If you take some time to head outside each evening for a month and make some close observations of the stars, you will notice two things. Firstly, the stars make a complete rotation in the sky roughly every 24 hours. Secondly, the stars always keep the same positions relative to one another. Practically all of the ancient civilizations were profoundly, and even obsessively, aware of these two facts about the heavens. For them, the heavens seemed the best place to start in order to discover the ultimate meaning of the universe, and so they gazed intently at the skies, looking for answers.
A persistent question naturally arose for the ancient stargazer contemplating these two star facts, their perfectly circular and seemingly eternal rotations: what makes the stars go round? In this article, we will consider three different answers: the ones of Aristotle, the medieval scholastics, and Newton.
Aristotle of Stagira, affectionately dubbed “The Philosopher” by his fans throughout the ages, had an answer for just about everything. As such, there was no chance that he would leave the celestial motion question unaddressed. His answer, in itself, was simple: Love makes the world go round. The philosophical argumentation behind the answer, however, takes some effort to untangle.
The question, for Aristotle, was not so much, “What moves the stars?” as “Where does movement as such come from?” One’s answer to the first question would only be satisfactory if the second question could be addressed.
To discover the origin of all motion that exists in the entire universe, The Philosopher had to think very carefully about what motion is in itself. By all accounts, he did a great job. According to Etienne Gilson, “No one has ever better discerned the mystery that the very familiarity of movement hides from our eyes.” Aristotle formulated a key principle for understanding causes of motion, the principle that “Whatever is moved is moved by another.” In other words, all motion requires two aspects, one mover and the other moved, one active and the other passive. If mover and moved are the same in every respect, then movement does not take place.
Consider my fingers typing this article. For my index finger to move, it has to receive a nerve impulse that has its origin in my brain. If I were not to send that impulse to my finger and instead waited for my finger to move itself, I would be waiting forever. The reason is that my finger cannot be both what moves itself and what is moved by itself at the same time. For that to be so, it would have to be both active and non-active at the very same moment, and in the same respect, which is a contradiction.
Back to The Philosopher, he leveraged his motion principle to discover where motion comes from. “If everything in motion, he reasoned, has a certain dependence on a mover, then all of the motions that I see around me must have their source in something independent of movement, in something unmoved.” The ultimate source of motion must be an Unmoved Mover, a being that causes movement in other things, but himself does not move. Unless such a being exists, there is no way for us to account for the existence of any movement at all.
But how do you move other things without being in motion yourself? Easy, says Aristotle. You do it by way of attraction. Consider a painting hanging on the wall at The Louvre. It is not moving. It is just existing there, in its artistic beauty. Even though it does not move, however, it has the power to move everyone in the art gallery. They are attracted to the painting and so walk towards it, without the painting having done anything other than hang on the wall.
Thus, Aristotle concludes, the ultimate reason for the movement of the heavens is the attraction exercised by the First Unmoved Mover on the stars. This is “the love that moves the stars” of which Dante speaks in the last line of his Divine Comedy.
It is one thing for your ideas to hold sway for a few years. It is another thing for them to hold sway for a few millennia. Such was the case with Aristotle’s notions of motion. When the scholars of the Catholic Middle Ages started translating Aristotle into Latin and studying him closely, they found his ideas extremely compelling. To their credit, however, they did not become “Aristolators” or worshippers of Aristotle who thought he was some demi-god gifted with philosophical infallibility.
The reason they did not follow Aristotle in every iota of his works and pomps was that he contradicted the Catholic Faith in some of his teachings. For instance, to prove that a First Unmoved Mover must exist, The Philosopher first tries to prove that the motion of the universe is eternal. The Catholic Faith, meanwhile, teaches that the universe came into being at some limited time in the past.
Scholastic philosophers, then, had to do some sifting when they encountered the Aristotelian corpus. When they came to his teaching on physical motion, they did not like the idea of stars moving by an innate attraction. They knew from Genesis that the heavenly bodies are lower than human beings. Mars, Mercury, and Jupiter cannot not be closer to the deity than souls endowed with sanctifying grace. As such, the heavenly bodies are surely not striving to imitate God. The reason for their motion must be assigned to some other cause.
The best way to please both Aristotle and be faithful to the faith was to say that the angels were moving the heavens. “Most [medieval natural philosophers] assumed that [God] assigned an angel to move each orb.” This idea enabled medieval scholastics to keep Aristotle’s system of astronomy, wherein the heavenly bodies had the same motions he had envisioned. It was just the cause of the motion that was different.
One medieval thinker of the 14th century, however, realized that we do not need the angels to explain celestial motion. His name was Father Jean Buridan and he taught at the medieval University of Paris. In one of his works, Buridan sets out to explain why rocks move the way that they do when we throw them.
Aristotle, of course, said that a certain attraction explains their movement (as we saw, he explained the totality of motion in this way). Rocks really, really want to be on the ground. When they are thrown, the air surrounding the rocks keeps them off the ground for a while until their attraction for the ground overcomes the air, enabling them to return to terra firma.
For Buridan, this did not make much sense. With all due respect to Aristotle, he said, it is obvious that air does not help rocks to fly, but rather hinders them. Just like when we are running, we are held back by the wind, so too the air around a rock must slow it down. The real reason why the rock stays above the ground, Buridan claimed, was that our hand imparts an impetus to the rock that remains in it for a time, before slowly dissipating.
This makes sense and seems quite reasonable, especially to modern ears, accustomed to scientific notions of force. But Buridan went further. He realized that physical motion on Earth is really not essentially different from physical motion in the heavens. Thus, he said, perhaps the heavenly bodies are moving in space for the same reason that rocks move in space: they have been pushed. Perhaps, at the moment of Creation, God pushed the planets and, since there is no friction or contrary winds in space, they have been moving ever since, without needing the angels to keep them going around in circles.
When the apple dropped out of the tree to plunk down on the head of Isaac Newton (1642–1727), in the perhaps apocryphal story, Newton simply repeated the same reasoning process that Buridan had performed centuries before. The physical force that makes the apple fall to the ground—gravity—must also exist in the heavens. Perhaps gravity, then, is what is making the planets turn.
This apple-induced idea led Newton to construct mathematical formulas for the way gravity works on earth. Then, he applied those formulas to the movements of the planets around the sun. What he discovered was that his formulas—what today are called Newton’s three laws—were able to describe planetary motion almost perfectly.
With his achievement, Newton was able to provide a purely physical explanation for the motions of the heavens, without, by any means, excluding the need for divine causality. God provides to the planets their being and the natural laws by which they operate. And they dutifully follow those laws.
For Aristotle, stars move because of their attraction to the First Unmoved Mover. Today, we know that this is incorrect, thanks to the Catholic Middle Ages. Using ideas taken from divine revelation, medieval scholastics were able to formulate ideas about forces that pushed the planets forward. The impetus from these thinkers was what ultimately pushed the human mind forward to give birth to what we now know as modern science.
Author’s Note: The subject of this article is covered in detail in chapters 4 and 5 of my book The Realist Guide to Religion and Science (Gracewing, 2018).