Epicycles

Understanding Epicycles

• Epicycles were an important part of the ancient geocentric model of the universe.
• They were small circles in which the celestial bodies moved while simultaneously making a larger circle round the Earth.
• This idea was proposed by Claudius Ptolemy to explain the observed motion of planets.

Function of Epicycles

• The key point of epicycles is that they help to account for retrograde motion, where planets appear to move backwards across the sky.
• By using a combination of an epicycle (small circle) and a deferent (large circle), Ptolemy’s model could make predictions about where planets would be in the sky.
• In order for this model to be accurate, the speed at which a planet moved on its epicycle had to be linked to the speed of its deferent.

The Downfalls of Epicycles

• While the concept of epicycles was innovative, it was also complex and required constant adjustments based on observation.
• The prediction of the position of a planet required the size, speed and direction of an epicycle to be accurately known.
• Over time, this meant adding more and more epicycles, which was unwieldy and complicated.

Influence of Epicycles

• Despite their problems, the concept of epicycles influenced the development of astronomy.
• This notion prompted astronomers like Copernicus, Kepler and later Newton to seek more straightforward models to explain the motion of celestial bodies.
• Epicycles can be seen as a stepping stone in the evolution of our current understanding of the solar system.