GEOCENTRIC MODEL (EARTH- CENTERED MODEL)  Explains the daily motion of the planets, constellations and stars (East / West; appearance and disappearance)  Lasted years  Describe by Ptolemy in ancient Greece  Could NOT explain retrograde motion HELIOCENTRIC MODEL  Described by Nicolaus Copernicus and supported by Galileo  Based on observation of the Galilean moons revolving around Jupiter and observations of Venus’ phases (like the Moon)  Could explain retrograde motion

1. Interstellar dust clouds (nebula) Where does interstellar dust come from? 2. Made primarily of H and He 3. Low density -> High density 4. Spinning and accelerated rotation (centripetal force) 5. Once conditions (temperature and pressure)cause H to fuse into He, a star is born 6. Spinning causes heavier/high melting point materials to stay close to the center and lighter/low melting point materials to be further from the center (OR DO THEY?)

P 2 = a 3  Where P = Earth years and a = length of semi- major axis in A.U.  Describes the relationship between the size of a planet’s ellipse and its orbital period (year length)

 Newton mathematically described gravity as an attractive force between 2 objects that depends on mass and separation. F = Gm 1 m 2 r 2  Where F is force (newtons),  G is the universal gravitational constant ( x m 3 / kg·s 2 ),  m1 and m2 are measured in kg  r is the distance between the bodies in meters.

v 2 = Gm r  where v is velocity (m/s)  G is the gravitational constant  m is the mass in kg  r is the distance between the two bodies in meters