<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> <P> Kepler's work (published between 1609 and 1619) improved the heliocentric theory of Nicolaus Copernicus, explaining how the planets' speeds varied, and using elliptical orbits rather than circular orbits with epicycles . </P>

Who proposed elliptical orbit of the planets around sun