Episode 18: Aristotle, Plato's Other Student

June 14, 2022

Two of Plato's students were notable astronomers. We looked at one of them, Eudoxus, in the last two episodes. In this episode we turn to Plato's other student, Aristotle. Aristotle embellished Eudoxus's model of planetary motion, but also developed a comprehensive physics and cosmology that ultimately became the standard model of the universe during the High Middle Ages.


Good evening, and welcome to the Song of Urania, a podcast about the history of astronomy from antiquity to the present with new episodes every full moon. My name is Joe Antognini.

Two episodes ago we learned about one of Plato’s students, a man by the name of Eudoxus who developed the first sophisticated model of planetary motion, an idea that today goes by the name “the theory of homocentric spheres” because all the spheres in this model were centered on a single place, the center of the Earth. And towards the end of the last episode we learned about how Callippus, one of the students of Eudoxus, had modified this theory of homocentric spheres. Callippus was probably too young to have learned from Eudoxus directly, but was taught from Eudoxus’s other students and then developed the model further by adding in more spheres to fix some of the deficiencies that Eudoxus’s model had, particularly when describing the orbits of Mars and Venus. In this episode we’ll turn to another student of Plato’s who also added his own embellishments onto Eudoxus’s theory, but had a lot of ideas on a lot of things to boot. As you will have seen from reading the show’s title, this man was, of course, Aristotle. It’s quite hard to overstate the influence of Aristotle on the western intellectual tradition, and in some ways the history of western philosophy can be viewed through the lens of an eternal competition between Plato and Aristotle’s philosophical visions.

But before we get to these lofty matters, I think we should spend some time with the man himself and the broader culture he was working in. Aristotle was born in 384 BC, so was about 40 years younger than his teacher Plato, and born about twenty years after all the turmoil that resulted when Sparta beat Athens in the Second Peloponnesian War and had installed the Thirty Tyrants who instigated a reign of terror. He was born in a small town called Stagira which was located in a relatively unimportant region in the north of Greece called Macedonia. Or at least, at the time of Aristotle’s birth, the Kingdom of Macedonia was of no real importance. It had existed for several centuries as one out of the multitudes of Greek statelets and around the time of Aristotle’s birth was more or less struggling for its survival and was being harassed on all sides by its neighbors. But Aristotle’s life coincided almost perfectly with the unbelievable rise of this podunk kingdom in northern Greece to the largest empire the world had seen to that point. Aristotle died just one year after Alexander the Great, who by his death at age 32 had expanded his empire all the way east to India. You may recall from Episode 8 that the history of ancient Greece is traditionally divided into five eras: the Dark Ages, the Archaic period, the Classical period, the Hellenistic period, and the Roman period. The death of Alexander the Great in 323 BC is the canonical demarcation between the Classical period and the Hellenistic period, so Aristotle’s life came at the tail end of the Classical period.

Now, Aristotle wasn’t just any old Macedonian, he had close connections to the Macedonian royalty. His father, a man named Nichomacus, was the personal physician to the Macedonian king Amyntas III. We have some evidence that as a child he probably helped his father out in his work which may help to explain his later particular fascination with biology. But he did not have the opportunity to learn too much from his father because both his parents died at the time he was 13. He stayed in Macedon for the remainder of his childhood, but when he came of age at 17, he travelled south to Athens where he joined Plato’s Academy. And there he stayed for the next 19 years of his life. We don’t really know too much about this rather substantial chunk of his life. From his later writings it seems that he entered the Academy as an enthusiastic disciple of Plato, but over the years began to have certain doubts about some of the fundamental principles of Plato’s philosophy. But this all is a bit of conjecture because none of his surviving works are thought to have been written during this two decade long span. At least as an author, Aristotle apparently got a rather late start. We do know for certain that Aristotle admired his teacher tremendously. He wrote that Plato “was a man whom the bad have not even the right to praise — the only man, or the first, to show clearly by his own life and by the reasonings of his discourses, that to be happy is to be good.”

Well, after Plato died in 348 BC, Aristotle left Athens for the city of Assos in northwestern Anatolia. There are a few guesses about his motives in leaving Athens. One possible reason for his departure is that he was not happy with the new head of the Academy, a philosopher named Speusippus. Speusippus seems to have deviated from Plato’s teachings, especially on Plato’s theory of forms, but this doesn’t really seem to be a good enough reason for Aristotle to have become so upset as to leave since by this point Aristotle himself was almost certainly also having his own doubts about Plato’s theory of forms. So it is conjectured that perhaps he was upset with more prosaic aspects of Speussipus’s management of the day-to-day life in the Academy. But another possible reason that Aristotle may have thought it was prudent for him to put some distance with Athens was a rise in anti-Macedonian sentiment in the city around that time.

Now, to understand why resentment had been building in Athens against what had up to now been a marginal kingdom in a marginal region of Greece, I’m going to momentarily pause the biography of Aristotle and take a detour towards what had been going on in Macedonia at the time that took it from being irrelevant to a military juggernaut in just a few decades. So, to rewind the clock back to the time of Aristotle’s birth in 384 BC, at this point Macedonia was not in a good way. I mentioned earlier that it had existed as a small kingdom for centuries, but wasn’t really of too much importance in the larger geopolitics of ancient Greece. It was in the northern part of Greece, sort of nestled in where the country starts to hooks east over the Aegean Sea. Surrounding Macedonia to the north and the east was the region of Thrace, and to the south was the region of Thessaly, maybe today best known for St. Paul’s two rather apocalyptic letters to the Thessalonians. Further south was a region of mountainous terrain that was difficult to navigate with the exception of the crucial pass at Thermopylae. And even further south of that, separated geographically, was the real center of Greek culture and politics: the cities of Athens, Thebes, and Sparta.

Now more than a hundred years earlier had been the first invasion of Greece by the Persians under King Darius. In the eyes of the rest of Greece, or at least in the eyes of Athens and Sparta which had formed the backbone of resistance to the Persians, Macedonia had not exactly covered itself in glory in this conflict. They capitulated to the Persians almost instantly and for the next century or so acted as enthusiastic lackeys for the Persian government. Their royalty intermarried with high ranking Persians, and they took advantage of their good relations to borrow the Persian military from time to time to try to expand their borders into neighboring Greek kingdoms. And during the second Persian invasion of Greece, they actively fought on the side of the Persians against Athens and Sparta. I mentioned in an earlier episode that in popular consciousness we often think of the Greeks as heroically resisting the invading Persians, but in reality most Greek city-states were neutral in the conflict and a large number of them actively fought on the side of the Persians, and Macedonia was one of these. It’s no surprise then, that in Athens, opinions of Macedonians were not exactly high. After the Persians were expelled from Greece in 480 BC, a century before Aristotle’s birth, Macedonia, rather sheepishly, went to Athens and asked for friendly relations. Athens did not feel especially magnanimous in the moment and more or less told them to pound sand. So, over the course of the 5th century, as Athens’s power in Greece grew, Macedonia found itself in a progressively weaker and weaker position. Various kings in neighboring kingdoms tried to grab territory, Athens encouraged settlers to colonize regions in Macedonia, and Athens provided military support to anyone at all who wanted to revolt and challenge the Macedonian king’s right to the throne. So, over the course of the 5th century, Macedonia lurched from crisis to crisis. To try to find some friends with some influence, they tried allying themselves with Athens’s archrival Sparta. But during the Battle of Lynketis the Macedonian army ran away before the battle even began after they got spooked when they saw the size of the opposing army. After seeing the Macedonians scatter without even giving an appearance of putting up a fight, the honor-obsessed Spartans lost all respect for the Macedonians and decided to steal their supplies after the Macedonians retreated. So in the end the Spartans saw the Macedonians as cowards and the Macedonians were angry at the Spartans for stealing all their stuff.

The next century did not start off much better. In 399 BC the King of Macedonia of the time, Archelaus I, was assassinated and the kingdom fell into civil war for the next six years. Now around this time Macedonia’s old enemy Athens was now losing influence after its defeat in the Peloponnesian War. So, over the next thirty years or so, Thebes started to rise as the main player in Greek affairs. Thebes warred against, and eventually defeated Sparta, and during this time started to project its power north as well, into Macedonia. At one point, around the time of Aristotle’s birth, the king of Macedonia, Amyntas III was forced to flee to escape an invasion. Ultimately Macedonia became essentially a vassal state and was forced to provide royal hostages to Thebes as collateral to ensure that they didn’t get any bright ideas. One of these hostages was a brother to the king named Philip II.

Now, I’m really racing over centuries of history because, after all, this is a podcast about the history of astronomy, so you’ll have to forgive me for skimming over all the details in the remarkable turn of events that then occurred after the death of King Alexander II. Another brother of Alexander II succeeded him to the throne, named Perdicas III, but Perdicas died in battle. The throne then fell to Perdicas’s son, Amyntas IV, but he was an infant at the time, so Perdicas’s brother, Philip II, usurped the throne in 359 BC, around the time that Aristotle had been in Athens studying at Plato’s Academy for about five years. And this is where the fortunes of Macedonia really start to turn around. At this point the fact that Aristotle was a Macedonian was probably seen as a somewhat shameful quality in a man, but ultimately harmless. What was the worst that a Macedonian going to do to you — run away from you in battle?

Now, when he ascended the throne, Philip had been a hostage in Thebes for many years and an upside of his captivity was that he had had the opportunity to see how things were done in a city-state that wasn’t a complete disaster. He may have been a hostage of Thebes, but he was noble so it’s not as though he was thrown into a dungeon. The Thebans just wanted him around so that if his relatives back home did something to upset Thebes, Thebes could exert a little, ah leverage. So when Philip acquired the throne and was permitted to return home, he had gotten a few ideas for reforms to make from his time in Thebes.

He first started with his military. He instituted rewards and promotion for good service, generally good policy in any organization, and he introduced a new set of tactics. You may recall from many episodes back I talked a little bit about the hoplite as being the foundation of the Greek military. Hoplites were freemen who could afford the necessary equipment to join the military. The two most important pieces of kit were the spear and the shield. A hoplite held his shield with his left hand, and his arm went through a strap in the center of the shield and gripped a strap at its rightmost edge. Because the shield tended to cover only his left half, he relied on his neighbor to the right to protect his right side. Then with his right hand he held a spear about nine feet long. Philip’s innovation, inspired by what he saw in Thebes, was to give his soldiers spears that were about twice as long as the standard spear, so around 18 feet long. But because of their length, they were much heavier than the usual spear and had to be held with both hands. This meant that they couldn’t hold a shield, so instead they just hung their shields from their neck, making it more like body armor. But because their spears were twice as long as those of their opponents, their opponents had a hard time getting close enough so that they could take advantage of the smaller shields. To make matters worse, or better if you were a Macedonian, because the spears were longer, spears from ranks further back in the phalanx would extend past the front, which essentially doubled the effective number of speartips exposed to the enemy. This innovation turned out to be extraordinarily effective for Macedonia and was a big reason that they were the dominant military tactic for about 150 years until it was displaced by the development of the Roman legion.

Well, the military couldn’t turn itself around overnight, so Philip II made a number of deft diplomatic moves to give the military the time it needed to train for this new way of fighting. Philip essentially bought off his neighbors to bribe them into not invading him, at least for a little while. But within a year he took his shiny new army for a test drive against a contingent of Athenian mercenaries in the Battle of Methone. The Macedonians won handily which surely brought up morale since it was the first time in a long time that anything good had happened to them on the battlefield. Soon enough he started attacking his neighbors one by one in quick succession so that they didn’t have time join up and combine their much larger forces against him. Now, I mentioned earlier that Athens had been sending settlers up into Macedonia and several cities in the region were allied with Athens. In his effort to gain control over the region Philip II was able to take these cities but was always careful to leave them in good condition, and in particular to treat any Athenians very well in order to avoid needlessly antagonizing Athens, which was still a relatively formidable foe. Over the course of a remarkable five years, Philip II transformed the Kingdom of Macedonia from a state of near collapse to one which was fairly secure in its borders. Most kings I think would have been content with a turnaround like that, but this was really just the beginning for Philip. He then started to push his influence south and took advantage of a rather abstruse religious conflict that had been brewing for a few years. The essence of the conflict was that a group called the Amphictyonic League had been governing the Temple of Delphi, which was the most sacred religious site for the ancient Greeks. We’ve periodically heard prophecies uttered by the Oracle of Delphi, who was the high priestess of the temple there. Well, acting essentially as a puppet of Thebes, the Amphictyonic League had imposed an enormous fine on the Phocian confederation to punish them for failing to help them in a past war. But this fine was so big that the Phocians could never in a million years pay it and felt that they were left with no option but to go to war and seize the Temple of Delphi. This then set off a complicated regional war called the Sacred War, or, if you are counting all of the other ones which were less important, the Third Sacred War. I won’t get into the details of this messy conflict, but the various parties involved began asking their allies for support and Philip II saw that this was a great opportunity for him to swoop in and exert his influence in the region. So he joined the fray on the side of Thebes and Thessaly, to fight a proxy war against Athens who were supporting the side of the Phocians. Philip’s actions were decisive in ending the war, and as such the Thessalonians appointed him archon, essentially making him ruler not just of Macedonia, but of Thessaly as well, which, in effect, expanded the size of his army. This then allowed him to easily make his way all the way south to Thermopylae, which was the main pass to go between northern and central Greece, which then gave him the means to credibly threaten to invade Athens itself.

So, when the Sacred War ended 346 BC, Philip II had now essentially become the undisputed ruler of much of northern Greece, and held control of the main pass into central and southern Greece, which meant that he could easily continue to march straight on down to Athens. It’s at this point that Athens starts to get really alarmed. They had been seeing the power of their city-state decline for about half a century now, and an upstart king from the boonies was now nearly at their doorstep. Well, right as all this was happening, in 348 BC, Plato died, so it’s not hard to see why anti-Macedonian sentiment was starting to simmer in Athens and Aristotle may have started to wonder what exactly was keeping him in this increasingly hostile city now.

As it happens, Athens couldn’t keep to the peace that it had agreed to, and six years later they went to war with Macedonia again, but this time were decisively defeated. Now, it’s worth pointing out that through all of this Philip II always gave Athens good terms in his peace settlements. He seems to have felt that it was more important to not antagonize them too much so that he could be on their side in the future, and, in particular, take advantage of their world-class navy. At any rate, after this last defeat, Philip II did what no Greek before him was capable of doing — he united Greece in what is called the League of Corinth. Being a proud lot, all the city-states nominally retained their independence, but Philip required that they stop attacking each other, ally with each other, and guaranteed that they would meet with his army if any of them stepped out of line.

Well, during the uneasy peace before this last war, Aristotle left Athens. Now, as I mentioned at the outset, Aristotle was raised in the Macedonian court and was well known to Philip II. Philip asked Aristotle to travel to a city called Assos in northwestern Anatolia in the hopes that Aristotle might build good relations between Macedonia and Assos since it was in a strategically valuable location. So Aristotle travelled to Assos with his friend, a philosopher named Xenocrates. It’s maybe a bit ironic that Xenocrates of all people was the one who accompanied Aristotle on this quasi-diplomatic mission for Macedonia because although Xenocrates wasn’t Athenian, he was from the East, a Chalcedonian, he was no great fan of Macedonia and was known for opposing Macedonian influence in Athens. There is a tale that later in his life Alexander the Great sent him 30 talents of gold, maybe the equivalent of 10 or 20 million dollars in today’s money, and Xenocrates sent it back saying that kings are the ones who need money, not philosophers. Well, in Assos he was hosted by the tyrant, a man by the name of Hermias. Hermias had a rather unique background for a ruler. He spent his youth as a slave. His slaveowner, a tyrant named Eubulus evidently recognized his intelligence and had sent him to Athens to be educated by Plato, which is how Hermias had met Aristotle. Eubulus died not too long after Hermias returned to Assos and by no small feat, Hermias acquired not only his freedom, but political power as well and stepped into the power vacuum left by Eubulus. So, by the time Aristotle and Xenocrates arrived at Assos, Hermias had turned his life around quite nicely. Aristotle spent the next three years enjoying Hermias’s hospitality and apparently occupied himself with scientific investigations into marine biology at the coast.

These three years were not so pleasant for Hermias, unfortunately. He ended up getting caught between the great powers in the region, Athens, Macedonia, and Persia. Persia had long been a threat to Hermias since his lands had previously been Persian territory prior to their retreat eastwards after the Second Persian invasion. Naturally the Persians had been angling to recapture this land. Hermias didn’t have anywhere near the military strength of Persia, so he was quite keen on building up his relationship with Macedonia so that he could count on them for protection. But Athens was none too happy with this relationship since they were afraid that if Philip II were ever given the excuse to provide military aid to Hermias, this would give him a foothold in Asia Minor from which he could grow his area of de facto control. So Athens, in an ironic turn of events, threatened to ally with Persia and attack Macedonia, a fellow Greek kingdom, if Philip provided any military support to Hermias. Now, throughout this the later authors tell us that Aristotle was continuously writing letters to King Philip requesting that Philip help his friend Hermias defend himself from the Persians. But Philip decided that it was just not worth it and left Hermias high and dry. Persia ultimately hired a mercenary by the name of Mentor who managed to kidnap Hermias and torture him for information about Macedonia’s plans to invade Asia Minor. Hermias apparently told his captor nothing and is reputed to have said before dying, “tell my friends that I have done nothing shameful or unworthy of philosophy.”

After the death of Hermias, Aristotle found it prudent to move once again and this time went to the island of Lesbos, bringing along with him Hermias’s daughter Pythias, whom he married within a few years. Pythias must have been quite young, probably in her mid teens, because at the time of her father’s death she was not yet of age and Aristotle married her only a few years later. What the marriageable age was in Assos is not known, at least to me, and the marriageable age did vary substantially in ancient Greece. It was especially young in Athens at 12 or 13, but the Spartans waited until 18. In most regions was between 14 and 16. At any rate, Aristotle was likely around 25 years older than she was.

Well, during his time in Lesbos, Aristotle was accompanied by another friend, one we have heard of on a few occasions, a philosopher named Theophrastus. Theophrastus was the one who wrote a comprehensive treatise on the history of physics which was tragically lost, but which formed the basis for several later works which did survive, one of which, St. Hippolytus’s Refutation of All Heresies, followed it so closely that we can reconstruct the chapters in Theophrastus’s original work. Aristotle spent two years in Lesbos continuing his biological investigations and then received a request from King Philip that he return to Macedon and became a tutor to his son Alexander. Aristotle was certainly not the boy’s only tutor, we learned last month of Menaechmus who taught the boy geometry, and almost certainly there were many other tutors besides. Nevertheless, given the enormous stature of Aristotle in the intellectual realm, and the enormous stature of Alexander the Great in the realm of politics and warfare, generations of historians haven’t been able to help but wonder what exactly Aristotle’s influence was on Alexander — to what degree was Alexander’s incredible talent at empire building a consequence of the wisdom imparted to him by one of the greatest philosophers humanity produced? From what we can tell, Aristotle was keen on the idea of subjugating Persia, so there was almost certainly some influence there, but I will leave it to others to speculate about the details.

Well, Aristotle spent eight years in his homeland as a tutor to Alexander. But in 336 BC Philip II was assassinated during the feast at his daughter’s wedding, and Alexander ascended to kingship and so had no more need of book learning. So the next year in 335 BC Aristotle finally returned to Athens. There he founded his own school. Since he was a metic he couldn’t buy land, so he rented some buildings right next to a public exercise area dedicated to Apollo Lykeios, meaning Apollo of the Wolves, and so the school came to be known in Latin as the Lyceum. And over time this moniker came to apply to schools in general and, in fact, today, in French the word for high school is lycée which ultimately derives from this Greek word for wolf.

Now it was Aristotle’s habit to spend his mornings walking up and down the walkway next to this exercise area with his students and discuss the more abstruse points of philosophy with them. Since they were always seen pacing up and down, Aristotle’s philosophical school became known as the Peripatetic School. In the afternoons Aristotle would settle down and teach in one spot to a larger audience on subjects of broader interest, things like politics and rhetoric.

The mode by which Aristotle operated his school was that a leader would be elected to run the school for a 10 day period. What that entailed was that the leader would essentially have to stand his intellectual ground against any and all comers. Philosophers would ask for his opinion on a particular subject and he would have to then defend his position against their objections. This mode of debate came to be of tremendous influence in Western civilization since centuries later the medieval universities consciously mimicked this style, where a professor would set forth a series of theses and be obligated to defend them in debate against any objections that would be raised by the students.

Thanks to Aristotle’s close relationship with Alexander the Great, the Lyceum also became the first major library in ancient Greece. According to one account, Alexander granted him 800 talents to fund the library. As always converting these ancient units to modern currency is a dicey affair, but nevertheless, this was a prodigious amount of money, somewhere in the hundreds of millions of dollars today, so this was an extravagant collection. And calling it a library doesn’t quite capture the extent of it, it was maybe more like a museum, too, since it apparently also had a huge number of natural specimens thanks to Aristotle’s great interest in biology. To help him acquire these artifacts Alexander issued a proclamation throughout his empire that if any hunters or fishermen or anyone else stumbled across something unusual, they were to alert Aristotle so that he could collect the strange object for the library.

What exactly came to be of this great library is not entirely known and what is known is a long and winding story, too long and winding, believe it or not, even for this podcast, but the contents of the library were passed down to a succession of different men over the ages, some of whom were less scrupulous and diligent than others and either lost works or allowed them to decay, and ultimately it seems that they ended up in the hands of the Roman general Sulla as booty after the Sack of Athens in 86 BC and he then probably auctioned off whatever contents remained.

Aristotle remained at his school in Athens for 12 years, but in 323 BC his patron Alexander died at the tender age of 32. Suddenly all things were once again possible politically and Athens became a dangerous place to be for a Macedonian of his stature. A pair of Athenian priests, Eurymedon and Demophilus, jumped at the opportunity and tried to bring charges of impiety against the philosopher, so, always looking out for number 1, Aristotle got out of dodge while the getting out was still good. He is said to have remarked that he “saw no reason why Athens should be allowed to sin against philosophy twice.” He moved to the island of Chalcis, which is where his mother was from and there died of natural causes the following year.

Well, it is generally thought that it was in those 12 years or so at the Lyceum that Aristotle composed the works that he is known for today. Aristotle’s surviving works are notorious for their difficulty and terseness. But this reputation seems to be an unfortunate accident of history. Aristotle apparently wrote a tremendous number of works, most of which did not survive. They’re broadly grouped into two categories, the so-called exoteric works and the esoteric works. The exoteric works were works intended for publication to the general public, basically books and treatises. The esoteric works, by contrast, were essentially lecture notes and were just intended for use within the school. Not because they were secret, they were just rough and ready. They were there if, say, some students were unable to attend a lecture and would want to know what was discussed. But by a cruel and improbable twist of fate, all the works that were broadly published were lost, and only those of limited circulation within the school survived. So all of Aristotle’s surviving works are in this esoteric category, and none are in the exoteric category. This is quite the tragedy because Aristotle was apparently a tremendously gifted writer. Plato is seen as one of the greatest authors in ancient Greece, but Cicero said that if Plato’s writing was silver, then Aristotle’s was a river of gold. But you wouldn’t know it from what survived.

Now, all that said, relative to the other astronomers we’ve learned about so far, really a tremendous number of works of Aristotle survive, around 31 in all. Maybe somewhat surprisingly, though, only two of these works survived continuously in the West: Categories and On Interpretation. The rest made their way eastward sometime toward the end of Antiquity and were translated into Arabic during the Islamic Golden Age at the tail end of the Early Middle Age in the 9th through 11th centuries. During the Crusades, Western scholars came in contact with Islamic scholars and became acquainted with the rest of Aristotle’s surviving works and began a project of copying them into Latin in the 11th through the 13th centuries. The introduction of Aristotelian philosophy into Europe had the effect of instigating an intellectual revolution as scholars tried to reconcile these new ideas with Christian theology, with St. Thomas of Aquinas being at the forefront of this effort. But of course I will have much more to say on this topic when the time comes because it was a critical element in the development of science in Western Europe during the Late Middle Ages.

Well, the vast majority of Aristotle’s work deals with philosophy, and of course we are not interested in philosophy on this podcast, only astronomy. But because his astronomy was guided by his philosophy, it would be no good if I didn’t at least make an embarrassingly brief sketch of the character of his philosophy.

Probably the most immediately striking feature of Aristotle’s works is their encyclopedic quality. Now partly this is a consequence of the works which happen to survive, but it is nevertheless clear that Aristotle was obsessed with classifying things. The major groupings of knowledge that we recognize today, like logic, mathematics, biology, and so on, were set forth by him. You may recall in the episode about Plato that I had to provide a caveat that when Plato uses a word like arithmetic or logic, it doesn’t have the modern meaning. But in most cases Aristotle’s meaning is the same as the modern meaning, or more correctly, the modern meaning of these subjects is the same as Aristotle’s because it came from Aristotle.

Now, this obsession with classifying things came from a deeper philosophical conviction. If you read virtually any philosopher, it won’t be long before you’re reading about bizarre thought experiments like minds in vats and demons altering your perception. One of the perennial problems in philosophy is what is known as the “problem of skepticism.” How can we be sure that we know anything at all? How do we know that everything we see is not an illusion? I believe myself to be at my desk speaking into a microphone right now, but maybe I am really a brain in a vat and a mad scientist is feeding electrical signals into my brain to trick me into thinking that I am speaking into a microphone when in reality no microphone or mouth exists at all. How would I know the difference? Descartes thought hard about this problem and famously asserted that he could at least be sure of one thing, namely that he was conscious, and then took it from there. And the other greats down the ages from David Hume to Soren Kierkegaard to Elizabeth Anscombe have all grappled with the problem in various ways.

But Aristotle had no time for any of that nonsense. To him, the senses were basically dependable. The point of philosophy was not to waste your time worrying about whether or not your eyes were deceiving you, but to go out and actually use your eyes and ears to actually learn something about the world around you. So he’s a very practical sort of guy, especially for a great philosopher.

This puts him in contrast with Plato. Earlier on I said that when Aristotle was studying from Plato at the Academy, he was initially an enthusiastic supporter of his teacher, but came to have his doubts as the years passed. You may recall from Episode 15 that central to Plato’s philosophy was the theory of forms. To put it a little simplistically, the forms existed in an ethereal realm which contained the perfect versions of all things. The reality around us is just a pale shadow of the realm of the forms. Although we can’t see them, to Plato, the forms were much more real than the stuff around us. To Plato, we had to be wary of the senses and could only rely on pure reasoning to be sure that we were arriving at the truth.

Over the years Aristotle eventually came to the conclusion that this was backwards. To Aristotle, the realest things were in fact the stuff around us, and when we saw it with our eyes, we more or less are seeing it as it really is. He essentially takes observations at face value which is a rather refreshing characteristic in a philosopher. The downside of his approach is that it can lead him to be, shall we say, overconfident and at times even downright gullible. Now, Aristotle also held that forms existed, but to him, they were sort of patterns that emerged from objects in the real world. So the form of the cat emerges from the pattern of the millions of real-life cats across the world. To Plato the form of the cat takes ontological precedence, but to Aristotle this is not the case. Things in the world exist at the intersection of form and matter. The relationship between the worldviews of these two great Greek philosophers is on display allegorically in Raphael’s painting Philosophy, more commonly known as the School of Athens. I’m sure you’ve seen it, it’s the famous Renaissance era painting of a bunch of philosophers standing around in a great forum talking to each other. Well front and center, Raphael places Plato and Aristotle. Plato has his right arm raised up to the heavens to indicate the primacy of the forms, and Aristotle has his arm outstretched to indicate the primacy of objects in the world.

Well, to start to steer this ship towards astronomy, one of the central characteristics of Aristotle’s philosophy is that it is highly teleological. We don’t often come across the word teleology today because it’s a somewhat unfashionable idea in the modern world. But teleology is all about an innate direction towards some final purpose. A teleological explanation for a thing’s behavior is that it is in the nature of that thing to behave in the way that it does because the end state was in some sense its goal, its purpose for existing. So, in Aristotle’s philosophy, rocks fall towards the ground because it is in the nature of the element Earth to sink to the center of the universe. Now, put this way it seems a little circular or simplistic, but it is easy to slip into teleological explanations even if we think we know better. In a modern context they oftentimes crop up in evolutionary explanations. We might say something like “giraffes evolved long necks so that they could eat the leaves at the tops of trees.” But as a Darwinian explanation this is fundamentally wrong. Giraffes never evolved long necks for any particular purpose. Over the millennia giraffes were born with necks of all sorts of lengths, and those who happened to be born with longer necks had an easier time eating more leaves and reproduced more so over time more and more giraffes ended up being born with long necks. But that is fundamentally a different sort of explanation than saying that they evolved long necks in order to eat leaves at the tops of trees. One is an explanation of natural selection, the other is a teleological explanation, saying that the evolution occurred in order to achieve some purpose. The teleological explanation can often be a helpful shorthand for the real natural selection explanation, but we need to keep in the back of our mind that it’s not the real explanation.

I think in some ways teleology has been rather unfairly maligned in modern times. We can also talk about modern physics in a somewhat teleological manner. An extremely powerful formulation of classical mechanics uses what is called the principle of least action. You can define a quantity called the action, and you will find that a system always behaves in such a way as to minimize this quantity, or strictly speaking for the pedants, it will find an extremum of this quantity. Well why does the action get minimized? Well, it’s not really a question of physics anymore. We can’t really give a more satisfying explanation than that it is just in the innate nature of things to behave that way. Now you can derive the principle of least action from Newton’s laws of motions, but that just begs the question — why does matter obey Newton’s laws? Modern physics tells us that it is in the inherent nature of bodies to obey Newton’s laws or to minimize the action, and Aristotelian physics tells us that it is in the inherent nature of the element Earth to fall to the center of the universe.

Now, before we can really get into his astronomy we do have to flesh out his physics somewhat because his ideas on matter and motion really did inform his astronomy. At a high level, Aristotle’s physics had two components: matter and motion. His theory of matter was fairly straightforward: he more or less adopted Empedocles’s theory of the four elements. There are four fundamental elements: earth, water, air, and fire. All matter around us is composed of these four elements in varying degrees. But, for reasons I’ll get to momentarily, Aristotle also added in a fifth element, called aether, which only existed in the heavens. Now, Aristotle fleshed out Empedocles’s theory a little bit by drawing out two axes. There was the hot-cold axis, and the dry-moist axis. So fire is hot and dry, air is hot and moist (think of it being more like steam), water is cold and moist, and earth is cold and dry. Now according to Aristotle the pure versions of these elements are not necessarily our ordinary experience interacting with these elements. So a flame is not in fact pure fire. Pure fire does not have a flame, the flame only comes about due to the proximity of the fire to a moist element like air. In fact as a general principle, no matter we actually interact with can be completely pure — all four elements are always present, just in different proportions. These two axes, hot-cold and dry-moist also form their own axis of active vs. passive. The hot-cold axis is active and the dry-moist axis is passive. So the quality of dryness or wetness in an element is acted upon by the heat or cold of another element. But because each element has qualities along both axes — because it is hot or cold and dry or wet, each element has both active and passive qualities. This means that every element has the ability to act upon every other element and be acted upon by every other element. This then puts in place the prerequisites for dynamics, for a theory of motion.

Now Aristotle’s theory of motion is very different than the theory of motion of Galileo and Newton that developed during the seventeenth century, and, as a consequence, quite unlike the ideas around motion that we are used to today. But although Aristotle’s ideas around motion sound strange to those of us who have learned some basic physics, the only reason this is the case is that the first thing any basic physics class will do is try to beat out of you your intuitions around motion. The benefit of Aristotle’s theory of motion is that, unless you’ve already had your intuitions beat out of you, it is extremely intuitive, which is probably a big reason why it persisted as the dominant theory of motion for several centuries in Europe after Aristotle’s work was introduced around the 12th and 13th centuries. To Aristotle there are two fundamental kinds of motion: linear motion and circular motion. Linear motion is the kind of motion inherent to the matter around us on Earth. Anything composed of the four elements earth, water, air, and fire, will naturally move in a straight line. But this motion is necessarily finite because Aristotle believed that the universe is finite in size for reasons that I’ll get to. If you set an object in motion in a straight line, it couldn’t go along that straight line forever because eventually it would bump into the edge of the universe and could go no further. So any ordinary matter would naturally move for a while in a straight line and then slow down and stop. And of course this looks to be in good agreement with what we observe around us. If you push a rock to the side, it’ll stop moving when you stop pushing it, and the heavier the rock the quicker it’ll stop. If you start walking and suddenly stop moving your legs, you don’t just magically keep moving forward. Without any effort on your part, you stop moving.

Now Aristotle argued that a consequence of this is that to move an object continuously, something needs to be in continuous contact with it to push it along, otherwise it would stop since that’s its natural state. This might seem a little weird — if I throw a baseball it seems to move through the air all on its own — I’m not pushing it the whole way through the air with my hand until it reaches the catcher’s glove. But according to Aristotle what is actually happening here is that as you throw the ball, you create a disturbance in the air around the ball, and this disturbance propagates in such a way that the air surrounding the ball ends up pushing the ball forward. Presumably if you tried throwing a ball in a vacuum, it wouldn’t move forward at all, but that’s an absurd thought experiment for Aristotle anyway because he didn’t believe that it was possible for a vacuum to exist.

So the four elements tend to move in straight lines, but they have different destinations. Earth and water both tend to move toward the center of the universe whereas air and fire tend to move away from it. Furthermore, earth has a stronger tendency to move towards the center than water does, and fire has a stronger tendency to move away from the center of the universe than air does. This is why the ocean and rivers and lakes sit on top of the Earth and why fire rises up through the air.

So that in a nutshell is how motion works for one of the two kinds of fundamental motion, linear motion. But according to Aristotle there is a second kind of fundamental motion which is circular motion. We can clearly see that objects around us behave in one way, you drop a ball and it always falls straight back down to Earth. But the celestial objects do not behave in the same way. They perpetually move in circles around the Earth. Since the natural motion for the four terrestrial elements is linear, there must be a fifth element that inherently has a circular kind of motion. Aristotle calls this fifth element aether. Unlike linear motion, circular motion is eternal because it never runs into any constraints. Once a planet or star returns to the same point of the circle everything is exactly the same as it was when it began and the motion repeats. So the stars move around the Earth in circles and will continue to move around the Earth indefinitely.

This fifth element aether was different than the others not only in the quality of its motion. It also didn’t possess the qualities of heaviness or lightness. And it was superior to the other elements, both in a sort of normative sense of just being a better element, but also literally in a physical sense in that it existed above the other elements. The higher you went in the universe, the more the universe was dominated by aether. This identification of a sort of normative character with a physical location in space is very characteristic of Aristotle’s philosophy, though it was by no means unique to it. Lots of ancient cultures have had the idea of sacred locations and an intuition that higher places were closer to God or the gods — think of Moses receiving the Law from God on Mt. Sinai or the place of Mt. Fuji as a sacred location in Shintoism or Mauna Kea as a location where the gods of the Hawaiians dwelled, or in Ancient Greece, Mt. Olympus, which was both a physical mountain and the mythical dwelling place of the Greek gods.

According to Aristotle, the boundary between the heavens and the terrestrial realm was the sphere of the moon. So everything we know in our everyday life belongs to the sublunary realm. The Moon is the first of the heavenly spheres that contains the element aether, but it also contains a good deal of earth as well. As you get to more distant spheres the proportion of aether increases.

This then fits in very nicely with Aristotle’s theory of planetary motion. The natural tendency of the terrestrial elements is toward stasis whereas the natural tendency of the aether is circular motion. The most distant sphere, the fixed stars, contains aether almost exclusively, so its motion is perfectly circular. For Saturn, there is a higher proportion of the terrestrial elements, so there is now a combination of linear and circular motions, so Saturn’s motion is somewhat more complicated. Jupiter has even more terrestrial elements so its motion is more complicated still, and Mars even more so. But by the time you get down to the Sun and the Moon, the general tendency of the terrestrial elements towards stasis tends to dampen their motion, so they return to relatively simple, mostly circular motions.

Aristotle thought that Eudoxus’s model of homocentric spheres was essentially correct and adapted most of Callippus’s embellishments. You may recall from Episode 16 about Eudoxus that it’s not entirely clear how literally he believed his theory. Was it a physical model of the universe or was it just a mathematical device to predict the positions of the planets? But with Aristotle there’s no question at all. He believed his model of the planets to be a literal, physical model of spheres rotating in space.

Now, because Aristotle wanted a physical model he realized that there were some problems if he just used Callippus’s model right out of the box. Take a planet like Jupiter. It has four spheres to describe its motion: one for its daily rising and setting, one for its roughly 12 year motion through the zodiac, and then another two to explain the fact that sometimes it was a little ahead of where it was supposed to be in the zodiac and sometimes it was a little behind. In this model, you have these four spheres nested inside of each other and the poles of one sphere are attached to some point on the sphere right outside it. Okay, that’s all fine, but what is the outermost sphere attached to? The sphere outside it is Saturn, and Saturn has its own complicated motion, totally independent of Jupiter! If the outermost sphere is attached to one of Saturn’s spheres, you would see the motion of Jupiter be the superposition of this normal motion I’ve described plus the motion of Saturn. And what about Mars? Mars’s outermost sphere would then be attached to Jupiter’s sphere, so Mars’s motion would be even more complex because it would be the superposition of Mars’s motion plus Jupiter’s motion plus Saturn’s motion. By the time you got to the Moon it’s motion would look totally chaotic since it would be the superposition of the rotations of 34 spheres.

Aristotle’s idea to fix this was in the same vein as Callippus — just add more spheres. His basic idea was that in between, say, Jupiter and Saturn, there would be an extra set of three spheres, called reacting spheres. Each of these spheres would just rotate in the opposite direction of one of the spheres of Saturn in order to cancel out its motion. Then the lowest of these spheres would have the same rotation as the fixed stars. Then you could attached the outermost sphere of Jupiter to that sphere as normal.

Now it’s clear from his statements that Aristotle was not a geometer on the level of Eudoxus because later authors noticed that he made a few elementary mistakes in his theory. Since all the spheres are attached to the outermost sphere of the fixed stars, he didn’t need a sphere associated with each planet to produce the daily rising and setting. The outermost sphere would do just fine. So he had six extra spheres in his model that he didn’t need. Also, to be perfectly consistent, he should have added three extra spheres beneath the Moon to cancel out the Moon’s motion because according to Aristotle features like comets and meteors belong to the realm just beneath the Moon. But since comets clearly rotate with the fixed stars the lowest sphere has to rotate with the fixed stars, too, which means you need three spheres to cancel out the motion from the Moon’s sphere.

Well unlike Eudoxus, Aristotle’s astronomy goes well beyond just a model of planetary motion. One of the reasons that Aristotle became so influential in Europe in the Middle Ages was that his astronomy was really quite comprehensive. Almost any astronomical question you could think of, he had an opinion on it. Why do the stars shine, for example? To Aristotle the element of aether is not visible. The stars are made of aether, but as they move through the heavens, they displace air and fire and this displacement causes a visible burning.

He had an extensive cosmology, too. He believed, in contrast to some of the earlier philosophers, that there was only a single universe. His argument for this was almost tautological. In essence, any matter necessarily had to be a part of the universe. The natural place of matter was within the universe, there is Aristotle’s teleology again. If you wanted to move anything outside of the universe, you’d have to use some force to do so since it’s currently a part of the universe. But remember that in Aristotle’s physics the only way to apply force is through direct contact. So if one piece of matter was to push another piece of matter outside of the universe, that piece of matter would have to be outside the universe as well. So something would have to be outside the universe first, but this isn’t the case, so everything must be inside the one universe. Now, I’d say that it’s a rhetorically slick argument, but maybe not an especially convincing one for moderners.

Aristotle also believed that the universe was finite in size. He had a few justifications for this stance. One was essentially just intuition. He felt that infinities in general caused all sorts of trouble, which, to be fair, they do, but at least to us moderners we don’t see that as being a good enough reason to reject them out of hand. But, he argued that if the universe were infinitely large, this would mean that the stars were infinitely far away, and this would mean that they would rotate infinitely fast, which he regarded as absurd. Another serious problem with an infinite universe is that it had no center. But clearly the Earth is the center of the universe, which we can see because water and the element earth always move toward it, so the universe had to be finite in size.

The universe also had to be spherical in shape. After all, it clearly rotates, but if it had some other shape, say, a cube, then the corners would be in one place now and in another place a little while later. But what exactly are the corners rotating into? After all, the universe contains all the space there is. There’s no other space for the corners of the universe to rotate into. So if the universe were not spherical it would not be able to rotate. But since we do observe it to rotate it must be spherical. Now Aristotle actually made a subtle mistake here because this argument, clever though it is, isn’t quite right. It’s possible to have shapes other than spheres that can rotate without occupying any more space. Any solid of revolution would work, not just a sphere. A prolate or oblate spheroid where the distance between the north and south celestial poles is different than the distance to the celestial equator would also be perfectly valid. But Aristotle had deeper aesthetic reasons for preferring spheres since the heavens, with their outermost reaches, the fixed stars, in particular, being the highest, most incorruptible realm, they therefore had to be associated with the most perfect shape, which was the sphere.

Well Aristotle also reasons about the shape of the Earth and concludes that it, too, is spherical. On this question he really collected all the results at the cutting edge of astronomical research of his day. One argument he points to is that during a lunar eclipse the shadow cast by the Earth is always curved no matter the orientation of the shadow. If the Earth were a disc or some other shape, you would expect that its shadow would sometimes be curved, but other times might be flat or some other shape. But we never see that. No matter how many lunar eclipses we see, the shadow is always curved, even though sometimes it’s on the lower side of the moon, sometimes on the upper side, sometimes to the left and sometimes to the right. The only way the Earth can cast a curved shadow no matter its orientation with respect to the Moon and Sun is if it is spherical.

But, as is typical of Aristotle, he tried not to rely on a single argument for his position. He also noted that the sky appears slightly different depending on your latitude. Different stars are visible on the southern horizon in Egypt, in particular Canopus, the second brightest star in the night sky. And some stars in northern Greece always stay above the horizon but rise and set at lower latitudes. Aristotle concluded that this not only meant that the Earth was spherical, but that it couldn’t even be all that large, since the distances people could travel could noticeably change the appearance of the sky. Now, Aristotle wasn’t the first to realize this, we saw back in Episode 16 that Eudoxus had also pointed this out. And Aristotle actually cites the work of some people he calls “the mathematicians,” who had actually used this fact to try to estimate the size of the Earth and had arrived at a circumference of 400,000 stades. This is the first quantitative estimate of the size of the Earth that we know of. Now, unfortunately, the length of a stade isn’t known precisely, but it’s generally believed to have been a bit more than 150 meters though it could have been closer to 200. In modern terms this implies that by the time of Aristotle the Greeks had estimated the circumference of the Earth to be about 60,000 km, which is about 50% larger than its true value of 40,000 km. So, by modern standards this wasn’t an incredibly accurate measurement, but it’s still impressive that the mathematicians, whoever they were, were very much in the ballpark of understanding roughly how large the Earth was. As we’ll see in a future episode, within about a century Eratosthenes had vastly improved the accuracy of this measurement.

Well Aristotle wasn’t content to rely only on empirical measurements. He also provided a few physical arguments as to why the Earth had to be a sphere. Aristotle argued that if the Earth wasn’t a sphere, the heavier bits of matter on the extremities would push aside lighter matter until the Earth arrived at a spherical shape. The spherical shape was just a natural consequence of his physics — the element Earth tended towards the center of the universe, so things would shuffle around until all parts of the Earth were equidistant from the universe’s center. And from a modern perspective this argument isn’t all that far off from the truth. His theory of motion was all wrong, but it is at least similar to the modern picture that physical systems find the state of lowest energy and a sphere happens to be the lowest energy state because all parts are equidistant from the center.

Now we’ve heard form a few astronomers in the last few episodes who all held that the Earth is a sphere. But this doesn’t seem to have been the only opinion among astronomers of the day. Some had pointed to the fact that when the Sun sets, it is cut directly in half by the horizon. If the Earth were really round the Sun would form a sort of crescent as it sets. But Aristotle says that it just looks this way because the Sun and the horizon are so far away from us that the horizon looks flat instead of curved. This is maybe one of the few cases where Aristotle rejects the direct observation of his senses.

One other consequence of his theory of motion was that it implied that the Earth did not rotate. After all the Earth was composed of the element earth, and the natural tendency of all four terrestrial elements was to move in straight lines until they slowed to a stop. If you let go of a rock it falls straight down and settles to a stop as close to the center of the universe as it can get, there is no circular motion. Since the planet Earth is nothing more than a big blob of this element, all its parts, too, would fall straight down and settle as close to the center of the universe as they could get, so there would be no rotation. Even if, in the distant past, the heavenly spheres had imparted some circular motion to the Earth, that motion would have eventually died away because its not in the nature of the element earth to move in a circle.

Aristotle also thought carefully about other properties of the universe that we might today find a little strange. He spends a while wondering about whether or not the universe has a left or right side. Now, today we might see this as being kind of an odd question. Aristotle believed the universe to be spherical and how can a sphere have a left or right side? But Aristotle’s reasoning went like this. He said that there are three dimensions and we can call those up-down, front-back, and left-right. So a left-right axis exists in some sense but the question is can we define it uniquely? Now to Aristotle the axis about which the universe rotated was clearly the up-down axis. Furthermore he asserts that the right side has the quality of the beginning of motion — things inherently move from the right to the left — here again is that good old Aristotelian teleology. The stars rise in the east, so the east must be the right side. Now we have to imagine lying on the ground — in Greece of course — with our head towards the north pole and our feet towards the equator. If we do this, the east will be on our left and the west on our right, so we’ll see that the stars rise on our left side and move toward our right side. Now in modern English we would be perfectly content with calling this motion “to the right,” but at least in Aristotle’s Greek this does not work. He calls this motion to the left. But because Aristotle already established that the right had the inherent property of the beginning of motion, this means something is wrong. So the only way he could reconcile this was to say that the problem is that you are oriented wrong. The problem is that you have laid down with your head towards the north and your feet towards the south. If you turn around so that your head is towards the south pole and your feet are towards the north pole, everything will be fine. What this means then is that, contrary to the way we normally represent globes, the south pole is in the up direction and the north pole is in the down direction. Then the east direction is the right and the west direction is the left, and everything is as it should be.

So we see in Aristotle’s astronomy a curious blend of reasoning about properties of the universe some of which strike us as really quite modern and other arguments that sound like they came from a crackpot. On the one hand his arguments about the spherical shape of the Earth are generally sound and the size he quotes is really not too far off from the truth. But then you have this strange discussion about whether or not the universe has a left or right side that ultimately hinges on his own personal opinions about whether motion originates from the left or the right.

Well Aristotle also writes a great deal about phenomena which he did not consider to be astronomical in nature, but which we do today, namely the nature of comets, meteors, and the Milky Way. He doesn’t really talk about these phenomena in his main work on astronomy De Caelo, On the Heavens, but instead in a separate book called Meteorology. The title of that book gives some indication of how he thought about these phenomena. To him, comets, meteors, and the Milky Way were things high up in the atmosphere, but beneath the sphere of the Moon. Since they were in the sublunary realm, they did not contain that special fifth element aether, but were fundamentally terrestrial phenomena just like the weather is. In fact, if you’ve ever wondered why meteorology is the study of weather and not, well, meteors, the truth is that when Aristotle coined it it was the study of meteors. The Greek word meteor just means “high up in the air”, so meteorology is the study of things high up in the air, which to him meant clouds and rain and wind, but also comets and meteors.

As is typical of Aristotle before giving his explanation of what comets and meteors were he went through the opinions of other astronomers and knocked them down one by one. You might remember that Anaxagoras and Democritus held that comets were the conjunctions of two planets. But Aristotle says that this clearly cannot be the case since otherwise we would only see comets in the zodiac and frequently we see them outside of it. Other astronomers had argued that what we thought were comets was actually just another planet, but we just don’t see it very often because it’s usually too low on the horizon like Mercury is. But Aristotle said that this also can’t be the case because sometimes we see two comets at the same time.

To Aristotle the mechanism that produced comets was the same that produced meteors. Every now and again a puff of dry air rises up from the Earth in what Aristotle calls an “exhalation” and contacts the Moon’s sphere. The rapid motion of the Moon’s sphere rubbing against the pocket of air then ignites it, producing a burst. Depending on the qualities of this puff of air, how warm or cold it was, it would ignite differently. So sometimes it produces a short burst, other times a pocket of cold air gets squeezed out and propelled in one direction which causes a bright streak in the sky which we would see as a meteor. But, if the properties of the puff of air are just right, which doesn’t happen very often, it will produce a slow, steady burn and this we observe as a comet. Now a corollary of Aristotle’s theory of comets is that because they are produced by these hot, dry exhalations, they are more common when the weather is dry and windy, so comets are a warning of potential drought to come.

Actually, though, Aristotle identified two types of comets: sublunary comets and superlunary comets. These slow, steady burns of pockets of air hitting up against the lunar sphere are the sublunary comets. But occasionally a star itself can produce one of these puffs of air, which then glows due to the rapid motion of the spheres and follows the star around like a kind of halo. Now, on this point, the terse, dense Greek of Aristotle’s writing really bites us because he seems to say that the puff of air, and therefore the comet itself, is still in the sublunary realm, but just follows the star around. Specifically he says “when the exhalation is constituted by one of the fixed stars or the planets, owing to their motion, one of them becomes a comet.” But he doesn’t explain the mechanism of how the star “constitutes” exhalation and how the exhalation follows it around.

Now in the Middle Ages, St. Thomas Aquinas wrote a commentary on Aristotle’s Meteorology among other works and interpreted Aristotle’s explanation of superlunary comets as being exhalations that happened to collect underneath a star and weren’t really due to any change in the star itself. This interpretation was necessary in medieval European cosmology where the superlunary realm, the heavens, were held to be absolutely unchanging. This was why Tycho Brahe’s observation of a new star in the sky in 1572, which we now know to have been a supernova, was so revolutionary. If this new object was in fact in the superlunary realm, it indicated that the superlunary realm was not unchanging after all. But I think that this rather absolutist viewpoint seems to have been more a medieval position than it necessarily was Aristotle’s own opinion. As far as I can tell, Aristotle was not explicit in saying that the heavens were absolutely unchanging. Aristotle did believe, for instance, that the some amount of the earth element was present in the superlunary realm, it was just highly diluted and the dominant form of matter was the aether. But in his physics if the element Earth was present, some amount of change should be possible. And in the case of superlunary comets, his wording really does suggest that the cause of the comet ultimately is in something that happens from the star itself, which seems to indicate that something in the superlunary realm has to change, even if he doesn’t say exactly what.

Well, Aristotle also assigns the location of the Milky Way to the sublunary realm as well and basically gives the same mechanism for its formation as for the formation of comets. Just as sometimes individual stars can form superlunary comets by collecting matter in the right constitution in the upper atmosphere that then ignites upon contact with the moving sphere of the Moon, a large group of stars collectively can also have this effect in a more permanent kind of way across a broad region of the sky.

Okay, well I have attempted to at least touch on the major features of Aristotle’s astronomy, but again, given the extent of his writing and influence, even though this has been one of the longest episodes yet it’s still a fairly superficial exposition of his astronomy, and a laughably cursory exposition of the rest of his philosophy. But never fear, as I’ve alluded to, many of the basic features of his astronomy came to dominate Western thought during the high and late middle ages, so when, in a decade or two, this podcast finally arrives at that era, we’ll be able to explore the implications of this worldview in more depth. In particular, one bit that I had to skip over in the interest of time has to do with the concept of the cosmogony of the unmoved mover, which he invoked to explain the motion of the outermost sphere of the fixed stars, and which was enthusiastically embraced by St. Thomas Aquinas as evidence for the existence of God. But all that will have to wait for the future.

Aristotle is an especially interesting figure because, on the one hand he has had more influence on Western intellectual life than almost anyone else, and on the other hand, his reputation has see-sawed over the ages. He was, of course, respected in his day, but was rather neglected at the close of antiquity and in the Early Middle Ages when all but two of his works were unknown in the West. After his works came into the West via Arabia, his philosophy became all the rage. St. Thomas Aquinas simply calls him “The Philosopher.” But in the Renaissance, humanist scholars rejected what they felt was the stifling nitpicking that characterized Scholasticism, which was the philosophy that developed out of Aristotelianism. The roller coaster of his reputation went up again in the Enlightenment and the 19th century as scholars started to focus more on his work in biology and political philosophy. But then his reputation came crashing back down in the 20th century as the intellectual currents once again drifted against him and he was seen as having instituted an orthodoxy that stifled innovation and creative scientific thought for centuries. Were it not for Aristotle, many 20th century thinkers mused, how much sooner the scientific revolution would have got going and by now we would all have flying cars on Mars.

Well, regardless of how dogmatically later thinkers adopted his astronomy, it is certainly the case that without Aristotle we would know much less about Greek astronomy than we do because many of the ideas of earlier astronomers only survive because Aristotle was thoughtful enough to write them down so that he could prove them wrong. So as historians of science we have no choice but to show him some gratitude.

Aristotle in many ways marks a turning point. His career lasted through the tail end of the Classical Era of Ancient Greece. He died just one year after the death of Alexander the Great, which traditionally marks the start of the Hellenistic Era. So now we will start to turn to the astronomers of that era. One of the consequences of Alexander’s conquests was that there was far more interaction between the Near East and Greece. So while we have heard innumerable stories of Greeks travelling to Egypt or Babylon and learning astronomy there, in the Hellenistic Era, along with the Roman Era, Greek astronomers start to gain access to Babylonian astronomical records which were an incomparable treasure trove of data for Greek astronomers trying to model planetary motions. But that will be a tale for another full moon. I hope you’ll join me then. Until the next full moon, good night, and clear skies.

Additional references

  • Sir David Ross, Aristotle
  • Henry Babcock Veatch, Aristotle: A Contemporary Appreciation