Kepler applies his laws of planetary motion to the specific cases of Mercury, Venus, Mars, Jupiter, and Saturn.

The book is a fascinating narrative of failure and discovery. Unlike modern scientific papers, which often present a sanitized final result, Kepler walks the reader through his "fruitless journeys." He details his initial attempts to fit Mars into a circular orbit, calculating and recalculating, only to find discrepancies of 8 minutes of arc.

Kepler’s work was "new" because it introduced into the study of the heavens, which had previously been treated as a branch of geometry.

: For those who don't read 17th-century Latin, Green Lion Press offers a downloadable PDF introduction that contextualizes its "astonishing originality".

He was the first to argue that physical forces (like magnetism or "anima motrix"), rather than mathematical abstractions, drive planetary motion . 📥 PDF Resources & Access