And as such, these celestial motions required a cause in the form of an unbalanced force. Circular and elliptical motion were clearly departures from the inertial paths (straight-line) of objects. Newton knew that there must be some sort of force that governed the heavens for the motion of the moon in a circular path and of the planets in an elliptical path required that there be an inward component of force. Even more troubling was the circular motion of the moon about the earth. To Newton, there must be some cause for such elliptical motion. Newton was troubled by the lack of explanation for the planet's orbits. There was however no interaction between the planets themselves. To Kepler, the planets were somehow 'magnetically' driven by the sun to orbit in their elliptical trajectories. Kepler could only suggest that there was some sort of interaction between the sun and the planets that provided the driving force for the planet's motion. The cause for how the planets moved as they did was never stated. While Kepler's laws provided a suitable framework for describing the motion and paths of planets about the sun, there was no accepted explanation for why such paths existed. (Further discussion of these three laws is given in Lesson 4.) The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun.An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time.The paths of the planets about the sun are elliptical in shape, with the center of the sun being located at one focus.Kepler's three laws of planetary motion can be briefly described as follows: Kepler's three laws emerged from the analysis of data carefully collected over a span of several years by his Danish predecessor and teacher, Tycho Brahe.

In the early 1600's, German mathematician and astronomer Johannes Kepler mathematically analyzed known astronomical data in order to develop three laws to describe the motion of planets about the sun.

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