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Plato-Aristotle: Teacher-Student

Plato (our left) and Aristotle (right)



About 150 years after Pythagoras' heyday Plato (427-347 BC) left Athens because of the excesses of Athenian political life and the execution of his friend and teacher Socrates (see page 26). "Plato" is said to have been a nickname for Aristocles meaning ‘the broad’ and derived from the width of Aristocles’ shoulders, the result of his training for wrestling, the size of his forehead, or the breadth of his style. Traveling in Egypt, Sicily and Italy, Plato came to appreciate mathematics by studying with, among others, the disciples of Pythagoras.

Returning to Athens Plato founded a school of learning in a grove once belonging to the Greek hero Academos (hence "academy"). Over the academy’s door was written: "Let no one unversed in geometry enter here." Flourishing for over 900 years (from 387 BC to 529 AD when it was closed by the Christian emperor Justinian who claimed it was pagan), Plato’s Academy is still the longest existing university on the face of the earth. The grove had been continuously inhabited from prehistoric times.

Plato also believed, as did the Pythagoreans, that the stars, planets, Sun and Moon moved around the Earth attached to the surface of crystalline spheres which slid over one another while circumscribed about the five regular solids. And as they moved past each other they created a sound in the cosmos called the music of the spheres (Pythagoras himself was reportedly the only one who could actually hear this music).

Plato’s academy: 387 BC-529 AD


Plato’s greatest pupil Aristotle (384–322 BC) postulated that the universe is perfect, that the Earth is the center of this perfect universe, and that everything in the universe revolves around the Earth. He believed the heavens were composed of 55 concentric, rotating crystalline spheres to which the stars and planets were attached. He also believed that the outermost sphere, known as the "Prime Mover," imparted motion to the sphere of stars that was located just within it, which in turn lent its motion to the next inner sphere, and so on from sphere to sphere until all the spheres were rotating.

As appealing as Aristotle’s geocentric system was, it still did not explain observed changes in the brightness of the planets or their occasional backward motion among the stars. The observed brightness of the planets changes because they and we all revolve around the Sun. Therefore their distance from us, and with it their apparent brightness, changes over time. And the periodic backward movement of the planets amongst the stars ("retrograde motion") occurs because of changes in the relative position and velocity of the point of view (the Earth) to the planet being viewed, as shown in the diagram below.

To account for these inexplicable observations while still allowing the planets to move in uniform circular motion (another of Aristotle’s ideas that later misled Kepler), Apollonius of Perga (262-190 BC) developed the idea that the planets actually move in small circles called "epicycles," the centers of which revolve about larger circles called "deferents." Even this was not enough to explain all observed phenomena, so Claudius Ptolemy (90-168 AD, a Roman citizen of Egypt who wrote in Greek and was famed for his Tetrabiblos said to "have enjoyed almost the authority of a Bible among the astrological writers of a thousand years or more") offset the Earth from the center of the deferent, which he then in turn had revolve around an "equant" (see the diagram below).


Planets moved on epicycles, whose center’s moved around the center of a deferent. The deferent’s center then moved around an equant, which was as far from the deferent’s center as the Earth. So let's see if I've got this right: The Earth and the equant are unmoving and relatively close to each other, but the Earth is at the center of everything. Around the equant revolves the center of a larger circle called the deferent. On the deferent the center of a smaller circle, the epicycle, revolves. And on each epicycle revolves a planet. And I think that each planet has its own equant, deferent, and epicycle.

Most of this was to explain the apparent—and differing—retrograde motion of the inferior (closer to the Sun than the Earth) and superior (farther from the Sun than the Earth) planets. In Ptolemy's geocentric cosmology the epicycles of the inferior planets Mercury and Venus were on the same line as the Earth and the Sun, while the epicycles of the superior planets Jupiter and Saturn were not.

By the Middle Ages philosopher-theologians like Thomas Aquinas had synthesized Christianity and Reason, thus wedding Aristotle’s newly rediscovered philosophy to the medieval church. Aristotle’s outermost sphere, or Prime Mover, became identified with the Christian heaven, and the Earth at the center reflected god’s central concern for mankind. No matter that the Catholic Church now embraced ideas that had originated with pagan Greek philosophers; to challenge them had become heresy.

This Plato and Aristotle-History of Astrology page and much of this 600-page website are excerpted from the fine art book You and the Universe.


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© Carl Woebcke, Plato and Aristotle: History of Astrology, 1991-2017. All rights reserved.