In astronomy, Kepler's laws of planetary motion, published by Johannes Kepler between 1609 and 1619, describe the orbits of planets around the Sun. The laws modified the heliocentric theory of Nicolaus Copernicus, replacing its circular orbits and epicycles with elliptical trajectories, and explaining how planetary velocities vary. The three laws state that:

1) The orbit of a planet is an ellipse with the Sun at one of the two foci.

2) A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.

3) The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.

The elliptical orbits of planets were indicated by calculations of the orbit of Mars. From this, Kepler inferred that other bodies in the Solar System, including those farther away from the Sun, also have elliptical orbits. The second law helps to establish that when a planet is closer to the Sun, it travels faster. The third law expresses that the farther a planet is from the Sun, the slower its orbital speed, and vice versa.

Johannes Kepler (27 December 1571 – 15 November 1630) was a German mathematician, astronomer, astrologer, and natural philosopher. He is a key figure in the 17th-century Scientific Revolution, best known for his laws of planetary motion, and his books "Astronomia nova", "Harmonice Mundi", and "Epitome Astronomiae Copernicanae".

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