How Galileo used the telescope to refute Aristotle and Ptolemy

And got himself in trouble with Pope Urban VIII at the same time

In the 17th century, Galileo Galilei (1564 - 1642) used the recent invention of the telescope to refute key Aristotelian and Ptolemaic beliefs about the solar system.

First, there are a few important facts about Aristotle’s and Ptolemy’s views we need to establish.

Aristotle (384 - 322 BC) maintained that there was a very important distinction between the so-called superlunary and sublunary realms. ‘Superlunary’ literally means ‘above the moon’, whereas ‘sublunary’ means ‘below the moon’. The idea is that the cosmos is divided into these two realms. We live in the sublunary realm, where there are four elements: earth, air, fire, and water. Each of these elements has its own natural direction: earth and water naturally go down; fire and air naturally go up. In the superlunary realm, the four elements don’t exist. Instead, there is only ether, and ether moves in perfect circles. Hence, the motions of the heavenly bodies that Aristotle thought we see: perfect circular motions, since they are made of ether.

Later in antiquity, there was Ptolemy (100 - 170 AD). Ptolemy is justly famous for having created the only mathematically consistent geocentric model of the solar system: every other attempt to put the Earth at the center of the solar system ran into serious mathematical problems.

Throughout the Medieval Period, the Church had come to adopt the Ptolemaic model of the solar system, which had in turn borrowed large parts of Aristotle’s views. As I said earlier, Galileo used the telescope in the 17th century to refute Ptolemaic and Aristotelian views — and got himself into serious trouble with the Church, in the process. The telescope had been invented in the Netherlands in the early 1600s, and within a year or two of its invention, Galileo was using it.

He put it to a slightly different use than the Dutch, who had used it to study things far away from them on land and at sea. Galileo wanted to point it up towards the stars, and the results were published in The Starry Messenger (1610) and Letters on Sunspots (1613).

Aristotle (and, following him, Ptolemy) had been convinced that the superlunary realm was entirely different from the sublunary realm: a world made of ether was incorruptible and perfect. But that isn’t what Galileo observed in the telescope. For instance, telescopic observations confirm that the Moon is a rock, filled with craters and mountains. This isn’t what an ethereal body should look like at all. The Moon’s topography is far too terrestrial to have been incorruptible.

In his Letters on Sunspots, Galileo is adamant that the existence of sunspots, confirmed by looking through a telescope, refutes the idea that the Sun is perfect and flawless. Sunspots are blemishes that just would not have been possible if Aristotle were right.

Galileo also believed that his observations showed that geocentrism was false. The Earth is not the center of the solar system.

Galileo was not the first person to claim this. In antiquity, Aristarchus of Samos (ca. 310 - 210 BC) had defended heliocentrism: the view that the Sun is the center of the solar system. But we don’t know exactly how Aristarchus supported this belief, since his views are merely reported by Archimedes (287 - 212 BC) who told us only what Aristarchus had concluded.

Much later, there was Nicolaus Copernicus (1473 - 1543), who had defended heliocentrism.

The problem is that Copernicus’ views were riddled with problems that he couldn’t resolve without the telescope. There were objections he couldn’t answer.

Galileo believed that he could answer them and thought that his observations vindicated Copernicus. He made it explicit that he was taking Copernicus’ position. It is not a coincidence that The Starry Messenger was the first work in which he publicly and explicitly sided with Copernicus and that its results were reached with the telescope.

Let’s consider an example.

If you believe, like Copernicus did, that the Earth revolved around the Sun, then what might you say about the Moon? Of course, we know that the answer is that the Moon revolves around the Earth, not the Sun, even though the Earth itself revolves the Sun. But centuries ago, it wasn’t obvious that it was possible for celestial bodies to revolve around different things: can the Earth really revolve around the Sun, while the Moon revolves around the Earth?

You didn’t have to answer that question if you followed the Ptolemaic model, but you did if you were Copernicus and were trying to overthrow that model.

The telescope helped to answer this question because it was used to decisively establish that not everything had to revolve around the center of the solar system (i.e., the Sun). That’s because Galileo used the telescope to discover that four moons, or satellites, revolved around Jupiter. This showed that it was possible for one thing to orbit a planet, while the planet, in turn, revolved around the Sun. Thus, the question that Copernicus could not answer was answered, and some degree of plausibility was added to the heliocentric view of the solar system.

Galileo’s work on this question got him into trouble with the Church, as I said earlier. Let’s talk about how.

There are passages in the Bible that had led the Church to conclude that the Earth stays still while the Sun and Moon revolve around it. For instance, in the Book of Joshua, it is written that “the Sun stood still and the Moon stayed in place until the nation of Israel had defeated its enemies” (10:13). In other words, God made the Sun and Moon be stationary until Israel had won its fight. This passage was interpreted to mean that the Sun and Moon are typically in motion around the Earth.

The Church had come to adopt the position that geocentrism was true and that reality was described by Ptolemy’s model of the solar system. The Church had not had a problem with Copernicus, not least of all because his views were immediately beset by challenges and objections that he could not answer, but also because the Church was not going through the difficulties brought upon it by the Protestant Reformation at that time.

By the time of Galileo’s life, the situation for the Catholic Church had changed completely.

In 1603, one of the earliest scientific communities, the Academy of Lynxes, had been started. The founders of this community had thought of the lynx as an animal able to see in the dark and therefore perceive things not visible to other people. It was an apt animal to choose as a mascot because the Academy of Lynxes was designed as a community aimed at discovering the truth even if it challenged religious propriety. Galileo was a member.

The problem is that the intellectual environment was not friendly to Academy of Lynxes’ approach to research.

Galileo knew that he might have been barking up a very dangerous tree. So, in 1615, in the Letter to the Grand Duchess Christina, Galileo lays out two principles that he used to make his views more palatable to Christianity.

The first is the so-called principle of accommodation. This principle was invented by Augustine (354 - 430 BC). It says that the Bible was written in a way that accommodates the beliefs and limited knowledge of its initial audience. People in the days of Joshua did not know that the Earth was in motion around the Sun, and so the Book of Joshua was written in a way that accommodated their false beliefs. It was intelligible to its original audience, and that’s what matters. A book written in a time when people knew about heliocentrism would reveal the same underlying truth but in a language that was consistent with heliocentrism, instead of a language that gives the impression of geocentrism.

(You can also think of passages in the Bible that talk about God as having fingers, hands, or human emotions such as jealousy. These are not true, but they are written to accommodate people who find it easier to think about people, actions, and motivations this way.)

The second principle that Galileo used is the view that while the Bible is right about spiritual matters, it is not the authority on other things, such as astronomy or cosmology. Our interpretations of the Bible shouldn’t give us ideas of what the natural world is like.

While Galileo’s attempt to defend himself might have worked earlier in European history, it came at the wrong time. As he was writing to the Grand Duchess Christina, Europe was teetering on the edge of the Thirty Years’ War (1618 - 1648), and the Church’s Counter-Reformation movement that had begun with the Council of Trent precluded Galileo’s attempt to defend himself.

That’s because, in the Council of Trent (1545 - 1563), the Church had more or less warned people not to interpret the Bible on their own, especially not in a way inconsistent with the teachings of the Church.

For instance, the Council of Trent held that “no one, relying on his own judgment and distorting the Sacred Scriptures according to his own conceptions, shall dare to interpret them contrary to the sense which Holy Mother Church, to whom it belongs to judge their true sense and meaning, has held and does hold, or even contrary to the unanimous agreement of the Fathers.”

This pronouncement doomed Galileo’s attempt to defend his views: sure, he could appeal to some principles to justify his interpretations, but the fact remains that he disagreed with the Church, and that’s the big problem.

This was confirmed in 1616 when a Church committee considered Copernicus’ heliocentrism in light of the Inquisition that the Catholic Church had formed to root out heresies. The Inqusition had found that the idea that the Sun was the center of the solar system was absurd, false, and contrary to the Church’s teachings of the Bible. That meant it was a formal heresy.

As a result, Galileo was personally summoned by Cardinal Robert Bellarmine and told not to affirm or defend Copernicus’ views. Copernicus’ On the Revolutions of the Heavenly Spheres was published in 1543 and had not received much attention. Now that its views had received some plausibility due to telescopic observations, it was suspended from publication until it could be “corrected.”

In 1623, Pope Urban VIII was elected. In 1624, Galileo met Urban VIII many times to discuss the state of his research. Urban VIII instructed him that he could talk about Copernican theories but only as one possibility among many. He could discuss it but not affirm it.

In 1632, Galileo published his Dialogue Concerning the Two Chief World Systems. Unfortunately for him, this text was interpreted by the Church as having violated the rule that he could not affirm Copernicus’ theories. The text was a dialogue between three characters: an Aristotelian, a Copernican, and a common person. The Aristotelian was named Simplicio. Simplicius (ca. 480 - 540) had been an Aristotelian philosopher, so the name made sense. However, it also suggested simple-mindedness. Readers got the impression that Copernicus’ theory was being elevated and affirmed. Worse for Galileo, Simplicio’s character voiced an argument that the Pope himself had given Galileo: namely, the argument that since God could move the heavenly bodies in any number of ways, it wasn’t up to humans to decide how the planets were being moved.

It sure looked like Galileo was calling geocentrists and even the Pope himself simple-minded.

This was a bad time for Galileo to be staunch in his views. The Pope was switching the Vatican’s support from the French to the Spanish during the ongoing Thirty Years’ War. He couldn’t have people disagreeing with him and outright disobeying his orders as Galileo had apparently done by affirming Copernicus’ theory.

So, in 1633, Galileo was sentenced by the Inquisition. His sentence was initially imprisonment, but eventually it was lessened to house arrest, and he had to recite psalms frequently as part of his penance.

What began as a heroic and revolutionary refutation of Aristotle and Ptolemy ended with his house arrest. According to tradition, he was alleged to have said “e pur si muove” — “and yet it moves.” This perhaps true, or perhaps apocryphal, story captures the idea that it didn’t matter what the Inquisition had tried to suppress; the Earth still moves.