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Claudius Ptolemy lived in Alexandria, Egypt from 

100CE-170CE. He was a famous astronomer. His 

life goal was to build upon the theories of the 

philosopher Plato in order to explain the movement 

of the planets in the Universe. 

Plato's model showed the Earth at the center, 

with each planet moving in its own perfectly 

circular orbit.  

Everything was studied without a telescope. Ptolemy 

used math based on the positions of the planets that he 

observed with his eyes to create his models.

Ptolemy had a geocentric model of the universe. That 

means that everything revolved around the Earth, 

while the Earth stayed in the same place. He had 

three ideas that helped explain this. 

First, he concluded that the Earth was not 

exactly in the center of the circle the planets 

made around Earth. This means that the 

system was eccentric. 

Ptolemy used an equant. An equant is a mathematical 

point that Ptolemy created to explain the speeds of the 

planets. The planets went different speeds at different 

parts of their orbit. The equant helped explain why this 

happened.     

Lastly, he thought that the planets moved in 

epicycles in their orbit. This means that they 

moved in small circles as they moved around 

Earth.

Epicycles explained retrograde motion. If the planets 

went faster in the epicycles then they did on the rest of 

their orbit, this would explain why sometimes the 

planets looked like they were moving backwards.

Despite the fact that Ptolemy was able to predict the 

position of the planets, none of his theories were 

correct. Ptolemy's model of the solar system was 

accepted for over 1300 years. How could people have 

believed him for so long if his model wasn't correct? 

Well, there were many factors that made Ptolemy's 

geocentric model so convincing.

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People believed Ptolemy because when they looked 

at the sky at night, the planets appeared to change 

positions, and the earth didn't feel like it was 

moving, so it made sense that the earth was the 

center of the universe and didn't move.

Ptolemy's model could also predict the positions of 

the sun, moon, and planets with math, which gave 

him credibility. 

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