Aristotle on Astronomy: Supporters and Challengers

From a lecture series presented by The Great Courses

Centuries after Aristotle’s death, his theories on astronomy were met with some surprising endorsements from the Christian church, most notably Thomas Aquinas. Discover how both Plato and Aristotle’s theories were used to support theological doctrine. Later on the historical timeline, though, both Copernicus and Galileo challenged Aristotle’s views on planetary bodies.

M41 is an open star cluster in Canis Major. It may have been discovered by Aristotle as early as 325 BC

Aristotle’s Views on Astronomical Bodies

According to Aristotle, extraterrestrial objects—the Moon, the Sun, fixed stars—consist of aether. Aether is a more perfect element than the earthly elements of earth, air, fire, and water, and, therefore, take on a perfect shape, the sphere.

Symmetry is not only beautiful, but also an indication of perfection. The sphere is the most symmetrical object and, thereby, the most perfect three-dimensional shape; so, all of the heavenly bodies are perfect spheres.

Their trajectories? The most perfect two-dimensional path, circles, are the trajectories of heavenly bodies. Because of the perfection of their constituent make-up, their behavior would be perfect, too. Aether is perfect, so it follows that their shape and their motion would be perfect.

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

The problem for Aristotle is that we knew from observation that we couldn’t use simple, single circles to account for the observations we collect. The planets, for example, will occasionally exhibit retrograde motions; their movements will seem to move backward as compared to the motion they previously exhibited, relative to the fixed stars.

Although the Sun does rise and set in the same way every day, the point on the horizon where it rises changes each day—moving forward and back through the year.

So Aristotle, as well as others of his time like Eudoxus—a fellow student at Plato’s academy—tried to multiply the circles, giving us epicycles, or circles on circles. We could imagine this as spheres rolling around each other.

Claudius Ptolemy, writing in Alexandra in the 2nd century, finished this project, creating the greatest work of the ancient world, his masterpiece, the Almagest. This book contains a complete account of the motions of all of the objects in the night sky viewable without a telescope, and perfectly describes the trajectory of each.

To get this degree of accuracy, Ptolemy relied on some intricate mathematical tools: epicycles—circles in circles and then circles in circles in circles; eccentricities—squished circles; and ecliptics—off-centered circles. We would combine these to create the great cosmic spirograph that never missed a prediction.

It was so complete and so accurate that it was given a name that combines the Arabic, al, which means “the” and the Greek, magest, which comes from the word for magisterial. So, the book was simply known as The Great Work.

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How Plato and Aristotle Informed Christianity

This was the state of science at the beginning of the Christian era. We find important advances during this period among Indian scholars—especially in mathematics, Chinese scholars—largely in medicine and mathematics, and Muslim scholars—in astronomy and physics.

These traditions developed in accord with their own cultural contexts, but as a result of global trade, there is still a Greek heart in the development that came after. In Western Europe, we see the Dark Ages at this time, during which little scientific interest emerged.

What’s important in the history of science, however, was the rediscovery of philosophy by the early church fathers. Augustine in the 5th century finds in Plato an intellectual foundation for Christianity, one that would systematize it and philosophically justify it.

Recall that in Plato’s thinking, he viewed a bifurcated world. On the one hand, we have the material world around us, a false corrupted representation of reality. Whereas, on the other hand, we have the world of forms, a nonmaterial world of perfection.

This picture was easily tweaked to map onto Christian theological teachings, in which reality is comprised of two realms, one, a sinful, diseased world of flesh and two, an eternal, perfect world of spirit. So, following Augustine there was a strong line of Christian neo-platonic thought.

But the rediscovery of Plato led to the rediscovery of Aristotle, and a division within the church arose over which Greek thinker ought to provide the philosophical basis for Christian thought.

Page from Summa theologica, pars secunda, primus liber, printed in Mainz by Peter Schöffer, 1471.
Page from Summa theologica, pars secunda, primus liber, printed in Mainz by Peter Schöffer, 1471

The Aristotelian line is most closely associated with the Dominican friar Thomas Aquinas, whose views Summa theologica and Summa contra gentiles shaped Aristotle to fit Christian belief. Aristotle, for example, has no creation event.

The circular orbits of matter made from aether have been happening for an infinite time and will continue to do so.

This, or course, would not do for Aquinas. So Aristotle’s prime mover, the ultimate cause of the circular motion, became a first cause, the Creator God of the Old Testament.

Soon after his death, Aquinas was condemned as a heretic for these views; his works were banned from being taught in Catholic contexts. But they persisted among the Dominicans. When the Dominican pope, John XXII, took control, Aquinas was made a saint and his works were made the official Church doctrine.

The view held of reality at this time period is one in which there is an Earth created by the Christian God; who operates as Aristotle described by God’s will. Aristotle’s view of the world was not only taught, it became enforced. The questioning of this Aristotelian worldview was the hallmark of the scientific revolution.

Copernicus Challenges Aristotelian Thought

Nicolaus Copernicus is the first major figure to present a substantive challenge. Ptolemy’s augmented Aristotelianism with its epicycles, eccentricities, and ecliptics was flawless in terms of predictions, but it was a bear to work with.

Monument of great astronomer Nicolaus Copernicus, Torun, Poland
Nicolaus Copernicus presented the first substantive challenge to Aristotelian thought.

Copernicus figured out an easier route: Move the Earth from the center of the universe and have it and the other planets orbit the Sun.

Not having the Earth as the center of creation was a heretical denial of Aristotle and an insult to God’s creation. So when Copernicus’s book, On the Revolutions of the Celestial Spheres, was published just before his death, it had an introduction added by a member of the clergy who warned that this should not be taken as a true description of the universe, but simply as a mathematical tool for easier calculation of observable astronomical phenomena.

The challenge was clear—it was simpler, it was more elegant. It didn’t do away with epicycles, but it needed many fewer and allowed the eccentricities and ecliptics to be jettisoned. This elegance certainly smelled of truth and not just utility.

This is not to say it was an open and shut case. There did seem to be good reason to reject the Copernican vision. Aristotle said objects move straight down when dropped. So, if you face in the direction that the Earth is moving, then everything that falls ought to drop behind you because the Earth would have moved forward while it was falling straight down. But we don’t see that.

Second, if a person is traveling by horse, wagon, or modern-day car, a rush of air can be felt, created by fast movement. If the Earth is moving, then there ought to be a constant wind in one direction. Yet there’s not. This sort of objection coupled with the theological view being pushed by the church then solidified the world view of reality as a stationary Earth at the center of the universe.

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Galileo Introduces New Views on Astronomy

But the Copernican view became more and more popular among the educated, with Galileo Galilei becoming a significant proponent. Galileo was bold and unrepentant in his scientific work. He wasn’t the inventor of the telescope, but he was the first to use it for serious scientific work.

In his book, The Starry Messenger, he documented all the observations he made that contradicted Aristotle. His detailed drawings of the surface of the Moon are astounding for their beauty and their detail.

He shows mountains. He shows craters. And since Aristotle said things made of aether had to be perfectly spherical, those defy the Aristotelian view that the Moon is made of aether, and, therefore, has to be perfectly spherical.

He discovered the moons of Jupiter, which showed that contrary to Aristotelianism, not everything orbits the Earth. And discovering the phases of Venus, he showed that shadows cast prove that it couldn’t be orbiting the Earth, but would be expected and easily explained if both Venus and the Earth were orbiting the Sun.

Galileo’s work on Copernicanism caused him to be brought before the Inquisition—twice. Convicted, he was sentenced to house arrest where he turned his attention from astronomy to physics, particularly the physics of motion, what we call mechanics.

His study of mechanics had been his focus earlier in life when he did work for the military on projectile trajectories, which was needed for aiming cannons.

But his later work was not just practical: It was mathematical and theoretical. He showed that objects fall at the same rate regardless of mass, and created equations to describe this motion.

From the lecture series Redefining Reality: The Intellectual Implications of Modern Science, taught by Professor Steven Gimbel

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