Before the 17th century, people generally believed that Earth was at the center of the universe, a system originally developed by Ptolemy. Galileo, however, was not afraid to challenge existing beliefs when he published his work in support of the Sun-centered, or heliocentric, theory developed by Copernicus. This video segment describes the two opposing worldviews and Galileo's observation that Venus exhibited phases, like Earth's moon, certain evidence in support of the Copernican theory. The segment is five minutes six seconds in length. A background essay and discussion questions are included.
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