EARLY COSMOLOGICAL THEORIES

The earliest cosmological theories known-from about 4000 BC-are those of the Mesopotamians, who believed that the Earth is the centre of the universe and that the other heavenly bodies move around it. The nightly motion of stars across the sky was explained by some ancients, such as Aristotle and the Greek astronomer Ptolemy, as the result of their being fixed on rotating crystalline spheres. The Greek astronomer Aristarchus of Samos maintained, about 270 BC, that the Earth revolves around the Sun. Mainly because of Aristotle's authority, however, the concept of the Earth as the centre of the universe remained generally unchallenged until 1543, when the Polish astronomer Nicolaus Copernicus published his theories in De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres). Copernicus proposed a system in which the planets revolve in circular orbits around the Sun, which he located at the centre of the universe. He attributed the rising and setting of the stars to the rotation of the Earth on its axis.

The German astronomer Johannes Kepler adopted the Copernican system and discovered that the planets move in elliptical orbits at varying speeds, according to three well-defined laws (since called Kepler's laws). Galileo, one of the first people to observe planets with a telescope, also rejected Aristotle's idea of the Earth as the centre of the universe and became a champion of the Copernican world view. The English mathematician and physicist Isaac Newton showed that Kepler's laws of planetary motion could be derived from the general laws of motion and gravitation that Newton had discovered, thus indicating that these physical laws were valid in the heavens as well as on the Earth.