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Telescopes from the Ground Up
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Views of the universe: Ptolemy vs. Copernicus

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

Illustration of Copernicus' heliocentric model of the universe.

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.

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

Illustration of Ptolemy's geocentric  model of the universe.

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.

NOTE: The outer planets, like Uranus and Neptune, are missing from both charts because they had not been discovered at the time. The planets are lined up to make the charts easy to read; they never line up this way in nature.

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