Dear kaz123,
Actually, Claudius Ptolemaeus (also known as Ptolemy) did not use the
epicyclic system and and the eccentric system, but a combination of
both. He did so because he had to reconcile certain views on the
nature of the universe regarded as basic facts with known
discrepancies and the results of his own observations.
The ancient Greeks believed that the supreme principles of the
universe were harmony and perfection. They were confident that any
planetary motion had to follow a circular course with regular speed,
for the circle was the most perfect shape. Furthermore, it was widely
accepted that Earth was the center all planets orbited, though the
idea of the Sun being the center of the planetary system also existed
among some scholars.
So the basic idea of the Greek astronomers was that the Sun, the Moon,
and all planets were moving around Earth on perfect, concentric
circular orbits. Aristoteles was one of the exponents of this
geocentric model. There were, however, some problems with that system:
Astronomers observed irregularities in the motions of the Sun, the
Moon, and the planets. The planets did appear to speed-up and to
slow-down, sometimes they even moved backwards. Also, the brightness
of the planets changed, which did not fit well with the belief that
they were perfect and thus constant in all aspects of their nature.
These irregularities could not be explained with the simple geocentric
model.
The Greek astronomer Hipparchus of Rhodes, whose work would three
centuries later have immense influence on Ptolemaeus, introduced the
idea of epicycles and eccentric circular movement by applying them to
the irregularities he had noticed in the motion of Sun and Moon.
Finally, Ptolemaeus further refined this model and applied it to all
planets that, as he thought, were rotating around Earth. Since this
highly complex model, based upon the assumed central position of an
immovable Earth, allowed projection of planetary motion within a
certain tolerance region that was not noticeable with bare eye, and
since it explained irrgularities in celestial movement, the combined
system of eccentics and epicycles indeed seemed to correctly show the
order of the planetary system. Only this combination could explain the
actual observations.
Therefore, Ptolemaeus without any doubt did not consider one of the
systems - eccentric or epicyclic - correct, but the combined model
that seemed proven through evidence.
Concerning the general structure of the universe, Ptolemaeus kept the
more philosophical than scientific model developed by Aristoteles. The
following paragraphs, slightly shortened quoted after the Rice
University website "Ptolemaic System" (see sources section), explain
this cosmologial system:
"[Aristoteles' cosmos] was divided into two distinct parts, the
earthly or sublunary region, and the heavens. The former was the abode
of change and corruption, the latter was the region of perfection
without any change. In the sublunary region, substances were made up
of the four elements, earth, water, air, and fire. Earth was the
heaviest, and its natural place was the center of the cosmos; for that
reason the Earth was situated in the center of the cosmos. The natural
places of water, air, and fire, were concentric spherical shells
around the sphere of earth. Things were not arranged perfectly, and
therefore areas of land protruded above the water. Objects sought the
natural place of the element that predominated in them. Thus stones,
in which earth predominated, move down to the center of the cosmos,
and fire moves straight up. Natural motions were, then, radial, either
down or up. The four elements differed from each other only in their
qualities. Thus, earth was cold and dry while air was warm and moist.
Changing one or both of its qualities, transmuted one element into
another. Such transmutations were going on constantly, adding to the
constant change in this sublunary region.
The heavens were made up of the fifth element aether, an immutable
substance. Heavenly bodies were part of spherical shells of aether.
These spherical shells fit tightly around each other, without any
spaces between them, in the following order: Moon, Mercury, Venus,
Sun, Mars, Jupiter, Saturn, fixed stars. Each spherical shell (or:
sphere) had its particular rotation, providing the motion of the
heavenly body contained in it. Outside the sphere of the fixed stars,
there was the prime mover (himself unmoved), who imparted motion from
the outside inward. All motions in the cosmos came ultimately from
this prime mover. The motions of heavenly bodies and their spheres
were perfectly circular and constant.
In this model of the universe, everything had its natural place, a
privileged location for bodies with a particular makeup, and that the
laws of nature were not the same in the heavenly and the earthly
regions. Eempty places or vacua did not exist. Beyond the sphere of
the fixed stars and the prime mover, there was nothing, not even
space. The cosmos encompassed all existence."
Ptolemaeus left the basic concept of Aristoteles' system untouched;
however, while Aristoteles' planets did not move within their spheres
of aether, but only with them, Ptolemaeus added the epicyclic motion
of the heavenly bodies inside the rotating spheres.
Had Ptolemaeus been convinced that the universe was simple and that
thus the simple model is the best, he would not have made an effort
like he did to develop a sophisticated system of planetary motion. He
would have accepted the small discrepancies and problems inherent in
the simple geocentric system with its concentric circular planetary
orbits. But he did not accept it; his goal was to find a system that
corresponded to the actual celestial observations and that offered the
possibility to calculate in advance and predict the courses of the
planets. The model he developed had all these qualities. And its
complexity reflects that Ptolemaeus indeed did not believe that
simplicity is the basic nature of the universe.
Ptolemaeus himself explains the goals of his astronomical efforts and
the approach he takes in his "Almagest":
"We shall try to note down everything which we think we have
discovered up to the present time; we shall do this as concisely as
possible and in a manner which can be followed by those who have
already made some progress in the field. For the sake of completeness
in our treatment we shall set out everything useful for the theory of
the heavens in the proper order, but to avoid undue length we shall
merely recount what has been adequately established by the ancients.
However, those topics which have not been dealt with by our
predecessors at all, or not as usefully as they might have been, will
be discussed at length to the best of our ability."
Sources:
Rice University: Ptolemaic System
http://es.rice.edu/ES/humsoc/Galileo/Things/ptolemaic_system.html
New Mexico State University: Ptolemy's Geocentric Theory
http://ganymede.nmsu.edu/tharriso/ast301/class04.html
Munich University: Universum von Aristoteles und Ptolemäus
http://www.physik.uni-muenchen.de/leifiphysik/web_ph11/geschichte/09epizyklen/weltbildaristoteles.htm
University of Tennessee: The Universe of Aristotle and Ptolemy
http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html
University of Oregon: Hipparchus of Rhodes
http://zebu.uoregon.edu/~js/glossary/hipparchus.html
University of Oregon: Ptolemy
http://zebu.uoregon.edu/~js/glossary/ptolemy.html
University of St Andrews, Scotland: Hipparchus of Rhodes
http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Hipparchus.html
University of St Andrews, Scotland: Claudius Ptolemy
http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Ptolemy.html
Science Daily: Geocentric Model
http://www.sciencedaily.com/encyclopedia/geocentric_model
University of New Mexico: Cultural Astronomy Curriculum Unit in the
Highland High School Physics Curriculum, by W. A. Glover (PDF file)
http://www.unm.edu/~abqteach/ArcheoCUs/99-01-04.pdf
Search terms used:
ptolemy "celestial motion" eccentric epicycles
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ptolemy simple
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ptolemy universe simple
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epicyclic eccentric
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hipparchus planets
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hipparchus ptolemy eccentric epicycles
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ptolemy epicycles ether universe
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ptolemy "simple model"
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Best regards,
Scriptor |
