Students study the heliocentric theory of Copernicus, the ideas behind it and the explanations which it displaced. A discussion of retrograde motion of planets helps students explain the logic underlying the ideas of both Ptolemy and...

Students trace the beginning of the heliocentric theory of the solar system--the idea that the solar system revolves around the Sun--to an observation by the Greek astronomer Aristarchus, which convinced him that the Sun was much bigger...

Learners explain how Hipparchus, around 130 BC, used a shift in the predicted location of a lunar eclipse to detect a slight shift in the path of the Sun around the sky. They examine the elliptical orbit in which the Earth travels around...