Kepler's third law of planetary motion states that:
"The ratio between the cube of the orbital radius of the planet and the square of the orbital period is constant". In formulas:
[tex] \frac{r^3}{T^2} = k[/tex]
where r is the orbital radius and T the orbital period.
Since this ratio is constant for every planet, we see that when the orbital radius r is larger (i.e. when the planet is farther from the Sun), the orbital period T is larger: this means the planet takes more time to complete one revolution around the Sun, so it moves slower.
Therefore, the correct option is:
A planet moves slowest when it is farthest from the sun.
