Omission and Invention: The Problematic Nature of Galileo's Proposed Proofs for Earth's Motion

Consider this little poem, from the children's book, C is for Ciao: An Italy Alphabet:

G is for Galileo,punished when he provedthat the sun was sitting stilland the earth's the one that moved.¹

The book continues,

Until the Polish astronomer Nicolas Copernicus discovered that the sun is the center of our solar system. … people since the second century had thought the sun revolved around the earth. . … In developing the telescope, Galileo was able to prove that Copernicus's theory was correct. This caused a problem with church leaders of the day, who—disrespectful of scientific facts—were offended by the idea that the earth was not the center of the solar system.

C is for Ciao provides a nice illustration of how a certain notion regarding Galileo can be found in surprising places. That notion is that Galileo, using his telescope, proved that the Earth circles the sun annually and rotates about its own axis diurnally. Other examples of this notion appearing in surprising places include: 1) a popular guide book for would-be tourists—"Galileo Galilei proved that the earth revolved around the sun, overturning Church doctrine. He was convicted of heresy in 1633";² 2) the front page of a major newspaper: "More than 350 years after the Roman Catholic Church condemned Galileo, Pope John Paul II is poised to rectify one of the Church's most infamous wrongs—the persecution of the Italian astronomer and physicist for proving the Earth moves around the Sun";³ 3) an English grammar book: "Sometimes, if the subordinate clause states a fact which is always true at all times, we may use the Present even though the principal verb is Past. Ex. Galileo proved that the earth moves round the sun; or, Galileo proved that the earth moved round the sun";⁴ and 4) the web page of a major business magazine: "With this discovery [of the moons of Jupiter] and his observations of the phases of Venus later that same year, Galileo gave us proof of a heliocentric universe. Earth was not fixed at the center of the cosmos, but rather moved around the Sun just as other planets did."⁵

A statement that Galileo proved the Earth's motion can even be found on the back cover of the standard English translation of Galileo's 1632 Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican: "Using the dialogue form, a genre common in classical philosophical works, Galileo masterfully demonstrates the truth of the Copernican system over the Ptolemaic one, proving, for the first time, that the earth revolves around the sun."⁶ Thus discussions that touch on the subject of Galileo and his telescopic discoveries—discussions in classes or public lectures, discussions of the news, discussions at universities or with children or for tourists—discussions that also usually touch on the Catholic Church in some manner—are likely to include, or even hinge upon, the idea that Galileo scientifically proved that Earth moved.

However, Galileo did not prove Earth's motion, despite this widespread (and very long-lived⁷) insistence that he did. On the contrary, Galileo's efforts to prove Earth's motion, or to suggest means by which Earth's motion might be proven, were scientifically problematic. In those parts of the Dialogue where he discusses the subject of proving Earth's motion, he tends to omit or invent key pieces of information. These omissions and inventions of Galileo are the subject of this article.

Phases and Moons

Observations of the sun, moon, and planets in Galileo's time could not prove Earth's motion. Prior to the invention of the telescope, the Danish astronomer Tycho Brahe had proposed a hybrid geocentric theory (a geocentric theory with heliocentric elements, often called geo-heliocentric) in which the planets circled the sun while the sun, moon, and stars circled the Earth. And long before Brahe, the ancient writer Martianus Capella had proposed a similar idea.⁸ Brahe's theory was mathematically and observationally identical to Copernicus...