... This lesson traces 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 than the Earth. ... This lesson plan supplements: "Aristarchus: Is the Earth revolving around the Sun?," section #9a : on disk Sarist.htm , on the web . ... What does the Sun-Earth-Moon angle (corner at Earth) at such times tell about the Sun's distance? ...