New Heavens, New Earth, and Worldly Politics: Galileo

Kepler had a friend deliver a copy of Cosmographic Mystery to the “mathematician named Galileus Galileus,” then teaching mathematics and astronomy at Padua, near Venice. Galileo Galilei (1564–1642) thanked Kepler in a letter that nicely illustrates the Italian’s views at the time (1597):

So far I have only perused the preface of your work, but from this I gained some notion of its intent, and I indeed congratulate myself of having an associate in the study of Truth who is a friend of Truth. . . . I adopted the teaching of Copernicus many years ago, and his point of view enables me to explain many phenomena of nature which certainly remain inexplicable according to the more current hypotheses. I have written many arguments in support of him and in refutation of the opposite view—which, however, so far I have not dared to bring into the public light. . . . I would certainly dare to publish my reflections at once if more people like you existed; as they don’t, I shall refrain from doing so.

Kepler replied, urging Galileo to “come forward!” Galileo did not answer.

At Padua, Galileo could not teach what he believed; Ptolemaic astronomy and Aristotelian cosmology were the established curriculum (see Interpreting Visual Evidence on pages 552–53). By the end of his career, however, Galileo provided powerful evidence in support of the Copernican model and laid the foundation for a new physics. What was more, he wrote in the vernacular (Italian) as well as in Latin, and his writings were widely translated and read, raising awareness of changes in natural philosophy across Europe. His discoveries made him the most famous scientific figure of his time, but his work put him on a collision course with Aristotelian philosophy and the authority of the Catholic Church.

INTERPRETING VISUAL EVIDENCE

Astronomical Observations and the Mapping of the Heavens

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One (often-repeated) narrative about the scientific revolution is that it marked a crucial break, separating modern science from an earlier period permeated by an atmosphere of superstition and theological speculation. But, in fact, medieval scholars tried hard to come up with empirical evidence for beliefs that their faith told them must be true, and without these traditions of observation, scientists like Copernicus would never have been led to propose alternative cosmologies (see “Ptolemaic Astronomical Instrument” on page 548).

The assumption that the “new” sciences of the seventeenth century marked an extraordinary rupture with a more ignorant or superstitious past is thus not entirely correct. It would be closer to the truth to suggest that works such as that of Copernicus or Galileo provided a new context for assessing the relationship between observations and knowledge that came from other sources. Printed materials provided opportunities for early modern scientists to learn as much from each other as from more ancient sources.

The illustrations here are from scientific works on astronomy both before and after the appearance of Copernicus’s work. All of them were based on some form of observation and claimed to be descriptive of the existing universe. Compare the abstract illustrations of the Ptolemaic (image A) and Copernican (image B) universes with Tycho Brahe’s (image C) attempt to reconcile heliocentric observations with geocentric assumptions, or with Galileo’s illustration of sunspots (image D) observed through a telescope.

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In the Ptolematic universe, the earth is at the center, symbolized with a landscape drawing in a circle. The next circles, radiating out, are labeled Lune, Mercurii, Veneris, Solis, Martis, Iovis, Saturni; then a string of stars; then symbols of the Greek zodiac.

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The diagram depicts concentric circles that are numbered from outside to inside. The outermost sphere is labeled Stellarum Fixarum and represents stars that don’t move. This surrounds circles for the orbits of planets labeled Saturnus, Iouis, Martis, Telluris (Terra and the Moon), Venus, and Mercury. Saturn and Jupiter have their rotating periods labeled as 30 and 12, respectively. At the very center is Sol.

Questions for Analysis

1. What do these illustrations tell us about the relationship between knowledge and observation in sixteenth- and seventeenth-century science? What kinds of knowledge were necessary to produce these images?
2. Are the illustrations A and B intended to be visually accurate, in the sense that they represent what the eye sees? Can we say the same of illustration D? What makes Galileo’s illustration of sunspots different from the others?
3. Are the assumptions about observation in Galileo’s drawing of sunspots (image D) applicable to other sciences such as biology or chemistry? If yes, how so?

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A diagram of Brahe’s universe that has two concentric circles, one large and one small, overlapping three concentric circles of similar size. Point A at the center of the smaller of the two circles represents the earth, point B at the top of the smaller of the two circles represents the moon, and point C where the two sets of circles overlap represents the sun.

In 1609, Galileo heard reports from Holland of a lens grinder who had made a spyglass that could magnify very distant objects. Excited, Galileo quickly devised his own telescope. He trained it first on earthly objects to demonstrate that it worked, and then dramatically pointed it at the night sky. Galileo studied the moon, finding features of an earthlike landscape containing mountains, valleys, and plains. His observations suggested that celestial bodies resembled the earth, a view at odds with the concept of the heavens as an unchanging sphere of heavenly perfection, inherently and necessarily different from the earth. He saw moons orbiting Jupiter, evidence that earth was not at the center of all orbits. And he saw spots on the sun. Galileo published these results, first in The Starry Messenger (1610) and then in Letters on Sunspots (1613). The Starry Messenger, with its amazing reports of Jupiter’s moons, was short, aimed at a wide reading audience, and bold. It only hinted at Galileo’s Copernicanism, however. The Letters on Sunspots declared it openly.

As a professor of mathematics, Galileo chafed at the power of university authorities who were subject to Church control. Princely courts offered an inviting alternative. The Medici family of Tuscany, like others, burnished its reputation and bolstered its power by surrounding itself with intellectuals as well as artists. Persuaded he would be freer at its court than in Padua, Galileo took a position as tutor to the Medicis and flattered and successfully cultivated the family. He addressed The Starry Messenger to them and named the newly discovered moons of Jupiter the “Medicean stars.” He was rewarded with the title of chief mathematician and philosopher to Cosimo II de’ Medici, the grand duke of Tuscany. Now well positioned in Italy’s networks of power and patronage, Galileo was able to pursue his goal of demonstrating that Copernicus’s heliocentric (sun-centered) model of the planetary system was correct.

This pursuit, however, was a high-wire act, for he could not afford to antagonize the Catholic Church. In 1614, an ambitious and outspoken Dominican monk denounced Galileo’s ideas as dangerous deviations from biblical teachings. Disturbed by the murmurings against Copernicanism, Galileo defended himself in a series of letters. He addressed the relationship between natural philosophy and religion, and argued that one could be both a sincere Copernican and a sincere Catholic (see Analyzing Primary Sources on page 556). The Church, Galileo said, did the sacred work of teaching scripture and saving souls, but accounting for the workings of the physical world was a task better left to natural philosophy, grounded in observation and mathematics. For the Church to take a side in controversies over natural science might compromise its spiritual authority and credibility. Galileo envisioned natural philosophers and theologians as partners in a search for truth, but with very different roles: the purpose of the Bible, he said, was to “teach us how to go to heaven, not how heaven goes.”

In 1616, the Church moved against Galileo. The Inquisition ruled that Copernicanism was “foolish and absurd in philosophy and formally heretical.” Copernicus’s De Revolutionibus Orbium Coelestium was placed on the Index of Prohibited Books, and Galileo was warned not to teach Copernicanism.

For a while, he did as he was asked. But when his Florentine friend and admirer Maffeo Barberini was elected pope as Urban VIII in 1623, Galileo believed the door to Copernicanism was (at least half) open. He drafted one of his most famous works, A Dialogue Concerning the Two Chief World Systems, which was published in 1632. The Dialogue was a hypothetical debate between supporters of the old Ptolemaic system, represented by a character he named Simplicio (simpleton) on the one hand, and proponents of the new astronomy on the other. Galileo gave the best lines to the Copernicans throughout. However, at the very end, to satisfy the letter of the Inquisition’s decree, he had them capitulate to Simplicio.

The Inquisition banned the Dialogue and ordered Galileo to stand trial in 1633. Pope Urban, provoked by Galileo’s scorn and needing support from Church conservatives during a difficult stretch of the Thirty Years’ War, refused to protect his former friend. The verdict of the secret trial shocked Europe. The Inquisition forced Galileo to repent his Copernican position, banned him from working on or even discussing Copernican ideas, and placed him under house arrest for life. According to a story that began to circulate shortly afterward, as he left the court for house arrest he stamped his foot and muttered defiantly, looking down at the earth, “Still, it moves.”

The Inquisition could not put Galileo off his life’s work. He proposed an early version of the theory of inertia, which held that an object’s motion stays the same until an outside force changes it. He calculated that objects of different weights fall at almost the same speed and with a uniform acceleration. He argued that the motion of objects follows regular mathematical laws. The same laws that govern the motions of objects on earth (which could be observed in experiments) could also be observed in the heavens—an important step toward a coherent physics based on a sun-centered model of the universe. Compiled under the title Two New Sciences (1638), this work was smuggled out of Italy and published in Protestant Holland.

Galileo believed that Copernicanism and natural philosophy in general need not subvert theological truths, religious belief, or the authority of the Church. But his trial seemed to show the contrary: that natural philosophy and Church authority could not coexist. Galileo’s trial silenced Copernican voices in southern Europe, and the Church’s leadership retreated into conservative reaction. It was therefore in northwest Europe that the new philosophy Galileo had championed would flourish.