Answer:
the semi-major axis of the orbit
Explanation:
Kepler's 3rd Law is T² = r³, meaning ...
The square of the Period (T) of a planet (the time it takes a planet to go once around the Sun) is directly proportional to the cube of the elliptical orbit's semi-major axis (r). Think of the semi-major axis as the average distance of the planet to the sun.
