Sphere
of fixed
starsSaturnJupiterMarsSunVenusMercuryMoonEarthEpicycle Earth EquantDeferentPlanetRetrograde motion
occurs herePtolemy’s great book Mathematical Syntaxis (c. AD 140)
contained the details of his model. Islamic astronomers preserved
and studied the book through the Middle Ages, and they called it Al
Magisti (The Greatest). When the book was found and translated from
Arabic to Latin in the 12th century, it became known as Almagest.
Uniformly rotating circles were key elements of ancient
astronomy. Claudius Ptolemy created a mathematical model of the
Aristotelian universe in which the planet followed a small circle called the
epicyclethat slid around a larger circle called the deferent. By adjusting
the size and rate of rotation of the circles, he could approximate the
retrograde motion of a planet. See illustration at right.
To adjust the speed of the planet, Ptolemy supposed that Earth was slightly
off center and that the center of the epicycle moved such that it appeared
to move at a constant rate as seen from the point called the equant.
To further adjust his model, Ptolemy added small epicycles (not shown
here) riding on top of larger epicycles, producing a highly complex model.
The Ptolemaic model of the universe shown below was geocentric
and based on uniform circular motion. Note that Mercury and
Venus were treated differently from the rest of the planets. The centers
of the epicycles of Mercury and Venus had to remain on the Earth–Sun
line as the sun circled Earth through the year.
Equants and smaller epicycles are not shown here. Some versions
contained nearly 100 epicycles as generations of astronomers tried to
fine-tune the model to better reproduce the motion of the planets.
Notice that this modern illustration shows rings around Saturn and
sunlight illuminating the globes of the planets, features that could not
be known before the invention of the telescope.
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