How did Kepler figure out the planetary motion?
I presume he based it on observations of planetary motion. At least two terrestrial planets, Mercury and Mars, have rather eccentric orbits i.e. their distances from the sun at perihelion obviously differ from that at aphelion. From that, it is obvious that their orbits are elliptical not circular. Moreover, they move more rapidly at perihelion, so the radius vectors of the planets sweep out equal areas in equal times.
