Popular science history paints a picture of the Greek geocentric model dominating astronomical thought beginning around the 3rd century BCE, and being the favored model for ~1,500 years. Then, suddenly (it suggests), astronomical thought was overhauled at the birth of the Renaissance by brilliant astronomers such as Copernicus, Kepler, and Galileo, all of whom rejected placing the Earth at the center of the cosmos.
But these sources are generally quiet on why this shift occurred. If mentioned at all, sources generally suggest that it was because the Ptolemaic geocentric model was too complicated – overly burdened with epicycle and equants. Heliocentrism, in comparison, was simple – elegant, even.
Yet, Copernicus’ heliocentric model was still rooted in the Greek philosophical principles of uniform circular motion. Thus, it too was forced to adopt many of the complications we’re regularly told were the reason for rejecting Ptolemy’s model – epicycles included.
So, why then, did Copernicus actually turn his back on over 1,500 years of astronomical thought?
The answers are an interesting glimpse into the astronomical paradigm of the 16th century.
To find out Copernicus’ thoughts, we can examine the first book of his masterwork, De Revolutionibus.
The Force Needed to Sustain Geocentrism
The first reason he gives applies to the forces involved:
Surely if [Ptolemy’s reasoning for the geocentric model] were tenable, the magnitude of the heavens would extend infinitely. For the farther the movement is borne upward by the vehement force, the faster will the movement be, on account of the ever-increasing circumference which must be traversed every twenty-four hours.
– Copernicus, De Revolutionibus, Book I, Chapter 8
Copernicus’ writing of De Revolutionibus predated Newton’s Principa by over 140 years. The notion that “an object in motion tends to stay in motion” was, therefore, not yet one in the scientific consciousness.
Instead, natural philosophers believed that the natural tendency of objects was that of rest and the only way an object could be kept in motion was through an application of force.
In the Ptolemaic geocentric model, the Earth did not rotate on an axis. Instead, the stars were all affixed to the surface of a sphere at an immense distance which rotated about the Earth every day along with the rest of the cosmos. Copernicus criticizes the absurd amount of force he supposed would be necessary since, “things to which force or violence is applied get broken up and are unable to subsist for a long time.”
In other words, Copernicus believed that the force that should keep Ptolemy’s geocentric model going would necessarily destroy it.
The heliocentric model avoids this by making the motion of the stars and planets around the sky every night not actual motion, but apparent motion caused by the rotation of the Earth about its poles. This would require a far smaller force since the Earth is smaller than the stellar sphere. Indeed, this completely removes the need for the motion of the stellar sphere, and now the planets and Sun can move far more slowly, and thus would have a much reduced force on them.
To be fair, various astronomers had considered the possibility that the cosmos was geocentric, but did allow for the Earth to rotate on its axis. However, the Ptolemaic cosmos with its static Earth was still the predominant model of the day, which is why Copernicus attacks it with little mention of other authors.
But, if you’re willing to accept that the Earth rotates on its axis, why wouldn’t you accept that it has other motions too?
Early Musings on Gravity
I myself think that gravity or heaviness is nothing except a certain natural appetency implanted in the parts by the divine providence of the universal Artisan, in order that they should unite with one another in their oneness and wholeness and come together in the form of a globe. It is believable that this affect is present in the sun, moon, and the other bright planets and that through its efficacy they remain in the spherical figure in which they are visible, though they nevertheless accomplish their circular movements in many different ways.
-Copernicus, De Revolutionibus, Book I, Chapter 9
To understand this, we should briefly examine Ptolemy’s thinking on gravity. In the Almagest, Ptolemy opines that there is some point in the universe towards which all things fall unless they are supported. Thus, the Earth, being unsupported by a celestial sphere, must fall towards this point and thus, is the center of the cosmos; ergo, geocentrism.
Copernicus suggests that, perhaps gravity is just an innate force, and it would have the property to make things round. And since the Sun and moon are obviously round, perhaps they too have gravity. This removes the need for the central point to the cosmos that Ptolemy relies on, undercutting Ptolemy’s argument.
Elongation of Inferior vs Superior planets
How unconvincing is Ptolemy’s argument that the sun must occupy the middle position between those planets which have the full range of angular elongation from the sun [i.e., Mercury and Venus] and those which do not [i.e., Mars, Jupiter, and Saturn] is clear from the fact that the moon’s full range of angular elongation proves its falsity.
– Copernicus, De Revolutionibus, Book I, Chapter 10
Here, Copernicus is taking aim at the argument that the Sun must be between Venus and Mars due to a division in the angular elongation (the distance from the Sun) inferior and superior planets are able to have. Specifically, Mercury and Venus are never more than 24º and 45º away from the Sun respectively. Meanwhile, Mars, Jupiter, and Saturn can be any angular distance from the Sun (although they are always found along the ecliptic).
Ptolemy explains this by matching the mean (or average) speeds of Mercury and Venus to that of the Sun. Therefore, their getting ahead of and falling behind the Sun’s motion is due only to their epicycles. The other three planets had mean speeds unrelated to the Sun, allowing their centers of motion to drift anywhere along the ecliptic relative to the Sun.
The Ptolemaic order of the planets was largely correct; Ptolemy had ordered them according to speed. Ignoring the Sun and moon momentarily, this meant the planets, in increasing distance from the Earth, were ordered Mercury, Venus, Mars, Jupiter, and Saturn.
The Sun was inserted between Venus and Mars, again based on its speed. But, this conveniently meant that the Sun’s sphere provided a division between planets which were fixed to the Sun (Mercury and Venus), and those that could obtain any elongation (Mars, Jupiter, and Saturn). And astronomers of the day used this division as evidence that that positioning of the Sun among the planets must be correct.
But the moon, Copernicus tells us, upends this argument, because the moon is the innermost sphere and it is able to have any elongation, just like the outer planets.
Keep in mind, the nature of the moon, Sun, and planets was still quite uncertain at this time. Quite frequently, the term “planet” can include all of them. Hence why Copernicus considered their nature all together in this point.
Apogee & Perigee are Aligned with the Sun
For, it is manifest that the planets are always nearer the Earth at the time of their evening rising, i.e., when they are opposite to the sun and the Earth is in the middle between them and the sun. But, they are farthest away from the Earth at the time of their evening setting, i.e., when they are occulted in the neighbourhood of the sun, namely when we have the sun between them and the Earth. All that shows clearly enough that their center is more directly related to the sun and is the same as that to which Venus and Mercury refer their revolutions.
– Copernicus, De Revolutionibus, Book I, Chapter 10
Copernicus’ next argument has to do with the position of the planets when at their farthest points to Earth versus their closest points. These are known as apogee and perigee, respectively.
What Copernicus is indicating is that planets always seem to have their apogee when they are nearest to the Sun. This is a natural consequence of a heliocentric model (because the planet is on the opposite side of the Sun), but the geocentric model has no special cause for this.
This is easiest to understand if we think about a superior planet, like Mars, in the context of the heliocentric model. If we think of the closest Mars can be to Earth (perigee), it occurs when the Sun, Earth, and Mars are all in a straight line, in that order. When that occurs, Mars would be rising in the evening, being highest in the sky around midnight.
Conversely, the furthest Mars could be from us, is when it is on the opposite side of the Sun. It’s still on a straight line, but this time the order would be Mars, Sun, then Earth. When this occurs, Mars is setting in the evening (although we couldn’t see it because it would be too close to the Sun to be visible).
What Copernicus is pointing out is that this is true for every planet – they’re all tied to the Sun in this manner. Thus, he tells us, the Sun clearly has some special privilege.
Venus’ Massive Epicycle
Moreover, there is the fact that the diameter of the epicycle of Venus – by reason of which Venus has an angular distance of approximately 45º on either side of the sun – would have to be six times greater than the distance from the center of the Earth to its perigee, as will be shown in the proper place. Then what will they say is contained in all this space, which is so great as to take in the Earth, air, ether, moon, and Mercury, and which moreover the vast epicycle of Venus would occupy if it revolved around the immovable Earth?
– Copernicus, De Revolutionibus, Book I, Chapter 10
Epicycles are often cited as one of the biggest problems with the Ptolemaic geocentric model. And that is precisely what Copernicus is taking aim at here. That’s not to say that Copernicus was fundamentally against epicycles. Indeed, his own adherence to uniform circular motion forced him to include epicycles in his model. But what Copernicus is criticizing here is the size demanded by the Ptolemaic model for Venus in particular.
As discussed above, the mean motion of Venus is tied to that of the Sun. So it can only deviate from that position based on its epicycle. Thus, to get 45º away from the Sun, it was going to need a massive epicycle. One so large, it would take Venus crashing through the spheres of both Mercury and the Moon. The latter was particularly problematic because of a belief about the nature of matter.
The natural philosophy of the time was still alchemical, with four terrestrial elements (earth, fire, air, and water) and one celestial element (æther, or quintessence). It was held that the celestial element was eternal and unchanging. “Incorruptible,” as they would phrase it, which is why the heavens were so pure and consistent. It was only on Earth that we had the other four classical elements, which were “mutable” or “corruptible”. But where does that division between the incorruptible and corruptible take place? Greek astronomers placed it at the sphere of the moon which was the closest to Earth in the geocentric model.
Conclusion
However, because Venus’ epicycle would be so big, it would cross into this realm. Thus, there becomes a logical contradiction as you’d have the celestial matter diving in and out of the terrestrial realm which was not something that was considered acceptable.
Ultimately, these arguments were only partially convincing to astronomers of the time. We know that Copernicus’ work was widely read. However, it was not quickly adopted.
Even after Kepler revised it, sweeping away the Ptolemaic equants and epicycles and replacing them with ellipses, geocentrism still took quite a bit of time to be fully dislodged. Newton’s theory of gravity gave a compelling theoretical reason to give centrality to the larger object, but it was the discovery of the aberration of starlight and the parallaxes of stars that finally disproved the geocentric model.