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#that planets move in Elliptical orbits with the sun at one Focus , and
#that planets do not move with constant speed along this orbit but their speed varies so that the line joining the centers of the sun and a planet sweeps out equal parts of the ellipse in equal times.

To these two laws Kepler added a third a decade later, in his book '' Harmonices Mundi '' (''Harmonies of the world''). The third law sets out a proportionality between the third power of the average distance of a planet from the sun and the square of the length of its year.