For the greater part of recorded history, the heavens and their motions have been subject to heated scrutiny and various hypotheses. At first glance, we observe several bodies that move in our sky and conclude that because of their regular appearance, they must be revolving around us. Claudius Ptolemy of Alexandria was a well known astronomer who held such a geocentric theory. Ptolemy was so opposed to the idea of the earth having any motion that he was forced to attribute various complicated motions to the remaining heavenly bodies for his theory to function. Enter Nicolaus Copernicus, Renaissance astronomer. Copernicus had a passion for exploring the motions of the heavenly bodies and studied Ptolemy’s geocentric model with the utmost care. Copernicus agreed with Ptolemy on several theories, mostly concerning the shape of the heavenly bodies. Their progression of proofs is similar, yet they eventually disagree on the motion and placement of those bodies. How do they reach different conclusions if their methods were so similar? To answer this question, both of their arguments must be examined.

To begin explaining the motion of any heavenly body, it is necessary to understand its shape and the shape of what contains it. Through observation of the heavens, Ptolemy states that they can only be described as spherical. If they were anything other than spherical, the distances between stars and their respective distances from earth would vary with each revolution. Instead, some stars are ever-visible and seem to rotate about a fixed pole. The stars that are closer to this pole are ever-visible and rotate in a smaller circle. The stars that are further away from the pole travel in a larger circle about the pole, to the point of vanishing into the earth until they reappear in the next revolution. Thus, the heavens are indeed spherical in shape and rotate about a pole. Copernicus approaches the shape of the heavens from earthly observations, ascribing the desire for continuity found in liquids to that of the heavens. Found in the theories of both, is the idea that the circle is the most perfect shape and that the sphere then, is the most perfect figure. Like Ptolemy, Copernicus feels a need to utilize the most perfect figure when describing the divine shape of the heavenly bodies.

The shape of the earth is likewise agreed upon as being spherical. Ptolemy refutes any other possible shapes by means of observation. If the shape were concave or plane, then either the people in the west would be the first to see sunlight, or the sun would rise at the same time for all, both of which are contrary to reality. Nor could the shape be any other polyhedra for the same problem would arise for each group of people on each face of the figure. A cylindrical shape is the only shape besides a sphere to accommodate the east to west motion of the sun, but in relation to the fixed stars there is still conflict for there would be no ever-visible stars. Therefore the only suitable shape for the earth is a sphere. Copernicus added that when traveling north, certain stars are no longer seen to rise in the south, while new stars are seen to rise in the north. This maintains that the earth is contained by poles that are seen to move in ratio to the observer’s travel on earth. Only a sphere possesses that quality. The qualities of the heavenly bodies have thus far been agreed upon, but now the theories diverge.

Ptolemy believed the earth to be fixed in the universe as well as being its center. For if it were not, he provides two possible examples. The earth is either (A) not on the axis of the universe but equidistant from both poles, or (B) on the axis but removed towards one of the poles. The former is not possible because either the equinox will never be experienced by the observer, or if it is, it would not occur half way between the summer and winter solstices. But it is agreed that these intervals are equal everywhere on earth, since everywhere the increment of the longest day over the equinoctial day at the summer solstice is equal to the decrement of the shortest day from the equinoctial day at the at the winter solstice. The latter, (B), is not possible either because then the horizon would bisect the heavens into unequal parts. This does not reflect reality because we observe six zodiacal signs above the earth at all times, therefore the horizon bisects the zodiac. A third option is given combining the two preceding it, but if neither are possible then a combination of the two is even less so. According to Ptolemy, the earth is at the center of the universe.

There is one crucial concept that is limiting Ptolemy’s conclusion thus far. The premise for his conclusion seems to focus on observations of the heavens. Ptolemy states that a relative displacement of the earth from the center of the universe would change our perspective so drastically that we would perceive not only a change in the sun’s path in the sky but even a shift in the celestial bodies. For this to be true, Ptolemy must neglect the immensity of the heavens and diminish it to a size more comparable to that of the earth. If the earth was moved far enough from the center of the universe, there might well be a change in the heavenly observations, but this is an extreme example. Ptolemy does not fully entertain the possibility of having the earth slightly off-center.

If the earth were to have motion, Ptolemy says it would be violent. The speed at which the earth would need to revolve to reflect night and day would be relatively faster than any other object in the sky, “neither clouds nor other flying or thrown objects would ever be seen moving towards the east, since the earth’s motion towards the east would always outrun and overtake them, so that all other objects would seem to move in the direction of the west and the rear.” (Ptolemy’s Almagest, 45). Therefore the earth is motionless, but it is also fixed in the center of the universe. He proves this by explaining motion on earth compared to the heavens. On earth, heavy objects fall in a straight line perpendicular to the earth’s surface, drawn to the earth’s center. Lighter objects do the opposite, rising in an upwards direction relative to the observer. Earth’s mass is such that it can sustain impact from objects dwarfed by its size, remaining motionless. Compared to the heavens, the earth in turn, is but a point in a sphere. Dwarfed by the heavens, the earth is pressed in from all sides, coming to rest at the center of the sphere.

If the earth is not responsible for the motions of the heavenly bodies, then those must be accounted for. Ptolemy sets out two principle motions. The first motion is a uniform rotation from east to west along circles parallel to the equator. This is known as the daily motion. The second motion is observed over a greater period of time, where all the planets rotate in the opposite direction from the first motion, about a different set of poles. Another aspect of this second motion is a constant deviation to the north and south. Ptolemy describes this motion as taking place on a circle inclined to the equator. Ptolemy admits that each planet has its respective motion which is much more involved than these primary motions, but to prove that the earth is the center of the universe he must make all the necessary accommodations to fit his theory. This is where the theory held by Copernicus thrives.

During the 1500s, opposing the geocentric model was bordering heresy. Copernicus claimed (cautiously) the possibility of a heliocentric model, with the earth having motion around the sun. To support his claim, Copernicus applied his logic to Ptolemy’s work. According to Ptolemy, if the earth were to have motion, it would necessarily be a violent one. If Ptolemy believed that and still ascribed some motion to the heavens, because of their great distance, their speed would have to be even more violent. “As a quality, moreover, immobility is deemed nobler and more divine than change and instability, which are therefore better suited to the earth then to the universe” (On the Revolutions, I.8). If motionless is akin to divinity, then it is the heavens that should be without motion. The earth then, can hardly be thought to rival the divinity of the heavens. It seems more likely that it be in motion. Then if it is in motion, it is hard to conceive it inhabiting the center of the universe.

Without the earth as the center of the universe, the next logical candidate is the sun. This certainly follows from a divine point of view since it is the provider of light for all the heavenly bodies and therefore its divinity supersedes motion. About this new center of the universe revolves the earth. These two astronomers have begun their arguments very much alike, but now they seem almost opposite one another. What caused this shift?

When a theory is being developed, eventually a decision will be made that does not make room for many other possibilities. What Ptolemy faced was a challenge in perspective. He perceived the heavens to have motion and he did not feel the earth moving. For me, this is relatable to being on a ferry docked next to another ferry. Ferries move so slow that motion is barely felt when they start moving. Several times I have been staring out the window at the ferry docked alongside, when suddenly I see motion. Since I can’t feel the motion due to the low speeds, I have to rely on my visuals. It is hard for me to distinguish which ferry is moving and which is stationary. Many times my heart has skipped a beat because I thought the ferry I was on was ramming into the dock, only to realize that the other ferry was moving.

The difference with Ptolemy is that he is too quick to deny the earth any motion, which then leads him to the conclusion that it must be in the center. This puts a lot of pressure on his theory. Now he must organize the explanations for the motions of all the other heavenly bodies in relation to the earth’s lack of motion. Copernicus is not so restrictive. He accommodates the motion of the earth, keeping his perspective relative to the celestial sphere. The earth is but a point in the sphere, so our view of the stars would not change so radically if we were not at the center of the universe.

Ptolemy attempted to place the earth not at the center, but did so to an extreme. If the earth was not at the center, then it must be so far removed from the center that our perception of the universe would dramatically change. If he had imagined the earth slightly off-center, then our perceived movement of the heavens around earth would not have been so affected. Yet he concludes that the earth must be in the center of the universe. This conclusion could be influenced by his extreme premises, or possibly by a some amount of geocentric predispositions.

Resources

Ptolemy, and G J. Toomer. Ptolemy’s Almagest. Princeton, N.J: Princeton University Press, 1998. Print.

Copernicus, Nicolaus, and Edward Rosen. On the Revolutions. Baltimore: Johns Hopkins University Press, 1992. Print.