Historic Models of Astronomy http://www.youtube.com/watch?v=UtOEnTiAZlU http://www.youtube.com/watch?v=iiBIFlvu-X0

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 system- Earth 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 and moon phases 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 (ovalish)

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