Final answer:
Kepler's observation that planets move faster when closer to the Sun is due to Kepler's second law, which describes that a planet sweeps out equal areas in equal times, leading to variable orbital speeds to conserve angular momentum.
Step-by-step explanation:
The cause of the variation in the orbital speed of planets, as observed by Kepler, can be explained by Kepler's second law of planetary motion. This law states that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. This means that when planets are closer to the Sun, they must travel a shorter distance to sweep out the same area, hence they move faster. Conversely, when they are further from the Sun, the distance they travel to sweep out an equal area is greater, so they move more slowly. Kepler's law describes this as the conservation of angular momentum, which dictates that in the absence of external forces, the angular momentum of a system remains constant.