Nicolaus Copernicus

The assumptions of the Copernican model are:

1. Universe is spherical but very large in size!!

2. All orbital motions are circular

3. Sun-centered; all planets orbit about the Sun.

Thus, to get around the lack of observed parallax, Copernicus simply said that the stars were far away (as opposed to saying the Earth was stationary). At the time there was no compelling reason to choose either explanation. Based on the simple assumptions given above, Copernicus correctly deduced:

• the correct "picture" for the size of the Solar System

• the correct explanation for retrograde motion

• the explanation for the phases, variable brightnesses, and sizes of the planets.

• ....

Despite all of the above successes, the Copernican model was not embraced because of some sound scientific reasons:

• the model did not explain the observations any better than did the Ptolemaic model (and assumed the stars were distant to explain the lack of observed parallax)

• there was no physical foundation for the model