For the better part of the past two millennia, most European and Arabian cultures accepted an explanation of gravity and of the universe that had been developed by the Greek philosopher Aristotle (384 BC-322 BC). According to Aristotle, the universe was made up of a group of concentric spheres (one sphere inside another) with Earth at the very center. Surrounding the sphere of earth was the sphere of water, then the sphere of air, the sphere of fire, and finally, encompassing all the others, the sphere of the heavens. Tied in with this concept was the idea that all matter was composed of four basic elements: earth, water, air, and fire. Aristotle said that if an object contained more of one element than of any other it would try to move to its respective sphere. In other words, using this type of thinking, the reason a rock falls when you drop it is because it contains more of the element earth and, therefore, tends to move towards the Earth.It is proof that Aristotle valued pure thought over experimentation when he claimed that heavier objects fall faster than lighter ones. This idea seemed so natural to him (and to practically everyone else) that he never even bothered to make sure it was correct. Instead, he supported it with the meager defense that a rock falls faster than a feather. In reality, it is a feather’s high air resistance, not its weight that causes it to fall slowly, making this a bad test.
Accurate or not, however, Aristotle’s theories were revered as solid fact up through the first half of the second millennium. The only notable amendment came in the second century AD when Claudius Ptolemy (approx. 100 AD-170 AD, pronounced Tolemy), a Greek astronomer and geographer, developed Aristotle’s concentric spheres universe into an elaborate cosmological model. Here again, Earth was placed at the center of the universe, while the Sun, moon, planets and stars all orbited around it, attached to their own respective spheres. However, Ptolemy could not come up with a good, scientific explanation as to what actually caused the planets to move.
