Circles Within Circles: The Ptolemaic Universe

800px-Bartolomeu_Velho_1568The history of science is littered with ideas that, in retrospect, seem pretty silly. While it is easy to mock ridiculous notions from yesteryear, it is good to keep in mind that our ancestors were as intelligent as we are; they only lacked the sheer weight of knowledge that we have today, mostly because they were the ones figuring it all out in the first place. They were starting from scratch, so they were bound to make a few mistakes along the way.

Which leads us to questions of cosmology. These days, it is generally accepted in the scientific community that the Earth is only one of billions and billions of planets orbiting an average star on a distant arm of an average galaxy. The universe, according to current understanding, is an unimaginably vast place full of all sorts of odd and interesting stuff (the Nazis didn’t agree, but nobody in their right mind listens to Nazis.)

But, to the ancients, the modern view of the universe would seem preposterous. One of the leading cosmologies of the ancient world was the Ptolemaic System. It was a geocentric model, meaning it posited that the Earth was center of the universe and all things orbited around it, including the sun. This system floated around long before Claudius Ptolemy,  who lived from about 90 AD to 168 AD in the Roman Empire, gave his name to it. He refined the system into a model that could accurately predict the motions of stars and planets, despite being completely wrong.


Circles within circles

The Ptolemaic System was built around three central concepts. The first and foremost was that the Earth was the center of the universe, and everything orbited around it. The second was that everything in the Heavens moved in perfect circles, because everything in the Heavens was perfect and thus it should move in a perfect way, and the ancients regarded the circle as the perfect shape. The third concept, which was related to the second, was that the objects in the Heavens were perfect and immutable. Which is to say that they wouldn’t change their intrinsic properties, like shape or brightness.

The problem with these strict ideas about how the universe ought to be was that the actual, observed movements of the stars and planets didn’t match. Mars, for example, appears to move backwards in the sky, a phenomena known as retrograde motion. And Venus appears to undergo phases, which it should not do if the properties of the spheres were immutable.

To account for this, Ptolemy and his supporters came up with the concepts of deferents and epicycles. The deferent was the orbit of the plant around the Earth. An epicycle was a circle that essentially orbited around a point of the deferent. Roughly speaking, the point on the deferent that the epicycle orbited was called the equant. Each planet had its own equant and epicycle, and in some cases some epicycles had other epicycles. Charts from the time are a mess of circles within circles, but surprisingly it worked pretty well in describing the motions of the heavens. While some argued for a heliocentric model–which put the sun at the center of the solar system– most stuck with the Ptolemaic model because it worked.


The sun takes its rightful place

This was the state of affairs, at least until the 1500s, when the heliocentric model began to gain steam as a series of scientific heavy hitters — Copernicus, Brahe, Galileo, and Kepler — argued in its favor. The painful system of circles here there and everywhere worked in making calculations, but obviously these were difficult to carry out. A simpler system would make calculations simpler. Plus, the old system did not account for the growing number of observations about the universe. For example, Galileo turned his famous telescope to the moon, and saw mountains, valleys, and craters. He also saw moons orbiting Jupiter, which gave a small glimpse of the complexity of the wider cosmos.

Copernicus had already set out a model of a heliocentric system before Galileo came around, but the problem was that it predicted the motions of the planets no better than the old Ptolemaic model. It was only when Kepler came around, using Brahe’s observations, and determined that the planets orbited not in perfect circles but rather ovals, that the heliocentric system was made workable and began to become generally accepted among the scientific community and the lay community alike. The Ptolemaic universe faded into history, discarded into the junk drawer of obsolete ideas.



“The Universe of Aristotle and Ptolemy.” The University of Tennessee, Knoxville. <>

Cessna, Abby. “Geocentric Model.” June 17, 2009. Universe Today. March 8, 2014. <>

“Geocentric Theory.” March 8, 2014. <>