1. I.Astronomer Ideas A.The Greeks thought Earth was a rotating dome- celestial sphere. B.Geocentric system- Earth at the center of the revolving planets. C.Astronomers 1. Ptolemy- (Greek) thought the planets revolved around the Earth on circles. 1,400 years people believed this to be correct. 2. Copernicus- (Polish)thought the sun is at the center of the planets, called heliocentric; included six planets he knew about: Mercury, Earth, Mars, Jupiter, Saturn.

2. 3.Galileo- (Italian) believed the heliocentric explanation to be correct. First astronomer to use the telescope. Found 4 moons revolving around Jupiter. Discovered Venus goes through phases similar to our moon. 4. Brahe- (Danish) observed the position of the planets for almost 20 years. Kepler- (German) concluded orbits of the planets were not circular they were oval shaped. elliptical II.Inertia and Gravity A. Explains how the planets stay in orbit.

3. B.Isaac Newton concluded that inertia and gravity- together- keep the planets in orbit. C.Inertia- a moving object will continue in a straight line or an object sitting still will stay still 1. the more mass an object has the more inertia it has. D.Gravity- the attractive force between two objects. 1. strength of the gravity depends on the mass of the object and how far away it is.