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How did the epicycle model account for irregular celestial motion?
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Hipparchus
“Six astronomers,” the frontispiece of Astronomica Institutio (1692) by Jan Luyts, engraving after drawing by Gerard Hoet. Hipparchus is seated at center with from left to right: Galileo, Johannes Hevelius, Tycho Brahe, Nicolaus Copernicus, and Ptolemy.
The epicycle model accounted for irregular celestial motion by combining multiple regular circular movements to simulate the complex paths observed from Earth. In this system, a planet revolves uniformly along a smaller circular path known as an epicycle, while the center of that epicycle simultaneously orbits Earth along a larger circular path called a deferent.12
This arrangement explained specific irregularities:
- Retrograde Motion: Because one half of the epicycle moves in the opposite direction to the deferent, the planet’s combined motion appears to slow down or reverse direction when it is at its closest point to Earth (perigee).13
- Varying Speeds: By adjusting the relative sizes and rotation rates of the epicycles and deferents, and by using eccentric circles (where the center of the orbit is slightly displaced from Earth), astronomers could represent the observed changes in a planet's speed as it moved through the zodiac.134
While Hipparchus successfully applied these geometric principles to model the Sun and Moon, he did not achieve a fully accurate predictive model for the planets; this was later refined by Ptolemy.13