Solar System Model

• Geocentric Model


  Earth was at the centre, and the other planets, moon and sun orbited the Earth.

  Everything orbited in perfect circles with a fixed background of stars
• Heliocentric Model


  Sun at the centre

  Main evidence was Mars ‘retrograde motion’

  Galileo observing moons orbiting Jupiter showed not everything orbited the Earth

  Kepler showed that the planets orbited in ellipses, and not circles
• Retrograde Motion 


  Earth orbits the sun faster than Mars, so we undertake it, so it appears to reverse its direction in the sky