Johannes Kepler (December 27, 1571 – November 15, 1630) was a German mathematician, astronomer, and astrologer.

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.

Kepler lived in an era when there was no clear distinction between astronomy and astrology, but there was a strong division between astronomy (a branch of mathematics within the liberal arts) and physics (a branch of natural philosophy). Kepler also incorporated religious arguments and reasoning into his work, motivated by the religious conviction and belief that God had created the world according to an intelligible plan that is accessible through the natural light of reason. Kepler described his new astronomy as “celestial physics”, as “an excursion into Aristotle’s *Metaphysics*“, and as “a supplement to Aristotle’s *On the Heavens*“, transforming the ancient tradition of physical cosmology by treating astronomy as part of a universal mathematical physics.

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

- 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.
- 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 close to being circles, and careful observation and calculation is required in order to establish that they are actually not perfectly circular ellipses. Using calculations of the orbit of Mars, whose published values are somewhat suspect, which indicated elliptical orbits, Johannes Kepler inferred that other heavenly bodies, 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 consequences of his own laws of motion and law of universal gravitation.

Kepler’s laws are part of the foundation of modern astronomy and physics.

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