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Animation showing Kepler's third law of planetary motion. Kepler's 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's orbit (red a) is directly proportional to the square of its orbital period (blue 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 (yellow P). The eccentricity of the orbits has been greatly exaggerated for clarity - in reality, the semi-minor axis of the Earth's orbit is 99.98% of the semi-major axis, making the orbit almost circular.
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