Animation showing Kepler磗 third law of planetary motion. Kepler磗 three laws were published in 1609 first and second laws and 1619 third law. The third law states that the cube of the semi_major axis of a planet磗 orbit red a is directly proportional to the square of its orbital period cyan P. The constant of proportionality is the same for any planet, so zooming out to show a more distant orbit, the same holds true: the cube of the semi_major axis purple a is proportional to the square of the period orange P. The eccentricity of the orbits has been greatly exaggerated for clarity _ in reality, the semi_minor axis of the Earth磗 orbit is 99.98 of the semi_major axis, making the orbit almost circular.