|Themes > Science > Astronomy > The Universe > The Development of Modern Astronomy > The Universe of Aristotle and Ptolemy|
(The diagram is not to scale, and the planets are aligned for convenience in illustration; generally they were distributed around the spheres.) There were additional "buffering" spheres that lay between the spheres illustrated. The sphere of the stars lay beyond the ones shown here for the planets; finally, in the Aristotelian conception there was an outermost sphere that was the domain of the "Prime Mover". The Prime Mover caused the outermost sphere to rotate at constant angular velocity, and this motion was imparted from sphere to sphere, thus causing the whole thing to rotate.
By adjusting the velocities
of these concentric spheres, many features of planetary motion could be
explained. However, the troubling observations of varying planetary brightness
and retrograde motion could not be accommodated: the spheres moved with
constant angular velocity, and the objects attached to them were always
the same distance from the earth because they moved on spheres with the
earth at the center.
Epicycles and Planetary MotionThe "solution" to these problems came in the form of a mad, but clever proposal: planets were attached, not to the concentric spheres themselves, but to circles attached to the concentric spheres, as illustrated in the adjacent diagram. These circles were called "Epicycles", and the concentric spheres to which they were attached were termed the "Deferents". Then, the centers of the epicycles executed uniform circular motion as they went around the deferent at uniform angular velocity, and at the same time the epicyles (to which the planets were attached) executed their own uniform circular motion.
net effect was as illustrated in the following animation. As the center
of the epicycle moves around the deferent at constant angular velocity,
the planet moves around the epicycle, also at constant angular velocity.
The apparent position of the planet on the celestial sphere at each time
is indicated by the line drawn from the earth through the planet and projected
onto the celestial sphere. The resulting apparent path against the background
stars is indicated by the blue line.
Now, in this tortured
model one sees that it is possible to have retrograde motion and varying
brightness, since at times as viewed from the earth the planet can appear
to move "backward" on the celestial sphere. Obviously, the distance of
the planet from the Earth also varies with time, which leads to variations
in brightness. Thus, the idea of uniform circular motion is saved (at
least in some sense) by this scheme, and it allows a description of retrograde
motion and varying planetary brightness.
More Sophisticated Epicycles: The Ptolemaic UniverseHowever, in practice, even this was not enough to account for the detailed motion of the planets on the celestial sphere! In more sophisticated epicycle models further "refinements" were introduced:
Medieval Aristotelian AstronomyBy the Middle Ages, such ideas took on a new power as the philosophy of Aristotle (newly rediscovered in Europe) was wedded to Medieval theology in the great synthesis of Christianity and Reason undertaken by philsopher-theologians such as Thomas Aquinas. The Prime Mover of Aristotle's universe became the God of Christian theology, the outermost sphere of the Prime Mover became identified with the Christian Heaven, and the position of the Earth at the center of it all was understood in terms of the concern that the Christian God had for the affairs of mankind.
Thus, the ideas largely originating with pagan Greek philosophers were baptized into the Catholic church and eventually assumed the power of religious dogma: to challenge this view of the Universe was not merely a scientific issue; it became a theological one as well, and subjected dissenters to the considerable and not always benevolent power of the Church.
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