The Copernican Revolution
The universe of the Greeks was a small sphere. The centre was the earth; the boundary, the thin shell of the fixed stars. Between, lay the orbits of the sun, the moon, and the planets. The whole heavens, it was believed, rotated once a day around the motionless earth. The daily rotation of the heavens, carrying along all the contents - sun, moon, planets, and stars - was directly responsible both for the small scale of the universe and for the thin boundary shell of stars. The larger the universe the more tremendous would be the speed of rotation. Deliberate efforts were made to keep the universe as small as possible. The orbits of the planets were packed together as closely as their supposed motions would permit; the stars were confined to a thin boundary shell, pressed tightly around the orbit of the outermost planet. The minimum radius eventually adopted was 20,000 times the radius of the earth, 80 millions of miles. Even so, a star on the celestial equator travelled 6,000 miles per second in its daily rotation.
The Greek conception of a small, closed universe was accepted almost without question until the Copernican revolution. Copernicus not only transferred the centre of the planetary system from the earth to the sun but also transferred the daily rotation from the heavens to the earth. No longer did the huge universe whirl madly about the motionless earth; instead, the earth itself, at a modest rate, was spinning in a motionless universe.
Thus Copernicus removed the necessity for a small-scale universe, and for a thin boundary shell of stars as well. He was primarily concerned with the planets and did not himself take the logical step of scattering the stars through infinite space. That step was reserved for an Englishman - Thomas Digges - who, forty-three years later (1576), assumed it as an obvious and welcome consequence of the new cosmology. The restless imagination, it seemed, had been loath to postulate a universal boundary but had been forced to that extremity by what appeared to be the hard facts of nature. Now there emerged a possible, alternative interpretation of those facts. A choice was presented between a small, bounded universe, centred around the observer, and a universe indefinitely large plus an unfamiliar principle of relative motion.
Definite observational evidence to guide the choice was slow to materialize. Nevertheless, the larger universe, with all its significance, was adopted quite early, on the grounds of simplicity and uniformity. It was recognized that the system of the planets, centred around the sun, was isolated and lonely in space; that the sun itself was but one of the myriads of stars scattered at vast intervals through the universe.