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.
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.