Unit 2 : Activity 2 : Ptolemaic Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit Exam









The Copernican Model
(a.k.a Sun Centered)

As we discussed in the previous subunit, the Earth-centered model of the universe, refined by Ptolemy, was set firmly in place in the early part of the first millennium. It was not until 1543 that it met serious competition in the Sun-centered model of Nicolas Copernicus.

Copernicus was born in 1473 in Poland and studied, among other subjects, mathematics and astronomy. He is mainly remembered for formally introducing the idea that the Sun is the center of our solar system. This heliocentric concept (sun-centered concept) was a radical idea for his time. Nearly all contemporary astronomers had adopted the Greek Earth-centered model. It was so radical a concept, in fact, that Copernicus waited until the year of his death to publish his famous essay titled, “On the Revolutions of the Heavenly Spheres.”

Copernicus had two main reasons for asserting that the Sun was the center of our solar system.

1. While the Ptolemaic model was very good at predicting the positions of the planets, it wasn't precise, and over the centuries its predictions got worse and worse.

2. Copernicus didn't like the fact that the Ptolemaic model had big epicycles to explain the retrograde motions of the planets. He knew that this could be explained instead by having the Earth also moving around the Sun.

The true motion of the planets around the Sun is not uniform circular motion, so Copernicus' model still needed to have epicycles. He had 1500 years of post-Ptolemy data to work with, and needed quite a lot of epicycles to make a new set of accurate predictions for the motions of the planets.

The main simplification of the Copernican model was that the retrograde loops of the planets as seen from the Earth occur naturally as a result of the Earth's motion combined with the motions of the planets.You worked on this problem in the second part of Activity Two. Here are some illustrations to consider.