1599 – Kepler hired by Tycho Brahe
Work on the orbit of Mars
1609 – Kepler’s 1st and 2nd Laws
Planets move on ellipses with the Sun at one focus
The radius vector sweeps out equal areas in equal times
1618 – Kepler’s 3rd Law
The square of a planet’s orbital period P is proportional to the cube of its semi-major axis R.