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Eudoxus developed one of the first explanations of planetary movement which accounts for the
observable phenomena of retrograde motion. The mathematical construct he developed uses two
counter-rotating concentric spheres with offset axes of rotation. The combined motion of these
spheres causes a planetary motion called a "hippopede", which when combined with the planet's
normal easterly track across the backdrop of the fixed stars, causes the planet to move briefly
in the opposite direction, as demonstrated here.
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