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The ancient philosophers imagined the universe to resemble a complex clockwork consisting of concentric crystalline spheres, nested inside one another, which carried the sun, moon, and planets in their motions and made the "music of the spheres" as they revolved. Professional astronomers did not claim that such a mechanism physically existed; rather, they treated it as the hypothetical basis for constructing geometrical schemes that would allow them to make accurate predictions of the motions and future positions of celestial bodies.
However, the motions of the planets against the stars are not uniform and circular but exhibit a host of irregularities. For a superior planet (Mars and those farther from the sun), the most important of these is the planet's retrograde motion at the time of opposition. The planet seems to halt and then reverse its motion for a few months, so that its complete circuit of the ecliptic is attended by a series of yearly loops or switchbacks.
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