Kepler's Third Law relates orbital period to which parameter?

• A. Eccentricity
• B. Semi-major axis
• C. Orbital inclination
• D. Distance to the Sun at aphelion

Answer: (B)

Understanding that the time a body takes to complete an orbit is governed by the size of the orbit. Kepler's third law says that the square of the orbital period is proportional to the cube of the semi-major axis, meaning P^2 ∝ a^3. In practical terms for planets around the Sun, longer orbital periods correspond to larger semi-major axes, regardless of how stretched or tilted the orbit is. The eccentricity describes the orbit's shape, the inclination is its tilt, and aphelion distance is just a farthest point on the orbit—none of these by themselves sets the period in the simple law. So, the parameter that determines the orbital period is the semi-major axis.