Johannes Kepler's 1609 work, Astronomia Nova , fundamentally changed astronomy by establishing that planets move in elliptical orbits, rather than perfect circles, and introducing the law of areas. This seminal text, which details Kepler's analysis of Mars' orbit, formally broke from the centuries-old Ptolemaic tradition. Access the original Latin text via the Library of Congress.
The Paradigm Shift of Astronomia Nova Johannes Kepler’s Astronomia Nova (1609), or New Astronomy astronomia nova pdf