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When Tycho died in 1601, Kepler inherited a treasure trove of data: the most precise naked-eye observations of Mars ever recorded. While Copernicus had suggested the Sun was the center, his model still clung to circular orbits and epicycles. Kepler knew the data was wrong. For nearly a decade, he fought with the orbit of Mars.

A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

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

I then considered the speed of the planet. It was known that a planet moves faster when it is nearer the Sun and slower when it is farther away. But in what proportion?

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