What Is a Geocentric Model of the Solar System?
The geocentric, or Ptolemaic, model of the solar system is the astronomical model advanced by ancient people to describe the motions of the planets, Sun and moon. This model held that the Earth was the center of the solar system, and the other heavenly bodies orbited around it. Scientists now know that this model is incorrect and that the Sun is at the center of the solar system.
One problem with the geocentric model is that it only allowed for perfectly circular orbits, which reflected the gears and wheels with which these ancient astronomers were familiar. However, the path taken by the various planets are not perfectly circular. The Ptolemaic model accounted for the strange movements of the planets by asserting that they moved in two simultaneous orbits; one orbit took the planet in a small circle along its larger orbital path. Additionally, the theory held that the Earth was in an offset position within the circle.
However, modern scientists know that the reason such adjustments were necessary is that the planets do not orbit the Earth. Further, the planets and moons do not orbit the Sun in a path that traces a circle. Instead, the planets travel in elongated circles, called ellipses.