Ptolemy begins by drawing a distinction between practical and theoretical philosophy. The distinction is not defined here although Ptolemy gives “moral virtues” as an example of the former and “understanding of the universe” as an example of the latter. Likely, he is referring to Aristotle’s work, Metaphysics (Ptolemy almost never cited sources and Aristotle is the one name which is later cited which indicates how much Ptolemy drew from him), which defined three types of philosophy: The theoretical (knowledge for its own sake), the practical (morality), and the productive (works of utility and beauty).
Following Aristotle, Ptolemy breaks down theoretical philosophy down into three sub categories: physics, mathematics, and theology.
- Ptolemy defines theology as “the first cause of the first motion of the universe” noting it “is completely separated from perceptible reality.”
- Physics he defines as that “which investigates material… and which concerns itself with ‘white’, ‘hot’, ‘sweet’, ‘soft’ and such qualities.”
- Mathematics is that “which serves to investigate shape, number, size and place, time and such”.
Where Ptolemy breaks with Aristotle (who maintained theology was the primary philosophy as it dealt with the first causes and thus comes before all others) is that Ptolemy states that theology and physics as mere speculation,
theology because of its completely invisible and ungraspable nature, physics because of the unstable and unclear nature of matter; hence there is no hope that the philosophers will ever be agreed about them; and that only mathematics can provide sure and unshakeable knowledge to its devotees, provided one approaches it rigorously.
But while Ptolemy may dismiss physics and theology as conjecture, the Almagest is certainly a work very much about the physical world. His insistence on mathematics is merely a framework through which he approaches these subjects in an effort to give them what he views as more legitimacy.
With his foundation for his natural philosophies thus defined, Ptolemy moves on to several points he takes to be fundamental.
1. The heavens appear to move as a sphere.
This comes from simple observation – the paths of the stars trace circles on the sky with ones nearer the poles making smaller circles. Those circles may dip below the Earth where we cannot see them, but we still observe arcs as should be expected and so we should not assume that the stars are extinguished when absent from our sight only to be rekindled as they rise again. For Ptolemy declares it absurd that
one whole area of the Earth has a kindling nature, and another an extinguishing one.
The reason Ptolemy gives for this (somewhat obtusely) is that the stars appear different from different places (following from the Earth being round), and so there can be no such place. As such, the only reasonable explanation for their motion is for it to be spherical.
2. The Earth is spherical
This follows the observation that the stars do not rise for everyone at the same time. Similarly, the timing of eclipses does not happen for everyone at the same time. He also notes that
if we sail towards mountains or elevated places…they are observed to increase gradually in size as if rising up from the sea itself.
Contrary to popular belief, the sphericity of the Earth was not questioned in Ptolemy’s time (the 2nd century CE). It was well established some 300-400 years earlier by Eratosthenes who used the differing lengths of shadows in different cities to estimate the circumference of the Earth to a reasonable degree of accuracy. However, Ptolemy does not include this in his reasoning despite assuming the same conclusion.
3. The Earth is the center of the universe
While this is obviously incorrect his reasoning is as follows: If the Earth was displaced from the center of the universe, the equinoxes should not occur (assuming that the sun still orbits the center from which the Earth has been displaced) and the geometry of eclipses should not work out. Additionally, if the Earth were not centered, then the celestial sphere would not be evenly divided as it appears. The projection of the horizon and/or celestial equator would be greater for one side of the Earth than the other. Since we observe 6 zodiacal signs at all time (half of the 12 recognized), the division is equal and at odds with the Earth being displaced.
4. The Earth is negligibly small in relation to the heavens.
This one is entirely correct. The first reason Ptolemy gives is that the stars appear of roughly equal brightness at all times. If the Earth’s diameter was an appreciable size of the distance to the stars, they would brighten as they approach their highest altitude (at which point they would be closest to the observer) and fade as they move away. Although Ptolemy did not hit on it, the lack of visible parallax is also an indication that the stars must be distant indeed.
5. The Earth does not have any motion
His evidence here is that all objects fall towards the center of the Earth. Ptolemy did not suppose that the Earth had any natural sort of attraction and thus assumed that these objects were simply trying to reach the center of the universe. As reasoning behind this, Ptolemy suggested that heavy objects fall “down” meaning away from the surface of the celestial sphere. But should an object pass the center, then it would be falling “up” (towards the surface of the celestial sphere) and since we do not observe such motion, movement towards the center of the sphere must be the natural motion.
Thus, if the Earth were displaced, it would simply fall back towards the center of the universe. And, since the Earth is heavier, it should fall faster leaving all objects not tied down behind as they would fall more slowly. Obviously Ptolemy had little understanding of how gravity really works as the acceleration is the same for objects regardless of their mass (since the additional mass creates more inertia to resist the increase in force). Since the Earth isn’t acceleration away from us, it must already be at the center of the universe with its center coincident.
In addition, Ptolemy assumes that the Earth does not rotate around the axis. While he concedes that there are no celestial observations which would discount this, it does not seem in accordance with terrestrial observations. Since the clouds are not fixed to the Earth, Ptolemy saw no reason that they would necessarily rotate the same way. And if they didn’t, then the Earth should rotate beneath them in the same manner it was proposed it rotates beneath the stars, and thus, we should see a prevailing motion from East to West.
But what if the clouds and other objects not attached to the Earth were somehow fixed to the air which was fixed to the Earth so they might rotate together? Then the nature of that which fixed them would prevent them from moving. Since we have winds from many directions and the clouds are free to move, this must not be the case and the Earth cannot move.
6. There are two primary motions in the heavens
The first of these motions is the overall motion of the sky from East to West. The second of these is the motion of the sun, moon, and planets slowly West to East at different rates.
From here, Ptolemy has established a set of observations and axioms from which to work. However, before closing out this first book, Ptolemy goes through an introduction to some of the math that will be necessary later on, but I’ll split that into a seperate post.