Johannes Kepler

Johannes Kepler (1571–1630) was a German astronomer who discovered that planets orbit the Sun in ellipses and formulated three laws of planetary motion.

Kepler's laws of planetary motion: Three mathematical laws describing planetary orbits, orbital speeds, and the relation between orbital periods and distances. These laws are Kepler’s best-known account of planetary motion.
Mysterium Cosmographicum: Kepler’s 1596 book proposing that the distances of the six known planets reflected nested regular solids. This early work shows the geometric cosmology Kepler later revised.
Nicolaus Copernicus: A Polish astronomer who proposed a Sun-centered arrangement of the planets in his 1543 book. Kepler extended Copernicus’s Sun-centered astronomy with accurate orbital laws.
Celestial mechanics: The branch of astronomy that studies the motions of celestial bodies using physical laws and mathematical methods. Kepler’s laws became basic tools for calculating orbital motion.
Kepler problem: The mathematical problem of finding an orbiting body's position at a given time from its orbital elements. It formalizes the calculations required to predict motion along Keplerian ellipses.
Elliptical orbit: An orbital path shaped like an ellipse, with the central body at one focus. Kepler established that planets follow ellipses rather than perfect circles.
Astronomia Nova: Kepler’s 1609 book presenting his first two laws of planetary motion, developed through study of Mars. It records the breakthrough from circular models to elliptical orbits.
Galileo Galilei: An Italian astronomer and physicist who used telescopes to investigate the heavens and defended heliocentrism. Kepler and Galileo exchanged ideas while arguing for a Sun-centered cosmos.
Rudolphine Tables: A set of astronomical tables published by Kepler in 1627, using his laws to predict planetary positions. The tables demonstrated the improved predictions made possible by his orbit model.
Keplerian orbit: An idealized orbit governed by Newtonian two-body gravity, taking the conic-section form described by Kepler’s laws. The term carries Kepler’s orbital geometry into later celestial mechanics.

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