Aristotle’s theory of the solar system was a specific theory concerned only with what one can actually see moving in the sky. By contrast, the theory of mechanics together with the law of universal gravitation provides a general explanation, embedding the knowledge of our own planetary system in a theoretical explanation of all possible planetary systems. We look upon our planetary system as an accidental special case, one among an infinity of possibilities. There are, we now know, systems with more and fewer planets, with different masses, different relative spacings, differently shaped orbits. The same theory that explains the motion of our actual planetary system can explain the motion of any other. And, in addition to discovering planetary systems of other suns, we have created our own planetary system of artificial satellites about the Earth, governed by the same laws as are the planets moving around the Sun or around any star. For our own local planetary system, we can both predict and control.
Apart from its role in stimulating invention and exploration, our modern theory of mechanics engenders a characteristic point of view about nature quite different from the ancient view. We look upon our planetary system as something that exists by chance and moves as it does because it happened to get started in a certain way. Conceptually it is in no way unique, because we can imagine (and even observe) other planetary systems where the laws governing their motions are the same as the laws governing all motion. According to the Greek view, which persisted for a very long time, our planetary system is a unique system with a unique set of rules to explain its motion. All the emphasis was on finding out by careful observation how the planets do move and then explaining this motion in terms of a mechanism as simple as the observations and human ingenuity and human prejudice permitted. There was no reason why the explanation should explain anything else. It was a closed explanation, with the single goal of explaining a single structure (to be sure, a very elaborate and complex structure). By contrast, the modern explanation is open, enlarged by a general theory to encompass other kinds of motion, real and hypothetical, without limit.
For close to twenty-five centuries philosophers and scientists have dared to cope with the problem of understanding the structure of the universe, or, in the somewhat more modest terminology of the seventeenth century, the system of the world. The parts of the world known to the Greeks were the Sun, Moon, and Earth, five planets (Mercury, Venus, Mars, Jupiter, and Saturn), a multitude of stars, and occasional comets. No more constituents of the world were discovered until the late sixteenth century, when a “nova,” or new star, appeared. For about two thousand years, designers of world systems had an unchanging and seemingly unchangeable list of parts to assemble. The problem of designing a world system was looked upon as a mechanical problem: Is there a mechanism whose motion reproduces the observed motion? Throughout those two millennia, as the most fashionable or most successful world systems underwent many alterations, four unchanging assumptions dominated almost all thinking on the subject of the system of the world. (1) The Earth is at rest at the center of the universe. (2) Celestial motion is based on circular motion. (3) The universe is of finite size with well-defined boundaries. (4) There exists a simple and regular pattern of motion in the heavens that can be described mathematically and understood by humans. In short, celestial motion is neither capricious nor unduly complicated.
These assumptions shaped Greek thinking about the system of the world as early as 400 BC. In 1600, the challengers of some of these assumptions were only beginning to be listened to. By 1700, three of the four assumptions had collapsed.1 Only the last, the faith in simplicity, remained, and it remains today, strengthened by three centuries of astonishing progress in science.
1 Whether the universe is finite or not is still an open question. Nevertheless, the assumption that it must be finite disappeared from science after Newton.