# How Our Ideas Changed Over Time

## Ptolemaic model

## Copernican Model

## 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

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.