Part I — Chapter 3
Kepler & the Laws of Planetary Motion
From mystical harmonies to precise mathematical laws of the cosmos
3.1 Tycho Brahe's Data
Tycho Brahe (1546–1601) compiled the most precise naked-eye astronomical observations in history. His data, especially on Mars, became the raw material for Kepler's revolutionary laws.
3.2 Kepler's Three Laws
Johannes Kepler (1571–1630) discovered that planets move in ellipses with the Sun at one focus (First Law), sweep equal areas in equal times (Second Law), and that the square of the orbital period is proportional to the cube of the semi-major axis (Third Law).
3.3 From Geometry to Physics
Kepler's laws were the first precise mathematical description of planetary motion. They demanded a physical explanation — a force from the Sun — that Newton would provide half a century later with universal gravitation.