The civilization of Greece and Rome began to decline shortly after the time of Ptolemy. Fortunately, the discoveries of the ancient astronomers were preserved by Islamic astronomers, who found them and translated them into Arabic.
Astronomical knowledge was gradually reacquired in Western Europe. By the fifteenth century, the level of knowledge matched or exceeded that at the time of Ptolemy. The geocentric model of Ptolemy was almost universally accepted as the correct description of the solar system.
In the early sixteenth century, Copernicus proposed that the Sun rather than the Earth is the center of the solar system. In the heliocentric model, the daily and annual patterns of celestial motion are explained by the rotation and revolution of the Earth. Retrograde motion of the planets occurs whenever the Earth passes or is passed by another planet.
In the model of Copernicus the orbital distances of the planets can be found through observations and geometry. In contrast, the geocentric model makes no specific predictions about the relative distances of the planets.
Through his care in building and using astronomical instruments, Tycho Brahe was able to make observations of unparalleled accuracy. His regular observations of the Sun, Moon, and planets covered many years. His data replaced the ancient observations that earlier theorists had been using for centuries.
Tycho was unable to detect stellar parallax and thus rejected the model of Copernicus. Tycho proposed a model in which the Earth was orbited by the Sun and Moon but all of the other planets moved about the Sun.
Using Tycho's data, Kepler was able to discover the laws of planetary motion. His first law says that the planets move on elliptical paths with the Sun at one focus. The second law says that a planet moves so that a line drawn between the planet and the Sun sweeps out equal areas in equal amounts of time. This means that the product of transverse velocity and distance from the Sun remains constant as a planet moves about the Sun. The planet moves fastest when it is nearest the Sun.
Kepler's third law says that the square of the sidereal period of a planet is proportional to the cube of its average distance from the Sun. The third law implies that there is a common principle that governs the orbital motions of the planets.
Galileo's telescopic observations provided strong support for the heliocentric model. In particular, his observations that Venus shows all the phases from new to full could not be explained by Ptolemy's model of the solar system.
Galileo summarized his arguments for the heliocentric model in his book, Dialogue Concerning the Two Chief World Systems. The book put Galileo in conflict with church authorities and resulted in his persecution.
