## Johannes Kepler

Johannes Kepler was a German mathematician, astronomer, and astrologer.

Kepler is a key figure in the 17th-century scientific revolution. He is best known for his laws of planetary motion, based on his works *Astronomia nova*, *Harmonices Mundi*, and *Epitome of Copernican Astronomy*. These works also provided one of the foundations for Isaac Newton’s theory of universal gravitation.

In astronomy, Kepler’s laws of planetary motion are three scientific laws describing the motion of planets around the Sun.

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

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

- The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

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.

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.

Isaac Newton showed in 1687 that relationships like Kepler’s would apply in the Solar System to a good approximation, as a consequence of his own laws of motion and law of universal gravitation.