Kepler's Laws of Planetary Motion

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Physical Laws

Kepler's laws of planetary motion are a set of three physical laws that describe the motion of planets in the solar system.

First Law

Planets move around the sun in elliptical orbits, with the sun at one focus.

Second Law

The line joining the sun and a planet sweeps out equal areas in equal times.

Third Law

The square of the period of the orbit of a planet around the sun is proportional to the cube of its average distance from the sun.

Source of Name

This entry was named for Johannes Kepler.

Historical Note

Kepler derived his three laws of planetary motion in the early $1600$s from a concentrated study over the course of $20$ years of the colossal wealth of observational data which had been made previously by Tycho Brahe of the behavior of the planets of the solar system, and in particular Mars.

The first two of these results he published in his gigantic work Astronomia Nova.

The third appears some ten years later in his Harmonices Mundi of $1619$.

It was Isaac Newton who managed to interpret these three laws and so work out what is now known as Newton's Law of Universal Gravitation, from which Kepler's laws can straightforwardly be derived.