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[[Image:Ptolemaicsystem-small.png|right|thumb|Rendition of a geocentric system from the Middle Ages.]]

[[Image:Ptolemaicsystem-small.png|right|thumb|Rendition of a geocentric system from the Middle Ages.]]

The '''geocentric theory''' is a system for describing the universe with the assumption that [[Earth]] lies at its center. It was extremely popular from ancient times until the 1600s, as it had better agreement with observation than any alternative. [[Ptolemy]]'s model was particularly effective at cosmological predictions.

The '''geocentric theory''' is a system for describing the universe on the basis that [[Earth]] lies at its center. It was extremely popular from ancient times until the 1600s, as it had better agreement with observation than any alternative. [[Ptolemy]]'s model was particularly effective at cosmological predictions.

However, during the 1500s and 1600s, it became clear that the theory had some serious flaws in it. In particular, [[Galileo]]'s use of the [[telescope]] to look at the skies revealed that [[Jupiter]] had at least four moons, and Venus had phases like the [[Moon]].  The work of [[Johannes Kepler]] showed that planets had elliptical orbits. None of these observations could be adequately explained by a geocentric universe. When [[Isaac Newton]] applied his newly discovered [[gravitation|universal theory of gravitation]] to the data that Kepler collected, he found that a heliocentric model was the only one that could explain all observed phenomena. By the 1800s, the spectacular successes of Newtonian theory and [[Maxwell's equations]] for electromagnetism had convinced practically everyone that the Sun is a preferred frame of reference, and that the laws of physics must be applied in that frame. The geocentric theory was considered to be profoundly mistaken, and the [[Heliocentrism|heliocentric theory]] correct.

However, during the 1500s and 1600s, it became clear that the theory had some serious flaws in it. In particular, [[Galileo]]'s use of the [[telescope]] to look at the skies revealed that [[Jupiter]] had at least four moons, and Venus had phases like the [[Moon]].  The work of [[Johannes Kepler]] showed that planets had elliptical orbits. None of these observations could be adequately explained by a geocentric universe. When [[Isaac Newton]] applied his newly discovered [[gravitation|universal theory of gravitation]] to the data that Kepler collected, he found that a heliocentric model was the only one that could explain all observed phenomena. By the 1800s, the spectacular successes of Newtonian theory and [[Maxwell's equations]] for electromagnetism had convinced practically everyone that the Sun is a preferred frame of reference, and that the laws of physics must be applied in that frame. The geocentric theory was considered to be profoundly mistaken, and the [[Heliocentrism|heliocentric theory]] correct.