Johannes Kepler (1571 - 1630)
Johannes was born in Germany in 1571, into a poor family. He was awarded a scholarship to study religion, theology, and mathematics at university. He was interested in the work of Nicolaus Copernicus, and the heliocentric model of the Solar System - which, at the time, had no observational evidence to support it.
Johannes sought to find evidence for the heliocentric model. He contacted the astronomer Tycho Brahe, to access the most detailed studies of the motions of the planets. Tycho was a wealthy Danish nobleman, who had built an observatory in Prague. Here, he tracked the movements of the planets and maintained the most accurate observations of the Solar System of the time. In 1600, Tycho invited Johannes to work with him.
However, Johannes found Tycho to be suspicious and unwilling to share his work. He assigned Kepler with solving the 'problem of Mars'. Johannes said he would solve the problem in 8 days, yet it took nearly 8 years to understand the observations. Tycho died in 1601, and Johannes acquired the detailed notes he needed to solve the problem.
Mars appeared to move backwards across the night sky, in retrograde motion. Using the detailed observations of the planets, Johannes realised that the planets do not orbit in circles but in ellipses (this became Kepler's First Law of Planetary Motion). Earth had a small, almost circular orbit, and Mars had a more distant, more elliptical orbit. The Earth would come from behind Mars, catch up, and eventually pass the planet - causing Mars to appear to travel backwards through the night sky.
Johannes worked on other astronomical problems in his later years. When a planet is closer in its orbit to the Sun, the planet moves faster, and when it is further away, the planet moves slower. This became Kepler's Second Law of Planetary Motion. His Third Law states that the period of a planet (the time taken to orbit the Sun) is connected to the planet's distance from the Sun.