making good lenses was the hard part. Glass of the period was full of bubbles and streaks, and Galileo reported that only a small fraction of the lenses he ground were good enough for his best instruments. He also learned to cover part of the front lens with a cardboard ring, reducing blurring at the edges. Using such a telescope required patience: it had to be held steady on a support, and only a small patch of sky was visible, so finding and following an object was slow work.

What Galileo saw

From late November 1609 Galileo observed the Moon systematically and made a series of drawings. He saw that the boundary between its light and dark parts was ragged, with bright points appearing in the shadow before the sunlit area reached them. He correctly interpreted these as mountain peaks catching the sunrise and estimated their heights from the length of their shadows.

This contradicted the old Aristotelian idea that heavenly bodies were perfect, smooth spheres made of a different substance from the Earth. Galileo also explained the faint glow of the dark part of the crescent Moon, called earthshine, as sunlight reflected from the Earth.

Turning to the stars, he found far more than the eye could see. Around the belt and sword of Orion he counted dozens of new stars, and in the Pleiades about 36 where the eye sees six or seven. The Milky Way, he reported, was nothing but a mass of innumerable stars clustered together, and several cloudy patches, or nebulae, resolved into groups of stars.

The moons of Jupiter

The most dramatic discovery came on 7 January 1610. Looking at Jupiter, Galileo saw three small bright points in a straight line near the planet. Over the following nights their positions changed, and on 13 January he saw a fourth. He concluded that these were not fixed stars but bodies orbiting Jupiter, just as the Moon orbits the Earth.

This mattered for the debate about Copernicus. Critics had argued that if the Earth moved around the Sun it would leave its Moon behind. Jupiter, which everyone agreed was in motion, clearly kept four moons with it. The discovery also proved that not everything in the heavens circled the Earth.

Galileo recorded the positions of the moons night after night in his notebook, and Sidereus Nuncius printed dozens of small diagrams showing Jupiter and its companions in different arrangements. From these records he could already see that the moons had different periods, the closest moving fastest, just as the planets do around the Sun.

Galileo, keen to win a post in his native Tuscany, named the satellites the “Medicean Stars” in honour of the young Grand Duke Cosimo II de’ Medici and his three brothers.

Sidereus Nuncius

Galileo wrote up his findings at speed. Sidereus Nuncius (“Starry Messenger”) was printed in Venice by Tommaso Baglioni and published on 13 March 1610, its dedication to Cosimo II dated the day before. It was written in Latin for an international learned audience and illustrated with engravings of the Moon and diagrams of Jupiter’s moons.

The print run of about 550 copies sold out within days. Galileo also sent telescopes to princes and cardinals so that they could see for themselves, an important step because many sceptics either lacked good instruments or could not make out what he described.

Reception and consequences

Reactions were mixed. Some scholars refused to look through the telescope or claimed its images were optical illusions. The most important early support came from the imperial mathematician Johannes Kepler in Prague, who published an enthusiastic open letter in April 1610 and, after borrowing a telescope, confirmed the moons of Jupiter later that year.

The book secured Galileo’s career. In July 1610 Cosimo II appointed him Chief Mathematician of the University of Pisa and Philosopher and Mathematician to the Grand Duke, and Galileo moved to Florence in September. Later in 1610 he observed that Saturn appeared to have two “companions”, in fact its rings seen with a weak instrument, and that Venus showed a full set of phases, something impossible in the traditional Ptolemaic system.

In 1611 Galileo visited Rome, where the Jesuit astronomers of the Collegio Romano confirmed his observations and the Accademia dei Lincei welcomed him as a member. At a banquet in his honour that year the name “telescope” was proposed for his instrument.

Key people

Galileo Galilei (1564–1642) was a mathematician, physicist and astronomer from Pisa who combined skill as an instrument maker with a talent for persuasive writing. He was also keenly aware of the value of patronage.

Paolo Sarpi (1552–1623), a Venetian Servite friar, scholar and adviser to the Republic, kept Galileo informed about the Dutch invention and helped arrange his demonstration to the Venetian authorities.

Cosimo II de’ Medici (1590–1621), Grand Duke of Tuscany from 1609, had been Galileo’s pupil. By dedicating his discoveries to the Medici, Galileo secured a court position that freed him from teaching.

Johannes Kepler (1571–1630), imperial mathematician in Prague, gave Galileo the backing of Europe’s leading astronomer at a critical moment, and in 1611 he described an improved telescope design using two convex lenses.

Myths and facts

Myth: Galileo invented the telescope. He did not; it was a Dutch invention of 1608. His achievement was to improve it greatly and to use it systematically for astronomy.

Myth: Galileo was the first person to look at the sky through a telescope. Thomas Harriot sketched the Moon in July 1609, and others pointed early telescopes upwards. Galileo was the first to publish detailed, interpreted results.

Fact: he was not the only one to see Jupiter’s moons. The German astronomer Simon Marius later claimed to have observed them from late 1609. Galileo published first, but it is Marius’s names, Io, Europa, Ganymede and Callisto, that are used today.

Fact: the telescope alone did not prove Copernicus right. The moons of Jupiter and the phases of Venus undermined the Ptolemaic system, but they were also compatible with Tycho Brahe’s mixed model, in which the planets orbit the Sun while the Sun orbits the Earth.

By the numbers

Timeline

• 1608, 2 October: Hans Lipperhey’s patent request for a telescope is considered in The Hague.
• 1609, 26 July: Thomas Harriot sketches the Moon through a telescope in England.
• 1609, August: Galileo shows an eight- to nine-power telescope to Venetian senators.
• 1609, late November–December: Galileo observes and draws the Moon.
• 1610, 7 January: Galileo first sees three small “stars” near Jupiter.
• 1610, 13 January: A fourth moon of Jupiter is observed.
• 1610, 13 March: Sidereus Nuncius is published in Venice.
• 1610, July: Galileo is appointed mathematician and philosopher to the Grand Duke of Tuscany.
• 1610, late: Galileo observes the phases of Venus.

Why it matters

Sidereus Nuncius showed that a new instrument could reveal facts about the universe that no ancient authority had known. It shifted astronomy towards observation, made Galileo the most famous scientist of his day and opened the road to the bitter disputes over Copernicanism that ended in his trial in 1633. Together with the work of Johannes Kepler, it laid the foundations of modern astronomy.