Geocentric Model

The geocentric model of the solar system is a model that has the Earth at the center of the solar system and all the planets, Moon, and Sun orbiting around the Earth.

This model made sense to the ancients who could only see the sky from the ground.  Imagine if you didn't know aby better, it looks as if the Sun, the planets, and the Moon all orbited around us.  We know that only the Moon goes around the Earth. But to an ancient, it would make sense that the Sun and planets went around the Earth.  This is why they created a model called the geocentric model.

 

Let's define some terms:

Deferent - the actual orbital path of the planet, Sun, or Moon around the Earth. Everything moves on these orbits counterclockwise looking down on the North Pole of the Earth.

Epicycles - the smaller circles that are centered on a point on the deferent.  These are used for the planets to help explain retrograde motion in the sky.  Without these, retrograde would be difficult to explain.

There is also a line connecting Mercury, Venus, and the Sun so that they stay in relatively the same location in the sky.  This will be explained later.

Lastly, the Earth is offset from the center to account for the changes in size of the Moon and planets as they "orbited" the Earth.

The most famous geocentric model was produced by Ptolemy in the 2nd century.  This model persisted for 1500 years until Copernicus and later Kepler and Galileo were able to prove heliocentric model, which will be our next post.

Both images above are from ScienceU.com.