Illustration showing Kepler's laws of planetary motion. Kepler's three laws were published in 1609 (first and second laws) and 1619 (third law). The first law (top left) states the the orbit of each planet is an ellipse, with the Sun at one focus. Kepler's second law (top right) states that a line linking the planet to the Sun sweeps out an equal area in any given time. The third law states that the cube of the semi-major axis of a planet's orbit is directly proportional to the square of its orbital period.