Johannes Kepler (1571-1630) worked for six years with intense effort resulting in his book 'Astronomia Nova' which was published in 1609.
This included the first two of his three planetary laws.(Remember the telescope was only invented in 1609!)Ten years later in 'Harmonice Mundi Kepler published his third law:
 

'The squares of the times which planets take to complete their orbits are proportional to the cubes of their mean distances from the sun.'

.
This is a quite staggering achievement considering that there was no coherent law of gravity or even mechanics at this time. We use Newton's gravitational law in our simple analysis of planetary motion. Back To The Top