Ptolemy's Universe

Ptolemy placed Earth at the center of the universe, with the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn circling our planet.

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Planetary Motion

For most ancient astronomers, accurately predicting the positions of the planets was tantamount to understanding the workings of the universe. The far more distant stars were simply the backdrop against which planetary action took place. Ptolemy, the last of the great Greek astronomers of antiquity, developed an effective system for mapping the universe. Basing much of his theory on the work of his predecessor, Hipparchus, Ptolemy designed a geocentric, or Earth-centered, model that held sway for 1400 years.

 That Ptolemy could place Earth at the center of the universe and still predict the planets’ positions adequately was a testament to his ability as a mathematician. That he could do so while maintaining the Greek belief that the heavens were perfect—and thus that each planet moved along a circular orbit at a constant speed—is nothing short of remarkable.

 The greatest difficulties he had to overcome were explaining the changing speeds and the occasional east-to-west, or retrograde, motion of the planets. He accomplished this by having each planet move along a small circle, called an epicycle, whose center travelled along a larger circle, called a deferent, with Earth at its center.

 Although this scheme came close to accomplishing what he wanted, it still came up a little short. So Ptolemy made a couple of refinements. First, he placed Earth slightly away from the center of the deferent. (A slightly off-center circle comes very close to mimicking an ellipse.) And second, he had the center of the epicycle move at a constant angular speed around a third point, called the equant, which lay on the opposite side of the deferent’s center from Earth. These modifications allowed Ptolemy to predict the positions of the planets with reasonable—though far from perfect—accuracy.

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