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Hipparchus (c. 190-120 B.C.) is considered one of the greatest figures in ancient Greek astronomy. He was the first to offer a detailed explanation of how objects move throughout the solar system. For this reason he is considered the true author of the geocentric (Earth-centered) model. His diagram consisted of seven large spheres for the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn-with the Earth in the center. Hipparchus was the first to introduce the concept of "epicycles," small secondary orbits that accounted for the periods in which the planets appeared to move backwards (called retrograde motion) with respect to the Earth. The inclusion of epicycles in Hipparchus' diagram turned the planetary paths from circles into elaborate figure-eight patterns.
If there were any question as to the dominance of the geocentric model of the solar system, Ptolemy, around A.D. 140, put an end to that. He essentially retold Hipparchus' version of the universe in a 13-volume catalogue entitled Megale mathematike systaxis (Great Mathematical compilation), while claiming all the credit, for which many accuse him of plagiarism. Ptolemy's book was so influential that, although the geocentric model originated centuries before him, it is today known as the Ptolemaic model.
Greek astronomer Aristarchus had come up with the sun-centered theory as early as 260 B.C. But in A.D. 140, another Greek astronomer named Ptolemy had convinced the scholars that the Earth was at the center of the solar system. His theory was accepted as truth for over fourteen hundred years.
In 1507 Polish mathematician and astronomer Nicholas Copernicus (1473-1543) announced a revolutionary theory for the structure of the solar system. It had long been assumed that the Earth was the center of the solar system, and that the other planets and the sun revolved around it. Copernicus proposed that the sun was at the center. He also suggested that the Earth was a relatively small and unimportant component of the universe.
Basing his calculations on a heliocentric (sun-centered) model, Copernicus developed a much simpler table of planetary positions than had existed previously. According to the geocentric (Earth-centered) model, the other planets had to move in strange ways to account for their positions relative to the Earth. For instance, Mars, Jupiter, and Saturn but not Mercury and Venus-were said to move in a reverse direction from time to time. Copernicus explained that this backward motion was merely an illusion that occurs because of the different lengths of the planets' orbits. Since Mars, Jupiter, and Saturn were farther from the sun, their orbits were longer than the Earth's orbit. Thus Copernicus said, the Earth "overtook" the other planets as it circled the sun on its shorter path. By the same token, Mercury and Venus, closer to the sun than Earth, have shorter orbits and race around the sun several times during an Earth year. Copernicus mistakenly assumed, however, that planetary orbits are perfectly circular. It was only determined a century later by Johannes Kepler that the orbits are elliptical, or oval-shaped.
In 1577 Danish astronomer Tycho Brahe (1546-1601) observed the elongated path of a bright comet. This observation caused him to question the two current models of the solar system and to devise a new one. The Ptolemaic, or geocentric, model placed the Earth at the center of the solar system, with the sun and other planets revolving around it. The Copernican, or heliocentric, model placed the sun at the center, with the planets revolving around it. Both models claimed that the planets and sun were carried around the sky by "spheres," which Brahe felt could not exist, since the comet had crossed several planetary paths. In Brahe's model, the sun and moon revolve around the Earth and all the other planets revolve around the sun. He did away with the planetary spheres. In this way he kept the Earth at the center of the solar system, which was not just a scientific theory, but a belief held by the Christian church.