Though the geocentric model stood the test of time for nearly 1,500 years and could explain some observations of the cosmos as well as conforming to religious beliefs at the time, it was by no means 'simple'. Objects obeyed the rules of 'natural motion' meaning they orbited Earth at the same speed.All motions in the sky should follow circular paths as objects are attached to spherical shells.The planets, sun and stars revolve around Earth.The Earth is the center of the universe and is stationary.
If observations of a real event match predictions made by a model then we know the model is a good fit however, if the observations do not match predictions made, then the model needs to be reworked.Īccording to Penn State University, the ancient Greeks firmly held onto several scientific assumptions that formed the basis of the geocentric model: Scientific models are used to test our understanding of the laws of science by predicting the behavior of a system. (Image credit: via Getty Images) Assumptions of the geocentric model The geocentric model was refined by Claudius Ptolemaeus (also known as Ptolemy) in his treatise Almagest. Related: What's the story behind the stars? According to NASA, The geocentric model also conformed to religious beliefs at the time as many Greek philosophers and astronomers believed that the gods created man and the heavens are divine then of course we must lie at the center of it all. It could explain why stars appear to rotate around Earth once per day and why planets move differently to stars. As stars appeared to move much slower than the planets, they were placed in the outermost sphere, furthest away from Earth, according to Lumen Learning.ĭespite the complexity of the model, this view was widely accepted for many years. The order of the solar system with regards to the geocentric model, according to Penn State University is Earth (stationary and at the center), moon, Mercury, Venus, sun, Mars, Jupiter and Saturn.
#Retrograde motion of planets series#
They believed that the Earth was at the center with the sky or 'heavens' rotating around it in a series of layered spheres.Īccording to the educational website Lumen Learning, Ptolemy's complicated geocentric model stated that a planet moves in a small circle (known as an epicycle), the epicycle then moves around Earth in a larger circle (known as a deferent). When gazing up at the night sky from what feels like a fixed reference point and witnessing the stars and planets dance across the sky we can understand why the ancient Greeks adopted the geocentric view of the universe.
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