Galileo’s physics


Justus Sustermans, Portrait of Galilieo. Wikimedia Commons.

Galileo‘s numerous observations of the heavens – including his telescopic observations of the appearance of the moon and the phases of Venus, his discovery of the four moons Jupiter, and his observations of sunspots – convinced him that the Copernican heliocentric model of the cosmos was not just a useful calculating tool.  It was a true description of the physical reality of the universe.  Although Galileo’s discoveries – and those of Tycho Brahe – were serious blows to the authority of Aristotle, few people saw them as support for Copernicus’ model.  More commonly, they were taken as support for Tycho Brahe’s geo-heliocentric model.  Supporters of the Copernican system were hampered by the fact that there was no observational evidence that decisively proved that the earth was in motion and no satisfactory physics for a heliocentric cosmos. In the seventeenth century, there were attempts to create a new physics – that is, a new science of motion – that would replace Aristotelian physics. Two of the most important figures in this development were Galileo and the French philosopher René Descartes. Both were committed Copernicans and both proposed new ways of understanding motion in the heavens and on the earth that were compatible with a heliocentric model of the cosmos.

One of Galileo’s major challenges to Aristotle’s physics was his claim that objects could have more than one motion. Aristotle had asserted that an object, like a rock or a planet, could only have one motion. The only motion a planet could have was circular motion, as this was the natural motion of the ether. The natural motion of a rock, as an earthy body, was in a straight line down to the center of the cosmos, because the center of the cosmos was the natural place of the element earth. If you dropped a rock from the top of a tower, you would observe that it fell down in a straight line toward the ground. It was possible to impart a violent or unnatural motion to the rock, if you threw it up in the air or parallel to the ground. But at some point, the violent motion would dissipate and be replaced by the natural motion, and the rock would fall back toward the earth. Aristotle’s understanding of motion, and his assumption that objects could only have one type of motion at a time, were critical to his argument that the earth was stationary. If a person stands on the top of a tower and drops a rock, the rock falls straight down and lands next to the tower, directly under where the person released it. If the earth was spinning on its axis, Aristotle claimed, then the tower would move while the rock was falling and the rock would land some distance from the tower. Similarly, if the earth was in motion and you threw a rock in the direction of the earth’s motion, it ought to go further than if you threw it in the direction opposite the earth’s motion. None of these phenomena are ever observed and Aristotle took this as evidence for the stability of the earth.

One of Galileo’s major innovations was to assert that an object could have two motions. If a person stood on a tower holding a rock, and the earth was spinning on its axis, then the person, the rock and the tower were all moving with circular motion. If the person dropped the rock, the rock would fall straight down, but it would also continue to move in a circle. That is, the tower and the person continued to move with circular motion and the rock moved with a combination of circular and straight line motion. The rock only appeared to move in a straight line. In reality its path was a combination of circle and straight line (a path that Galileo later calculated was a parabola). Galileo could not prove that the earth actually was in motion, but he cast doubt on one of the main Aristotelian proofs that it was not. Further, Galileo began to break down the distinction between terrestrial and celestial motion that was fundamental to Aristotelian physics. He saw no inherent reason not to ascribe circular motion to the terrestrial realm.

Another of Galileo’s significant achievements was to begin to develop the concept of inertia. In Aristotelian physics, rest was a normal state; motion required an explanation. If you dropped an earthy object like a rock, it stopped moving when it reached the ground, because this was as close as it could get to its natural place at the center of the cosmos. Once an object reached its natural place, it had no tendency to move anymore. This was part of Aristotle’s argument for the stability of the earth. Since the natural place of the element earth was at the center of the cosmos, and the globe of the earth (composed largely of the element earth) was located at the center of the cosmos, it had no tendency to move. This had long been an argument against attributing a daily rotation on its axis to the earth, and it was certainly an objection to setting the earth in motion around the sun. Any advocate of the Copernican system would have to explain what made the earth move. Galileo proposed such an explanation. First, he argued that neither rest nor motion is “normal.” It was not the case that motion required an explanation and rest did not. Instead, what required explanation was a change in state. If an object at rest began to move, this required explanation. And if an object in motion stopped moving, this too required an explanation. In this way, Galileo began to move toward the modern (Newtonian) concept of inertia. Galileo suggested that the planetary orbs were perfectly smooth spherical surfaces, and that the planets themselves (including the earth and its accompanying atmosphere) were perfectly smooth balls. God had set all the planets in motion rolling around and around these orbs and, as there was no friction or resistance in the heavens, they continued moving and would continue to move until God chose to stop them. Galileo’s explanation of the motion of the earth and the other planets challenged traditional Aristotelian physics, but it required him to reject Tycho’s argument for fluid heavens and Kepler’s for elliptical orbits. Galileo’s physics, like Aristotle’s, required solid spherical planetary orbs. Galileo even rejected Tycho’s discovery that comets were celestial phenomena because his physics could not account for non-circular motion in the heavens.

In 1632, Galileo marshaled all his evidence and arguments – his telescopic discoveries and his new understanding of physics – and write a book in which he defended the physical reality of the Copernican system. This book was titled A Dialogue on the Two Chief World Systems. This was the book that got him in trouble with the Inquisition and led to his trial for heresy, his forced recantation of heliocentrism, and house arrest. For a good summary of the arguments in the Dialogue, see here.  We are now in a position to discuss WHY this book caused such a stir.

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Galileo, Dialogo (1632). Courtesy of OU History of Science Collections.

Galileo had already argued that the Copernican system was compatible with the Bible in his Letter to Benedetto Castelli of 1613 and his more famous Letter to the Grand Duchess Christina in 1615. The “Letter to the Grand Duchess” played an important role in the Catholic debate over this issue. In the end, however, despite the vigorous advocacy of Galileo and several other Italian Catholic astronomers and philosophers, the Catholic Church (more specifically, a committee appointed by the Roman Inquisition to decide whether the Copernican system was or was not heretical) rejected the argument for a non-literal reading of the Bible and, in 1616, placed Copernicus’ On the revolutions on the Index of Prohibited Books. It was placed on the Index “until corrected,” which meant that it was still legal for Catholics to own, read and even teach the book as long as a few offending passages were crossed out. These passages were ones that asserted that the earth moves. The mathematical models could still be used to make calculations useful for astrology and navigation. Galileo himself was personally cautioned not to assert the physical reality of the Copernican system in his teaching or writing.  The image below is a page from the copy of Copernicus’ On the revolutions held by the History of Science Collections at OU.  Like many sixteenth-century copies once owned by Catholics, this copy has corrections.  In this case, the phrase De triplici motu telluris demonstratio (On the demonstration of the triple motion of the earth) has been replaced with the handwritten De hypothese triplicis motus terre, et eius demonstratione (on the hypothesis of the triple motion of the earth and its demonstration).  In some copies, the offending line has been completely blacked out and replaced with the corrected line.  But in this copy, the owner underlined the offending passage, but did not obliterate it.

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Copernicus, De revolutionibus (1543). Courtesy of OU History of Science Collections.


Lucas Cranach, “Martin Luther” (1528) Wikimedia Commons.

Let us consider WHY the Church made this decision in 1616. After all, the argument that not every passage of the Bible could be read literally was universally accepted by theologians and had been clearly articulated by no less an authority than the great Church Father Augustine (354 – 430). There was even some precedent for non-literal readings of the passages that referred to a stationary earth. In the fourteenth century, the French theologian Nicole Oresme (ca. 1325 – 1382) had argued that a moving earth was compatible with both scripture and physics. Oresme did not believe that the earth actually did move, but he denied that one could use passages of scripture to prove that it did not. One reason the comparison with passages about a flat earth did not work is that the evidence in favor of a moving earth was nowhere near as strong as the evidence for a round earth. After all, anyone could observe (or potentially observe) that the shadow cast on the moon by the earth during an eclipse was round or that as a ship sailed out to sea the last glimpse of it a person on shore would have would be the top of the mast disappearing over the horizon. This was simply not true of a moving earth. No observation or experience was consistent with a moving earth and not with a stationary earth. But undoubtedly an even more significant factor in the decision of 1616 was the crisis of authority in which the Church found itself.

Almost one hundred years earlier the German monk Martin Luther (1483 – 1546) had nailed ninety-five theses to the door of the university chapel in Wittenberg, unleashing the enormous religious, political and social upheaval known as the Reformation. The Catholic Church was still dealing with the effects of the Reformation, and there was a felt need to reassert the authority of the Catholic Church to be the sole arbiter on questions of biblical interpretation, and to defend the interpretations of traditional authorities like Augustine and Thomas Aquinas, men who had presumed the earth was stationary. Some modern writers have assumed that one reason the Church opposed Copernicanism was because it removed the earth from its rightful place at the center of the universe. However, as Ernan McMullin points out, there is no evidence that this formed any part of the deliberations in 1616, or in earlier or later attacks on Copernicus.

Although Galileo was explicitly told not to defend the reality of the Copernican system, none of his work, including his telescopic findings, was censured. Galileo’s telescopic discoveries certainly challenged the traditional Aristotelian model of the universe, but they hardly constituted proof that the Copernican system was the true physical structure of the cosmos. Indeed his observations fit equally well into the Tychonic system, and in the years following the ban on Copernicanism, many leading Catholic scholars adopted the Dane’s system. Even those, like the Jesuit Christopher Clavius (1538 –  1612), who remained loyal to the Ptolemaic system, worked to demonstrate that Galileo’s findings could be accommodated into the old geocentric cosmos.


Caravaggio, “Maffeo Barberini” (ca. 1598). Wikimedia Commons.

The events leading to Galileo’s trial for heresy were set in motion in 1623 when Cardinal Maffeo Barberini (1568 – 1644) was elected pope. Barberini, who took the name Urban VIII, was an old friend of Galileo’s. He was highly educated, and he was interested in astronomy. When he became pope, Galileo clearly thought he had a chance to reopen the question of the congruence of heliocentrism with scripture. He visited Urban in Rome and was granted six audiences with the new pope, during which he secured permission to write about the Copernican system so long as he treated it as an unproven hypothesis and so long as he gave equal weight to the Aristotelian/Ptolemaic system. The resulting work, The Dialogue on the Two Chief World Systems, followed the letter but not spirit of the pope’s injunction to treat the Copernican and Ptolemaic systems equally. The whole work was framed as a dialogue between three interlocutors: Salviati, who represented the Copernican view; Simplicio, who represented the Aristotelian view; and Sagredo, who acted as a neutral third party who asked questions of the other two. Although the name Simplicio could be taken to refer to a late antique commentator on Aristotle, Simplicius (ca. 490 – ca. 560), it also had the connotation in Italian of simpleton. And in fact, throughout the Dialogue, Simplicio does appear foolish and his arguments in defense of Aristotle are made to seem ridiculous. The pope was reportedly highly offended at Galileo’s temerity. Further, the Dialogue was not published until 1632, and by this time, Urban was under attack from several factions, both in Rome and abroad. He was under considerable pressure to defend both his own personal authority as pontiff as well as the authority of the Catholic Church. In this volatile context, the Dialogue was more than just tactless, it was positively incendiary. Galileo was summoned before the Offices of the Inquisition in 1633, tried and found to be “vehemently suspected of heresy.” He was forced to abjure, that is to deny the physical reality of the Copernican system, and sentenced to prison, though this was later commuted to house arrest. The Dialogue was placed on the Index of Prohibited Books.

Assignment: Read the excerpt from Galileo’s Dialogue on the Two Chief World Systems that is on D2L.  The entire excerpt is about 26 pages.  You are only REQUIRED to read pp. 282-303 (the passages from The Fourth Day on the tides).


Finocchiaro, The Essential Galileo

Nicole Oresme, “The Compatibility of the Earth’s Diurnal Rotation With Astronomical Phenomena and Terrestrial Physics,” translated by Albert D. Menut, annotated by Edward Grant, in Edward Grant (ed.), A Source Book in Medieval Science (Cambridge, MA: Harvard University Press, 1974), pp. 503-510.

Ernan McMullin, “The Church’s Ban on Copernicanism, 1616” in Ernan McMullin (ed.), The Church and Galileo (Notre Dame, Indiana: University of Notre Dame Press, 2005), pp. 150-190.

Michael H. Shank, “Setting the Stage: Galileo in Tuscany, the Veneto, and Rome” in Ernan McMullin (ed.), The Church and Galileo (Notre Dame, Indiana: University of Notre Dame Press, ), pp. 57-87.


Cristiano Banti, “Galileo facing the Roman Inquisition” (1857). Wikimedia Commons.



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