<Li> The orbit of a planet is an ellipse with the Sun at one of the two foci . </Li> <Li> A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time . </Li> <Li> The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit . </Li> <P> Most planetary orbits are nearly circular, and careful observation and calculation are required in order to establish that they are not perfectly circular . Calculations of the orbit of Mars, whose published values are somewhat suspect, indicated an elliptical orbit . From this, Johannes Kepler inferred that other bodies in the Solar System, including those farther away from the Sun, also have elliptical orbits . </P>

The square of the period of a planet is equal to what cubed