And yet from the time when it was first realized that it is the same Sun which rises and sets every day; that it is the same Moon, the same stars, the same constellations which revolve about us, those very facts forced one to admit with absolute certainty that there must be empty space underneath the Earth, to let the stars of the firmament pass from their setting to their rising. This first recognition was a turning-point. The admission of the Earth's isolation in space was astronomy's first triumph. It was the first step, and indeed the most difficult one. Think of it! To give up the foundations of the Earth! Such an idea would never have sprung from any brain without the study of the stars, or indeed without the transparency of the atmosphere. Under a perpetually cloudy sky, human thoughts would have remained fixed on terrestrial ground like the oyster to the rock. The Earth once isolated in space, the first step was taken. Before this revolution, whose philosophical bearing equals its scientific value, all manner of shapes had been imagined for our sublunary dwelling-place. In the first place, the Earth was thought to be an island emerging from a boundless ocean, the island having infinite roots. Then the Earth, with its seas, was supposed to be a flat, circular disk, all around which rested the vault of the firmament. Later, cubic, cylindrical, polyhedric forms, etc., were imagined. But still the progress of navigation tended to reveal its spherical nature, and when its isolation, with its incontestable proofs, was recognized, this sphericity was admitted as a natural corollary of that isolation and of the circular motion of the celestial spheres around the supposed central globe.