A Short History of Geocentrism
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The stars are like letters that inscribe themselves at every moment in the sky… Everything in the world is full of signs… All events are coordinated.
— Plotinus, from Enneads (c. 268 AD)
In astronomy, geocentrism refers to models and cosmologies that place the Earth at the centre of the universe. Geocentric models varied in their precise formulation, but they tended to assume that celestial objects — the Sun, Moon, planets, and stars — orbited around the Earth, which remained in a fixed position.
These systems, which predominated Western, Islamic, and Far Eastern cosmogonies for centuries, can seem primitive to us now that we know the truth. But we should not be so quick to pass judgement on our predecessors — not least because there’s an excellent chance that something we take for granted in our current understanding of the cosmos will end up being just as wrong. Instead, we should recognise that these early models of the universe represent that very human desire to understand the universe and our place in it.
Writing in the 13th century, William of Occam hit upon a profound wisdom when he formulated his philosophical razor — that the simplest explanation is often the best one. To the greatest minds of his time, a geocentric model remained the most simple explanation. After all, from our earthbound perspective the Earth feels entirely stationary, while the Sun and stars move across the sky as if they’re rotating around us. Similarly, the stars are so much further away from us than they could have ever imagined, so their reasoning that if the Earth did move they’d be able to see evidence of a change in star positions, was not so wrongheaded.1
The ancients believed the movements of celestial objects profoundly influenced events here on Earth. Thus, it was vital that they map the cosmos, tracking the Moon, planets, and star constellations as they arced across the heavens. In doing so, they developed complex mathematics, philosophies, and birthed the science of astronomy.
What follows is a short history of geocentrism, and how ancient astronomers in Babylon, Greece, and the Roman Empire made sense of their place in the cosmos, and how European Christendom adapted these ideas into its theology.2
Though references to celestial omens, star lists, and schemes of daylight length begin to appear in the archaeological record as far back as the 16th century BC, it was during the 8th and 7th centuries that Babylonians first developed an empirical approach to the study of the heavens. Their detailed reports on the movements of stars, the phases of the Moon, and eclipses of the Sun herald the beginnings of human science. In the words of Asger Aaboe, “we claim Babylonian mathematical astronomy as the common ancestor of modern efforts in the exact sciences.”
Unfortunately, the ancient Babylonians’ work comes down to us in fragments, scattered and piecemeal, so we’re not able to fully reconstruct their cosmology. However, the c. 7-8th century BC Imago Mundi, the first ever map of the world, suggest the Babylonians viewed the Earth as a flat disc surrounded by water.
Anaximander (c. 610–546 BC), an ancient Greek philosopher, likely drew on Babylonian ideas when he proposed one of the earliest known models of the universe. In his geocentric formulation, the Earth was envisioned as a flat, cylindrical disk floating motionless at the centre of the universe. The Sun, Moon, and planets were in fact openings in invisible wheels surrounding the Earth-disc, where the light of distant fires shined through. As the cosmic wheels rotate around the Earth, the celestial bodies appear to move across our sky. According to Anaximander, eclipses occur when the openings in the wheels temporarily close, blocking the light, while the phases of the Moon are caused by the regular closing and opening of the Moon-wheel's aperture.
Working a generation after Anaximander, it was Pythagoras of Samos (c. 570–495 BC) who first realised the Earth was not a flat disc, but a sphere instead (or at least, he’s the one the ancient Greeks credited with the idea). Thus, Plato (c. 427–348 BC) placed a spherical earth at the centre of the cosmos, with the celestial objects arranged outwards in concentric circles in the following order (out to in): the stars, Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon. These bodies moved along their circles as a result of the three Fates spinning the “The Spindle of Necessity” that was the universe.
Eudoxus of Cnidus (c. 390—340 BC), perhaps the greatest of all the classical Greek mathematicians, used less mythology in his own formulation of the cosmos. He borrowed Plato’s dictum that all celestial phenomena, even the erratic movement of the planets, can be explained with combinations of uniform circular motion, to construct a sophisticated model of their movements. He used three spheres to explain the movements and phases of the Moon, three more for the Sun, one for the fixed stars, and four spheres each for the five visible planets. Callipsus (c. 370—300 BC) went on to add seven more spheres to Eudoxus’ twenty-seven, before Aristotle (384–322 BC) went ring-mad and blew them both out of the water.
The Aristotelian system placed the spherical Earth at the centre of the cosmos, with 47-55 (depending on how you interpret his writing) transparent, rotating spheres surrounding it. These concentric, crystalline spheres were composed of an incorruptible substance called æther (the fifth element after earth, air, fire, and water), and moved at uniform speeds to create the revolution of the celestial bodies around the Earth. He believed in a primum movens, ‘prime mover’, who set off the motion of the spheres, and thus began the universe.
Aristotle, genius as he was, posited this model based on his assumptions about the density of the five elements: earth, the heaviest, must lay at the centre; the slightly lighter water formed the next layer (i.e. the oceans); then, lighter still, air and fire rose above the first two; and last, æther, lightest element of them all, was thus the furthest from the centre. Although we know his model to be wrong, we can still appreciate the logic of his deductions in formulating it.
By the time Aristotle was writing his grand treatise, On the Heavens, geocentrism was well-established in Western philosophy,3 but it hadn’t reach its final, long-lasting form. That would be left to Claudius Ptolemy (c. 100—170).
We’ve discussed the Alexandrian Greek polymath’s extensive contributions to the history of geography and cartography in other posts, but the great scholar had the same enduring influence on cosmography too.
Ptolemy’s mathematical and astronomical treatise, Almagest, may well be the most influential scientific text in history — or at least, it was once, before the work of Nicolas Copernicus superseded it in 1543. Just as in the Aristotelian formulation, the Earth sits motionless at the centre of the Ptolemaic Model, while nested transparent spheres carry each a celestial object around in a circular orbit. However, Ptolemy knew that this didn’t reliably account for the retrograde motion of the five visible planets, so he introduced Apollonius of Perga’s (c. 240—190 BC ), and Hipparchus of Rhodes’ (c. 190—120 BC), concept of epicycles to his model. Essentially, an epicycle is a smaller circular motion of a celestial body proceeding simultaneously to its orbit in a wider circle (deferent circle) around the Earth. That way from Earth it might appear as if a planet has reversed direction, but really it’s just rounding its epicycle loop.
Ptolemy drew on Hipparchus’ now lost work, Of Lines Inside a Circle, where the earlier astronomer-mathematician had invented the discipline of trigonometry by his use of the first known chord table, which he used to calculate the eccentricity (i.e. epicycle motion) of the lunar and solar orbits.4
However, even with his integration of epicycles, Ptolemy realised the actual movements of the celestial objects didn’t always match up to the predictions derived from the mathematics. Being the polymath that he was, he construed a new notion known as the equant. In essence, the equant was the point in space from which a planet’s epicycle would appear to move with uniform speed. It was placed so it lies directly opposite to the Earth’s position from the deferent’s centre, which is called the eccentric.
To make the model work, every planet’s equant had to be in a different position, and none of them were on the Earth - hence explaining why the planets seem to move so erratically from here. This may sound like quite a convoluted solution, and it certainly was (we’re quite far from Ockham’s Razor now!),5 but it was mostly accurate and help maintained the belief in uniform circular motion that Plato had first articulated four and a half centuries before.
The ancient Greeks, incredibly smart though they were, were so wedded to their idea of a perfectly uniform, circular cosmos, it prevented them from seeing things as they really were: the planets didn’t orbit the Earth, and their motion was elliptical, not circular. Regardless, Ptolemy’s fixes resulted in a margin of error far smaller than those of previous scholars, and his model became standard astronomy for 1200 years.
When Ptolemy was writing in the first century AD, a small Jewish sect was just beginning to spread its gospel across the Mediterranean. Though a growing enclave had taken root in his home city of Alexandria, he could hardly have foreseen its eruption into the hegemonic theology of Europe a few centuries later, or its adaptation of his ideas.
Just like Aristotle, Ptolemy’s outermost sphere was that of the Primum Mobile — first movable. This explained the daily motion of the stars around the Earth, and was the site of Empyrean (highest heaven), home to the First Mover that first set off the motion of the universe.
By the time the emperor Theodosius made Christianity the official religion of the Roman Empire in 380 AD, the Ptolemaic Model was taken as a given across the empire. It is no surprise then, that Christian theologians eagerly adapted Ptolemy’s system into their theology. The Prime Mover became the Christian god, and the Empyrean sphere became Christian Heaven. Below the throne of God, the “music of the spheres” rotated in perfect harmony — a music once heard by human ears. However, since Adam and Eve’s Fall, humanity, condemned to hardship within the sublunary sphere of Earth, could no longer hear God’s music. Only the righteous, the saintly, the most pious, could ascend to rejoin God in the outermost sphere of the cosmos, upon their death.
And that’s how the universe remained for over a thousand years, until a certain Polish astronomer began writing the treatise that would alter the very structure of the cosmos, in the 16th century.
The Earth’s orbit around the Sun actually does cause a slight change in the position of the stars in the night sky (a phenomenon known as a stellar parallax), but it’s so small it wasn’t observable until as late as the 19th century, when the first telescopes powerful enough to see it were developed.
I originally intended to include the history of geocentrism in Chinese, Indian, Islamic, and Mesoamerican thought in this post too, but it would have become far too long. Hence, the history outlined here is almost entirely Eurocentric, but I’ll tackle the other cultures some other time, I promise!
The exception was Aristarchus of Samos (c. 310—230 BC), who developed the first known heliocentric model of the universe, with the Earth rotating around the Sun. Though his ideas were common knowledge among ancient Greeks, they were rejected in favour of the geocentric models of Aristotle and Ptolemy. It would be left to Copernicus in the 16th century to convince the world of the truth.
For those interested, a brief history of the mathematics in question:
We don’t know the exact dating, but it seems the 360° circle came into mathematics after Aristarchus, but before Hipparchus, who deployed it in his table of chords. If you imagine an arc of a slice of a circle, the chord is the line that subtends the arc. If we draw a line from the centre of the circle to perpendicularly bisect the chord, then one half of the bisected chord is the sine of one half of the bisected angle (this is why the sine function is also known as the ‘half-chord’ function). Though the sine and cosine functions weren’t discovered until the 6th century by Indian mathematician Aryabhata (476—550 AD), Hipparchus, and thus Ptolemy, were able to use the equivalent chord function of the following:
Ptolemy developed Hipparchus’ work by introducing Ptolemy’s Theorem, a relation between four sides and two diagonals of a cyclic quadrilateral. In a cyclic quadrilateral with four vertices, A, B, C, and D, the theorem states that:
Ptolemy further derived the chord equivalent of the half-angle formula:
Ptolemy used these derivations to create his own trigonometric tables, and thus calculate the eccentric orbits of the Sun, Moon, and planets. He was a smart man.
Even if Ptolemy himself said, “We consider it a good principle to explain the phenomena by the simplest hypothesis possible,” in Book III of the Almagest.
Fascinating subject and a great capture! It’s hard to fathom Aristotle being wrong about anything! Ancient Indian mathematician of the 5th century, Aryabhatta, actually had elements of heliocentricism in his capture of the universe, but I have not studied it closely. Look forward to your Eastern version:)
So interesting. Thanks, Mikey.