The Astronomia Nova also introduces the Second Law: "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time." This was the first functional description of orbital velocity—a planet moves faster when it is closer to the Sun (perihelion) and slower when farther away (aphelion).
Kepler used the precise observational data of Tycho Brahe to formulate the first two of his three famous laws of planetary motion within this text: "Kepler: 'Astronomia Nova' ('New Astronomy')" - eCommons astronomia nova pdf
What happened next broke astronomy forever. The Astronomia Nova also introduces the Second Law: