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14-1 History of Solar System Study Wandering Stars Greeks called the stars that “wandered” planets, Greek for wandering star. Greek Ideas: Earth at the Center The Greeks thought that earth was in the center, and everything revolved around it (Geocentric Model). Ptolemy thought that the planets still rotated around earth, but in small circles around a big circle (pg 741). Copernicus’s Idea: Sun in the Center Heliocentric – The sun in the center, planets revolve around it. This is correct Galileo’s Observations Noticed moons around Jupiter Venus goes through similar phases as the moon Brahe and Kepler Discovered that the orbit of planets are ellipses (ovals) Inertia and Gravity Inertia Gravity Tendency for an object at rest to stay at rest and an object in motion to stay in motion The more mass, the more inertia The force that pulls all objects together These two factors keep the planets in orbit Astronomical Unit (AU) Average distance between the earth and the sun (150,000,000 km) Mars is 1.52 AU’s from the sun, or 1.52 times farther from the sun than earth.