Meantime western Europe had remnants of the Ptolemaic system left, but the dim light of monasteries was not sufficient to let the scholars see inaccuracies. Saint Paul had quoted Aratus when he preached to the Athenians from Mars Hill, but Aratus had believed that the world was round, and that was a little fact which Saint Paul forgot to mention.
The Renaissance is commonly supposed to have started with the fall of Constantinople in 1453, but long before that the monks had recovered some of the old books from the Arabians. In the eleventh century and throughout the twelfth, wandering scholars were traveling to Italy and Spain where the Moors had brought what was left of ancient science back into Europe. They owned a heterogeneous collection of books. The fleeing students from Alexandria in the first centuries of the Christian epoch had taken with them only such volumes as they could not live without, never dreaming that their great storehouse would be burned in the Arab invasion, and eight hundred years passed before even those books returned to Europe. But the European monks were never at a loss to piece together what they had, though they were slow enough in adding anything new. These books over which they worked were the Latin translations of the Arabian translations of the Syriac translations of the Greek originals. They had been known in Europe for over a hundred years before the first European scholar even suggested that he might find the Greek texts and study them instead. But the scholars managed somehow to resurrect the Ptolemaic system. In 1256, a Yorkshire student named John Holy-wood published a book on spherical astronomy which was received with applause throughout Europe, and was reprinted fifty-nine times after the invention of the printing press.
With the fall of Constantinople still more of the classics came into popular circulation. Men had relearned the Greek language in Europe, and some of the older writers came into fashion to displace the writings of Ptolemy and Aristotle which had held undisputed sway for so long a time. Apian, in 1540, still accepted the Ptolemaic system, with its stationary, spherical Earth, but there were whispers abroad, and he must have heard them for there are faint echoes in his works:
"Controversies have arisen as to whether the first point of the ecliptic (which is divided into 12 signs) should be put into the eighth, ninth, or tenth sphere."
and a little later:
"The second chapter teaches, if I mistake not, the movements of the eighth, ninth and tenth spheres."
The heliocentric theory was in the air; and three years after Apian's book was published, the whisper turned into a roar. Some fifty years before, in Rome, a secret group of men who called themselves "Pythagoreans" had been seriously questioning the stationary Earth, and to Italy in 1496, went a young Polish student whose name was Nicolaus Copernicus. His book was published in 1543, and a few years later the works of the first master of the heliocentric theory were also republished and Aristarchus had his laugh at long last.
Once the Copernican system was accepted by a few scholars, its progress was rapid, both along the lines laid down by its own premises, and in the new fields of inquiry which it opened up. In 1796, Laplace announced his famous "nebular hypothesis." Earlier cosmologists had taken the analogies for the Earth's creation from eggs and trees and the creation of life they saw about them. Laplace, in the spirit of his times, chose for analogy a chemical process. As crystals form out of solutions on nuclei that already exist, so do solar systems form on existing nuclei by condensations of the gases that fill space. According to Laplace, the nuclei become separated from other similar formations, and each nebula as it grows larger draws to itself the gases from a larger and larger area. Eventually they break apart and when the break comes, each nebula becomes to a greater extent self-contained, and large volumes of space almost devoid of matter exist between them.
In this way a continuous attraction is in process; the nebulae not only attract the outside matter, but exert an attraction within themselves, gradually shrinking in size; and as they shrink they begin to rotate, going slowly at first, then faster and faster, like giant whirlpools of matter, big enough to suck all the surrounding ocean into their midst, until there is nothing left except the whirling masses about their centers. Once this motion is started, it continues, and as they grow smaller and smaller they go faster and faster (following the law of the conservation of angular momentum) until finally they assume a coherent shape in the form of a lens.
But as their velocity increases, a counter motion sets in, and the smaller nuclei which have been attracted to the main one, are thrown off by centrifugal force, still whirling around the center, but carrying on an independent rotation of their own at the same time; again smaller nuclei are split off, creating motion within motion, each separate particle circulating around the one from which it was thrown, and each group still revolving around the center of the whole system.
If this history be granted, the effects can be deduced. The main nuclei become separated by vast distances and each nebula becomes an island of its own, as it were, like the Sun and the solar system. Several centers form like the planets, and around them are others, now moons or satellites, all more or less in one plane, and all rotating in the same direction.
Various other happenings occur as well. These gaseous masses cool off more rapidly as their sizes decrease, so that the planets became solid long before the central mass of the Sun grew cool enough to solidify, and the satellites cooled off before the planets. All bodies and, to a greater extent, gases, contract as they grow cold; and therefore spin faster. Any one of them in a semi-solid state might become unstable and fly to pieces. Such a celestial catastrophe seems to have overcome one planet, and as a result the asteroids were formed.
In the same way any of the satellites might burst into fragments at a certain stage. The particles would disperse, but stay within the range of the planet; hence the rings of Saturn.
Many of the facts which we know qualitatively are explained by this theory, and for about a century it was generally accepted; but keen mathematical analysis has found serious flaws, and there is one really disturbing factor. Instead of rotating more or less parallel to the ecliptic, the axis of Uranus is very nearly at right angles to it. Also there are a few small satellites that go around the wrong way. These might be strays, picked up by a planet and permanently attached.