How Our Ideas Changed Over Time

Ptolemaic model

In the second century, Ptolemy made a mathematical model of our solar system in which he believed the Earth was in the centre of the universe and all the other planets revolved around it.

Copernican Model

Over 1200 years after the Ptolemaic Model, Nicolas Copernicus developed a model, which had the sun in the centre, instead of Earth. The Roman Catholic Church rejected and banned his work.

Tycho Brahe

3 years after Copernicus’s death, Dane Tycho Brahe developed a model of the solar system that had the Sun circling around the Earth, and all the other planets orbiting the Sun.

Kepler – laws of planetary motion

In 1609, Tycho Brahe appointed Johann Kepler as his assistant. After Brahe died, Kepler took over his work and began developing his own ideas. He realised that the orbits of planets were elliptical rather than circular. He then made his laws of planetary motion:

  • The orbit of every planet is an ellipse with the Sun at one of the two foci.
  • A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  • The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.


Galileo lived at the same time as Kepler. He discovered Jupiter had four moons, which meant that objects could orbit around objects. He believed that if moons orbited other planets, then the Earth might not be the centre of the Solar System. He stated that the Sun was the centre, and all the other planets including Earth orbited around it.