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This lesson will lay out for you how ancient Greek philosophers came up with their models of the universe,. We’ll also look at who Ptolemy was and what he contributed, and whether or not the Greeks were correct.

Ancient Greek Philosophers

Ancient Greece is well-known for its huge contributions to the world in the fields of math, science, architecture, and government. Two great philosophers of ancient Greece, Plato and Aristotle, shaped the history of astronomy for many years to come.Plato and Aristotle tried to explain the structure of the universe by reasoning from first principle, something that is considered to be obviously true.

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If a principle is held as true, whatever is derived from it is also considered to be true. Such thinking was standard practice in their day. Of course, it is subject to quite a lot of falsehoods.For example, if we used this practice to say that everyone who has external female genitalia is obviously a woman, we’d be wrong, since this isn’t always the case.Therefore, this principle led ancient Greek philosophers, and millennia of countless others, astray from the reality of our universe.

Geocentrism ; Uniform Circular Motion

Plato argued that the heavens were perfect, the only type of motion that was perfect was uniform motion and that the only perfect shape was a sphere. Because of this, Plato believed that all motion in the universe is made up of circles turning at uniform rates, an idea called uniform circular motion.

Plato’s student, Aristotle, believed in a geocentric universe, a universe where the Earth was at the center of the universe. His model of the universe also employed Plato’s concepts of circular motion.The reason ancient astronomers believed the Earth did not move – something we know isn’t true today – was because they did not see any parallax of the stars. Parallax is a concept that explains the apparent motion of an object, which may actually be completely stationary, due to the motion of the observer.You can easily demonstrate this concept for yourself by taking out a pencil and holding it straight out in front of you. Now take this pencil and hold it still against the background of a tree. Close one eye and then switch eyes.

It will appear as if the tree shifts positions as you switch eyes back and forth. The reality is that you know the tree is standing completely still!Ancient astronomers believed that if the Earth actually moved, then you should be able to see the sky from different locations as time passes by and that parallax would distort the shapes of the constellations. The problem is that the actual parallax of the stars is way too small for people to see with the unaided eye and this led ancient astronomers away from the true nature of the universe.Because most people considered Aristotle to be the greatest philosopher of the ancient world, his ideas on the universe enslaved centuries of brilliant minds with the erroneous ideas of geocentrism and uniform circular motion.

The Ptolemaic Model

About 500 years after Aristotle, came along a man called Ptolemy. Ptolemy was a Greek who lived in Roman-controlled Egypt. He was a brilliant mathematician who didn’t take entirely to the first principles his predecessors ascribed to. He accepted the thought that the Earth was at the center of the universe and that there was circular motion involved, but he tried to tackle the motion of the planets a bit more precisely than his predecessors. Ironically, his model slightly undermined Plato’s and Aristotle’s classical ideas, but that’s neither here nor there.

Ptolemy created a model of the universe where a planet followed a small circle, called an epicycle, around a larger circle, called a deferent. By changing the sizes of these circles and their rate of rotation, Ptolemy was able to approximate the retrograde motion of a planet. You see, at certain times, planets seem to reverse and move westward, as opposed to eastward, before resuming their normal path.This is called retrograde motion; it couldn’t be explained well by Aristotle’s model of the universe, and therefore, Ptolemy created his own model to try to explain it through the use of things like epicycles.Ptolemy’s model was a bit more complex than what is shown below but I wouldn’t want to bore you with its incorrect details.

Ptolemaic model of the universe
Ptolemaic model of the universe

The Heliocentric Universe

Incorrect because, with the passage of time, his model accumulated lots of errors, and astronomers began to notice that uniform circular motion did not exist and that the Earth was not at the center of the universe.

The heliocentric universe, a universe with the sun at its center, eventually became accepted thought. Through the direct and indirect work of men like Copernicus, Tycho Brahe, Kepler, and Newton, scientists discovered how wrong 2,000 years of thought about our universe were and helped to reshape and rewrite astronomy into what it is today.

Lesson Summary

The ancient philosophers, Plato and Aristotle, tried to explain the structure of the universe by reasoning from first principle, something that is considered to be obviously true. This type of reasoning created models of our universe that were not only wrong but erroneously influenced the thoughts of many people to come.

Plato believed that all motion in the universe is made up of circles turning at uniform rates, an idea called uniform circular motion.Plato’s student, Aristotle, believed in a geocentric universe, a universe where the Earth was at the center of the universe.About 500 years after Aristotle, along came a man called Ptolemy, who created his own model of the universe. He tried to explain the motion of the planets that Aristotle’s model struggled to do.But it was all for naught, as all of it was shown to be incorrect. Long after Ptolemy, the heliocentric universe, a universe with the sun at its center, became accepted thought.

Not long after that, we found out uniform circular motion does not exist, either.

Learning Outcomes

Once you are done with this lesson you should be able to:

  • Recall Plato and Aristotle’s concepts of the universe
  • Compare Ptolemy’s model of the universe to those of his predecessors
  • Explain how first principle reasoning led astronomy astray
  • Describe the geocentric, Ptolemaic, and heliocentric models of the universe and the errors of each

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