Question

Compare the Ptolemaic and Copernican models. Explain in what ways each was an accurate model of the universe, and in what ways each was inaccurate. Describe retrograde planetary motion of the inner and outer planets and how those models explained it.

Answer 1

Ptolemy's model:
"Earth-centered," or "geocentric"

Ptolemy thought that all celestial objects — including the planets, Sun, Moon, and stars — orbited Earth. Earth, in the center of the universe, did not move at all.

Copernicus’ model:
"Sun-centered," or "heliocentric"

Copernicus thought that the planets orbited the Sun, and that the Moon orbited Earth. The Sun, in the center of the universe, did not move, nor did the stars.

Copernicus was correct about some things, but wrong about others. The Sun is not in the center of the universe, and it does move, as do the stars. Also, both Copernicus and Ptolemy thought the orbits of the planets were circular, but we now know they are elliptical.

Retrograde motion:

Retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Direct motion or prograde motion is motion in the same direction as other bodies.

Ancient astronomers like Ptolemy and Copernicus believed Earth lay at the center of the universe – went to complicated lengths to attempt to explain retrograde motion. Theirs was a complex cosmology in which each planet not only orbited Earth, but also spun around a moving point on their orbit. Imagine whipping a ball on a length of string around your hand while you rotated in place.