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Historic Models of Astronomy Greek Ideas: Earth at the Center • Thought they were inside a rotating dome with stars pasted on it • Called this dome the “celestial sphere” • Believed Earth was stationed at the center of the universe Greek Ideas: Earth at the Center Geocentric systemEarth is at the center of the revolving planets Ptolemy (140 AD) • Believed the geocentric model • Thought the moon and sun also revolved around earth Ptolemy (140 AD) • Said planets moved in little circles that moved in bigger circles • This explained why planets seemed to wander • Theory believed for 1400 years Copernicus’ Idea: Sun at the Center • Early 1500s • Developed another explanation for the motion of the planets • Thought the sun was at the center of the system of revolving planets • Called this the “heliocentric system” • Did not publish his ideas until after his death Galileo • Thought heliocentric explanation was correct • Used the telescope to prove • Saw four moons revolving around Jupiter • Saw Venus go through phases • Persecuted for his ideas Kepler Found that the orbit of each planet is an ellipse (oval) Newton- Inertia • Newton explained that inertia and gravity combine to keep the planets in orbit • The tendency of a moving object to continue in a straight line or a stationary object to remain in place is called inertia Newton- Gravity • Gravity is the force that attracts all objects to one another • Strength of gravity depends on – Masses of the objects – Distance between them • Newton figured out that the planets stay in orbit around the sun because the sun’s gravity pulls on them while their inertia keeps them moving ahead