Select the best introductory sentence for the paragraph from the following:

The Greek-Egyptian astronomer Ptolemy claimed that the earth stood still in space, with the sun, moon, and stars revolving around it. The bodies all moved at the same speed in perfect circular orbits. He believed that the universe was made up of a series of globes, one inside each other. Each globe was formed of a different kind of element. The earth was the innermost, surrounded by globes of air, fire, and gas.