Aristotle and the Geocentric Universe

The fact that multiple hypotheses can explain observations is so important to understand that it should be illustrated through a specific example. Clearly the most relevant one for us is the different explanations of the apparent daily motion of the Sun, Moon and stars in the geocentric and heliocentric theories. It helps to begin with a fresh look at the evidence, so take two minutes to watch this video:

By studying time-lapse footage, we see clearly that all the stars in the night sky move along concentric circles. This phenomenon was known as well to the early astronomers, who didn’t have the luxury of time-lapse videos, but who spent nights studying unpolluted skies.

The concentric circular motion of stars across the night sky is potentially explainable in two very different ways. According to geocentrism, the night sky’s motion is explained by assuming that the Earth is motionless at the centre of the Universe, with the celestial objects located on a surrounding sphere that rotates once a day. In this case, the Earth’s being at rest, and the night sky’s rotation, are supposed to be what is really going on; they are the assumptions which, if true, would be the basic cause of the motion we observe. According to heliocentrism, the Sun and stars are actually not moving, but the Earth spins on an axis once a day. The Earth’s spin is thus supposed to be the cause of the apparent daily concentric motion of the celestial sphere.

A good scientist would look at the evidence in the above video and say, first of all, that they are not able to decide which of the two explanations is actually correct. Beyond that, one should look for additional evidence to support one theory and not the other. With a little thought, you should see that this problem is closely linked to the determination of the shape of the Earth, since, if the Earth is in fact spinning, its shape should have an effect on the apparent motion of fixed celestial objects. Let us consider how Aristotle dealt with the whole problem.

As discussed in Book II of his On the Heavens, Aristotle believed the Earth was “a sphere of no great size.” One piece of evidence he used to justify his claim was that “in eclipses the outline is always curved; and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth’s surface, which is therefore spherical” (On the Heavens, 297b26-29). Then, in order to say that the Earth is not only spherical, but that compared with the distance to the stars it is not of great size, he noted,

quite a small change of position on our part to south or north causes a manifest alteration of the horizon. There is much change, I mean, in the stars which are overhead, and the stars seen are different, as one moves northward or southward. Indeed there are some stars seen in Egypt and in the neighbourhood of Cyprus which are not seen in the northerly regions; and stars, which in the north are never beyond the range of observation, in those regions rise and set. All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size; for otherwise the effect of so slight a change of place would not be so quickly apparent. (On the Heavens, 297b32-298a8)

All of this is indeed correct, and Aristotle (On the Heavens, 298a15) quotes a figure of “400,000 stades” from “those mathematicians who try to calculate the size of the earth’s circumference” (in fact, this is about 40% the actual value of 40,000 km).

Now, Aristotle knew all of this. He was aware that the celestial sphere apparently rotates about a fixed axis; that different stars are visible in different seasons near the equator, but in the north there are stars near the North Star which never set at any time of year. He therefore inferred correctly that Earth is a sphere of no great size when compared with the distance to the stars. He was even aware of the possibility that the Earth might actually be spinning “about the axis of the whole heaven” (On the Heavens, 293b31), as he noted that Plato had written this in Timaeus, and even argued against it.

The shape of the Earth was, for Aristotle, closely tied to his idea of geocentrism, as he thought both could be explained by observations of the substances, earth, water, air, fire and aether (the stuff he assumed the heavens to be made of). The substance “earth” was understood to always move towards the centre. As Aristotle (On the Heavens, 296b22-25) noted, “heavy bodies forcibly thrown upward return to the point from which they started, even if they are thrown to an unlimited distance,” and (On the Heavens, 295b21-23) “the place to which any fragment of earth moves must necessarily be the place to which the whole moves; and in the place to which a thing naturally moves, it will naturally rest;” therefore, he concluded that the Earth is at rest at the centre of the world, and that it must be a sphere.

Aristotle similarly argued that water moves towards the centre, but is lighter than earth and therefore sits above it. He held that air and fire naturally move upwards, fire being lighter than air, and that aether, of which the Sun, Moon, stars and planets were all composed, had to be something else entirely—a fifth element that is not of the Earth—which has a natural uniform circular motion about the centre, where the Earth rests. This is how Aristotle explained the apparent vertical motion of the Earthly substances along with the apparent uniform circular motion of celestial objects.

If you recall the above video displaying the concentric motion of the stars through time-lapse photos, it should take little imagination to understand how this principle of uniform circular motion of the cosmos came to be accepted by ancient philosophers. Plato argued that humans see only a distorted shadow of the truth, and that the heavens, with their uniform concentric motion, represent perfection. The idea was appealing, it appeared to fit with the facts from which it was derived, and, through Aristotle, it mostly stuck.

The first lesson that you should take away from this discussion is that Aristotle did carefully consider many options in constructing his theory, and that he used empirical evidence to support his own inferences. In fact, his usual method was to begin with a discussion of the many theories put forward by earlier philosophers, offering reasons why their ideas should be accepted or rejected. Then he would move on, in light of the ideas that had been proposed and the possible objections he could think of, to work out which principles seemed to best explain the phenomena. “Hence,” he said, “a good inquirer will be one who is ready in bringing forward the objections proper to the genus, and that he will be when he has gained an understanding of all the differences” (On the Heavens, 294b11-13).

The second lesson you should take from this discussion is that, even when theoretical physics is done with the greatest possible care, so that all possible explanations of the phenomena are considered in light of the best knowledge of the day before one in particular is selected, it may yet turn out that the theory is wrong in almost every detail. This was the case with Aristotle’s theory, as our discussion in module 3 of Galileo’s contributions to Astronomy will show.

And so, the lesson learned in science has been to maintain a sense of suspended disbelief. We have no access to the things that exist—not in and of themselves, anyway—but to the signals that they emit; the phenomena. We observe the things that happen, not the things that are. We know this, and have therefore developed a healthy sense of scepticism towards our hypotheses.

Our curiosity guides us as we seek to understand the world despite knowing we can never be sure we’ve got it right. We continually refine both our knowledge and our understanding—working from that knowledge to infer the most consistent understanding, and then working from the picture of the world that we think we understand to think up new questions that we can go out and investigate.

The greatest advances have been made, as we shall see, when the observations became detailed enough to show that things really aren’t happening quite as expected, given what we thought we understood. When that occurs, we finally have reason to stop believing that our hypotheses may be correct—to stop suspending disbelief for the sake of further inquiry—and revolutionary ideas are brought forward which fundamentally alter people’s worldviews. Indeed, even the word “revolution” comes from the title of Copernicus’s book, De Revolutionibus, which aimed to explain the motion of the planets by moving the Earth from the central location it had held since Aristotle’s reasoning put it there, to an orbit that revolves, along with all the other planets, about the Sun.