The curiously looping movements of the planets relative to the stars have presented all sorts of puzzles to keen, patient observers of the night sky.
In 1601, Johannes Kepler (1571-1630) undertook the challenge of deciphering the orbit of Mars and developing a mathematical theory of its motion to fit observations of the planet’s changing position in the sky. In assuming that Earth itself traveled around the sun, Kepler’s immediate hurdle was to find a way to disentangle Mars’ motion from that of Earth.
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