Kepler was able to determine that the planets move slowest when they are

farthest

Explanation:

Kepler used Brahe's observational data for writing three laws of planetary motion. According to the first law, all the planets revolve about the sun in elliptical paths. According to second law, the line joining the Sun and the planet sweeps equal areas in equal intervals of time. It also implies that planet closer to the Sun travels faster than the planet away from it. According to the third law, the square of the orbital period is directly proportional to the cube of the semi-major axis.

Thus, planets move slowest when they are farthest from the Sun.