Galileo Sidereus Nuncius 1610 moons of Jupiter

Galileo Galilei,
Sidereus Nuncius
(1610), description of
the moons of Jupiter

Galileo’s Messages from the Stars


Soon after sunset on Thursday, 7 January 1610, the planet Jupiter shone brightly in the southeastern sky over Padua. Galileo Galilei, the not-yet-famous mathematics professor at the university, turned his improved 20-power spyglass (the name “telescope” being a year in the future) to the king of the planets. He was surprised to see what he took to be three fixed stars arranged in a straight line near the planet. On the following evening, “guided by what fate I know not” as he later recalled, he looked again at Jupiter, and was even more surprised to find the planet to the left of all three small stars – surprised because he had assumed that Jupiter should have been moving to the right. By the end of that week he knew he was onto something big, a discovery for an international audience, and within two weeks he had delivered the first part of his manuscript to a printer in nearby Venice. In just over six more weeks, the printed book was ready. It was a book that would change his life and would change the world.

The Sidereus Nuncius or “Sidereal Messenger” divides into two principal parts: the first tells of the earth-like surface of the moon, the second of the four little planets revolving around Jupiter. This brief announcement was published in 550 copies, probably more than the first edition of Copernicus’s De Revolutionibus, but today Galileo’ s thin book is considerably rarer, a clear illustration of the aphorism “Bigger books linger longer.” Galileo targeted his book to Cosimo de’ Medici, the teenaged Grand Duke of Florence, on the hopes of getting a position at the Florentine court, and in this he succeeded. He apologized for the modest nature of the printing, but said he was in a hurry lest he be scooped on his discoveries, and he promised to bring out an edition on a nobler scale.

In a sense Galileo’ s Istoria e dimostrazioni intorno alle macchie solari of 1613 was the promised high-class publication, for it not only brought two more astronomical discoveries – the sunspots and the phases of Venus – but it featured a dazzling array of engravings of the sun, thirty-eight of them, giving an almost continuous sequence of daily images from 2 June through 8 July plus a trio from August. Such full-page engravings were expensive, but this time he had a patron to support a princely production. Galileo was not alone in finding the sunspots, but his elegantly illustrated book was clearly the most memorable and served to bring him the lion’s share of the credit. And the accurate images made a serious scientific point, for they showed how the sun rotated and how the spots themselves waxed and waned.

Neither the Sidereus Nuncius nor the Macchie solari were expressly Copernican tracts, yet each played a subtle persuasive role. One objection to the radical new heliocentric cosmology, expounded in Copernicus’s De Revolutionibus of 1543, concerned the question of how the earth could retain the moon in orbit around it if the earth was revolving swiftly about the sun in an annual orbit. In his Sidereus Nuncius Galileo pointed to the moons of Jupiter as an example to give the critics some pause, for in either the geocentric or the heliocentric systems Jupiter was in motion, and it didn’t lose its companions. But there were also some other subtle, throw-away lines, such as the comment that the little planets accompanying Jupiter revolved around the great planet just as Venus and Mercury revolved around the sun. The Macchie solari contained Galileo’s own first publication about the phases of Venus that he had observed in the fall of 1610, observations showing that Venus indeed did go around the sun as the Copernican system required, and as was not the case in the ancient Ptolemaic system. And the sunspots themselves revealed that the sun was not a perfect and unchanging celestial body, contrary to Aristotle’s cosmology. While this in itself did not prove anything about the mobility of the earth, it certainly helped to discredit the traditional world view.

With these discoveries Galileo became eager to educate the public about the heliocentric system, but the Roman hierarchy muzzled him. To bring an open discussion of the Copernican doctrine meant an open discussion of Scriptural interpretation of passages that seemed to speak to the stability of the earth, whereas the churchmen wanted to maintain a united Catholic front against the Protestants north of the Alps, and they did not want amateur theologians meddling with these matters. However, when a fellow Florentine and liberal supporter of the sciences was crowned as Urban VIII, Galileo gained permission to write a book comparing the various cosmological systems.

When Galileo’s Dialogo (Dialogue on the Two Great World Systems) finally appeared in 1632, Urban must have felt completely blindsided. Galileo had produced a popular account in the vernacular Italian, whereas the Pope had probably expected a dry geometrical treatise that gave a technical comparison of hypothetical cosmological systems. No doubt he had forgotten that when Galileo got his position at the Medicean court, he had insisted that his title be “Mathematician and Philosopher to the Grand Duke” – in other words, Galileo wanted to be credentialed not just to speak about theoretical mathematical models useful for computing the positions of planets, but to have authority to speak as a philosopher about how the cosmos was really made.

While the Dialogo did not bring new scientific results, it marshaled the arguments for the mobility of the earth, making heliocentrism intellectually respectable. It was, in effect, the book that won the cosmological war. It was printed in an edition of a thousand copies, and the Vatican authorities did not react quickly enough to prevent the great majority from being sold. The Pope was furious, convinced among other things that he was the model for Simplicio, the character in the dialogue who defends the Aristotelian viewpoint. Urban unleashed the forces of the Inquisition. Galileo deflected the charge of heresy, but was nevertheless sentenced to house arrest for disobeying orders by teaching the Copernican system. Perhaps the mere fact that the Dialogo was placed on the Index of Prohibited Books actually caused people to treasure and preserve their copies, for today it remains the most common and least expensive of the great works of the scientific revolution.

Owen Gingerich is Professor Emeritus of Astronomy and of the History of Science at Harvard University, and is associated with the Harvard-Smithsonian Center for Astrophysics. He wrote this essay in May 2003.

Next section: Classics of Astronomy by Johannes Kepler

Copyright © 2010 by Owen Gingerich
This page was last updated on 23 November 2010