G12. The “System Of The World”: How The Heavens Drove Mechanics

It is an interesting and at first a seemingly odd fact that the development of mechanics was stimulated more by the study of the heavens than by the study of simple everyday motion on Earth. There are several reasons why. First, let it be noted that everyday motion on Earth is in fact rarely very simple. The ever-present friction accompanying all terrestrial motion complicates matters, and many interesting examples of motion on Earth are too short-lived to be easily studied. Celestial motion, by contrast, is durable and frictionless. It was easier for scientists to accumulate accurate data on the motion of Mars than on the motion of a falling stone. Also the motion of the Sun and planets and stars had something to contribute to the practical goals of time-keeping and navigation, whereas the practical fruits of studying terrestrial motions were less obvious. Probably as important as either of these considerations were the psychological and religious factors. The heavens hold for humankind a natural fascination with which mundane motion cannot compete. A modern person who never thinks twice about the technological wonders that surround us may look skyward with awe, marveling at the structure of the universe. We humans have always been concerned about our place in the universe. Even before the conception of the universe expanded to its present enormity, people must have been conscious of themselves as a tiny part of the whole. The Greek philosopher who believed himself to be at the center of the universe must have been nearly as humbled by thoughts of the giant cosmos surrounding him as we modern humans are by thoughts of the inconspicuous corner of the universe assigned to us and our Earth.

For the ancients, the study of the heavens had a particular religious significance that it has, by necessity, now lost. The ancients wanted a physical location for the gods and things heavenly. For all ancient civilizations the celestial region was to some extent sanctified. At least until the early seventeenth century the idea persisted that the heavens were made of purer stuff than the Earth. Galileo with his telescope struck the most serious blow against the sanctity of the heavens when he took note of the scarred surface of the Moon. Newton completed the job of rendering the whole universe earthy.

Even before the time of Aristotle, ancient observers had accumulated remarkably accurate data on celestial motions, and Aristotle knew that his system of the world must conform to these data. At the same time there were no quantitative data whatsoever on aspects of terrestrial motion, and Aristotle apparently saw no need to accumulate any. In the centuries after him, astronomers made ever more refined measurements of the positions of the Sun, Moon, and planets. In step with these, scientists developed ever more elaborate and complex pictures of the structure of the universe. By the year 1600, the accuracy and extent of astronomical measurements were truly astonishing, and the mechanical structure of the then-known universe amazingly complex. At the same time there was as yet no general understanding of uniformly accelerated motion in a straight line, and there were no measurements of motion as simple as that of a swinging pendulum. Galileo was the first to deal carefully with these examples of terrestrial motion.

It is worth mentioning one other factor that contributed to the remarkable disparity in the degree of advancement of the celestial thread and the terrestrial thread in the study of motion. Before the seventeenth century, science was observational, not experimental. Scientists were interested in the planets, so they observed the planets. In modern science, the path to the things of interest may be much less direct. We approach the subject of mechanics now through spring balances, falling balls, and carts carrying weights not because these things are of much interest by themselves, but because they enable us to formulate concepts and laws which then give us dominion over a vast part of nature, including the planets and other things that are of interest. Galileo was perhaps the first scientist to see clearly the value of indirection. Before him the path to the planets was direct observation. After him it was indirect, via experiments and theories leading to a scheme of understanding that embraced the planets. Actually, because the knowledge of planetary motion developed sooner and more fully than the knowledge of terrestrial motion, it is equally true to say that the path to an understanding of terrestrial motion was via the planets. When Newton drew the celestial and terrestrial threads together, they reinforced each other. It was the accurate planetary data that provided the best confirmation of the terrestrial laws of motion.

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