Do the planets that are farther away actually move slower, or do they just take longer to orbit the Sun because they have farther to travel?
They actually move more slowly. You can see how that follows from Kepler’s second law: in a given time interval (say 30 days), a planet will sweep out a certain area (shown shaded in the diagrams I showed in class, which are also in your text). When the planet is close to the Sun, the sides of the “wedge” area are short, and the curved part of the path is longer. When the planet is far from the Sun, the sides of the wedge are long, so for the same area, the curved part is shorter. So when the planet is far from Sun, the distance traveled in a given time interval is shorter. Since speed is distance/time, the planet travels more slowly when it’s far away.
