JOHANNES KEPLER
The astronomer who redefined our understanding of the Solar System
Kepler was born on 27 December 1571 in the small town of Weil der Stadt in what is now the German state of Baden-Württemberg. He became a proponent of the Copernican system of planetary motion in the early 1590s, and in 1596 he published Mysterium Cosmographicum (The Cosmographic
Mystery), a detailed public defence of the theory, but it was a position that was still frowned upon. Kepler’s laws of planetary motion wouldn’t be formalised until after he began working with Danish astronomer Tycho Brahe in 1600.
Upon Brahe’s death in 1601, Kepler was given access to the entirety of Brahe’s observations of Mars, which allowed him to tackle the Martian conundrum that had been a thorn in the side of astronomers. The problem was that Mars appeared to move in a retrograde motion in the sky at certain times of year – it seemed to move backwards across the sky.
It would take Kepler several years to realise the reason for this was that the planets did not orbit the Sun in exactly circular orbits as Copernicus thought, but rather in slightly elongated ovals known as ellipses with the Sun not directly at the centre. This would become known as Kepler’s First Law. This accounted for the Martian problem in that Earth would regularly ‘catch up’ to Mars in its orbit, making it seem to drift backwards across the night sky.
From his First Law Kepler derived a second, both of which he published in 1609. His Second Law surmised that as a planet travelled further from the Sun in its elliptical orbit, it slowed down, and vice versa. Thus for a given period of time a planet would sweep out the same area if an invisible line were drawn between the planet and the Sun.
His Third Law, published a decade later, correctly recognised the relationship between the time planets took to orbit the Sun and their distance. He asserted that the square of the ratio of the orbits of two planets was equal to the cube of the ratio of their radius, a complicated law to understand but one that was vital for explaining how the planets related to one another.
Kepler would go on to further expand upon his theories in addition to making other notable discoveries, such as that the Sun rotates, but his work was not widely accepted at the time. In fact, the latter years of his life were not wholly enjoyable; he was ostracised from several places for a variety of reasons and had to defend his mother against charges of witchcraft. He died in Regensburg, Germany, in November 1630.