Kepler's Laws of Planetary Motion

Kepler's laws of planetary motion are three principles describing the orbits of planets around the Sun, formulated by German astronomer Johannes Kepler from the precise observational data of Tycho Brahe. The first law, published in Astronomia Nova in 1609, states that a planet's orbit is an ellipse with the Sun at one focus rather than a perfect circle; the second, published in the same work, states that a line joining a planet and the Sun sweeps out equal areas in equal times, meaning a planet moves faster when nearer the Sun. The third law, published in Harmonices Mundi in 1619, states that the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit. Isaac Newton later showed that all three laws follow mathematically from his law of universal gravitation, giving Kepler's empirical rules a theoretical foundation.