Geocentric Model

by Abby Cessna on June 17, 2009

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geocentric model
The geocentric model, also known as the Ptolemaic system, is a theory that was developed by philosophers in Ancient Greece and was named after the philosopher Claudius Ptolemy who lived circa 90 to 168 A.D. It was developed to explain how the planets, the Sun, and even the stars orbit around the Earth. The geocentric theory has existed even before Ptolemy though. This model has been described in various early Greek manuscripts, and as early as the 4th century B.C. Plato and Aristotle were writing about the geocentric model. 

As the Greeks noticed discrepancies between the way planets moved and the basic geocentric model, they began adjusting the model and creating variations on the original. In these models, planets and other celestial bodies move in circles that have been superimposed onto circular orbits around the Earth.

The Ptolemaic system, the most well-known versions of the geocentric model, was a complex interaction of circles. Ptolemy believed that each planet orbited around a circle, which was termed an epicycle, and the epicycle orbits on a bigger circle–the deferent–around the Earth. The center of the deferent is not the Earth, but a point near the midpoint of the distance between Earth and the equant. The equant was Ptolemy’s solution to some of the discrepancies that the geocentric model could not explain. The equant can be defined as the point at which an epicycle’s center always seems to move at the same speed. When an epicenter was at a different point on its deferent, then the planet moved at a different speed. To further complicate matters, each planet had a different equant. A diagram of the Ptolemaic system looks like a mess of overlapping circles.  

Despite its difficulties and complexity, the geocentric model survived well into the 16th century. One reason why the geocentric model held sway for so many years was because it fit in with a number of observations that the Greeks made. Those observations included the fact that things fall toward Earth and that – according to Venus’ brightness – it stays roughly the same distance away from the Earth. As theories evolved and more evidence was uncovered though, the geocentric model was slowly replaced by models developed by Copernicus and other astronomers.

Copernicus proposed a heliocentric model of the solar system – a model where everything orbited around the Sun. Today, with advancements in science and technology, the geocentric model seems preposterous. Simple tools, such as the telescope – which helped convince Galileo that the Earth was not the center of the universe – can prove that ancient theory incorrect.

You might want to check out one article on the history of the geocentric model and one regarding the geocentric theory

Here are links to two other articles from Universe Today on what the center of the universe is and Galileo one of the advocates of the heliocentric model.

Tune in to this podcast from Astronomy Cast for more information: the centre of the universe.

NASA Solar System Exploration
NASA Earth Observatory: Planetary Motion

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