The study of the structure of the universe is called cosmology. The ancient Greeks developed a cosmological model with the earth at the center of the universe while the moon, the sun, the planets, and the stars were points of light turning about the earth on large “celestial spheres.” This viewpoint was further expanded by the second-century Egyptian astronomer Ptolemy (the P is silent). He developed an elaborate mathematical model of the solar system that quite accurately predicted the complex planetary motions.
Then, in 1543, the medieval world was turned on its head with the publication of Nicholas Copernicus’s De Revolutionibus. Copernicus argued that it is not the earth at rest in the center of the universe—it is the sun! Furthermore, Copernicus asserted that all of the planets revolve about the sun (hence his title) in circular orbits.