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Was the Ptolemaic theory merely a matter of religious faith?
No, the idea that Earth was the center of the universe was not based just on religious belief. The Ptolemaic theory, constructed by Ptolemy (90-168 c.e.), did a fairly good job of describing both sensory experience and astronomical records and calculations that went back several thousand years. The movements of the heavenly bodies, which were themselves believed to be made of different and more ethereal stuff than Earth, could be more or less accurately predicted, according to this theory. It was also in accord with the existing natural philosophy that everything was made up of earth, water, fire, and air, in an ascending hierarchy. However, Ptolemy's assumption that Earth was stationary required a postulation of 80 "epicycles" to "save the appear-
What is an epicycle?
An epicycle is a type of circular motion that is not observed but, rather, theoretically postulated. From the postulation, what could be observed became predictable, which was how it "saved the appearances," or was consistent with what was observed. In the Ptolemaic system, the 80 epicycles were necessary to account for the different speeds and directions in the observed movements of the Moon, Sun, and five known planets. They also explained differences in how far the planets appeared to be from Earth at different times. The planets themselves were believed to move in small circles, which themselves moved along "deferents," or large circles. Both the epicycles and deferents moved counter-clockwise in planes approximately parallel to the plane on which Earth was situated.
ances," which means that new complicated postulations were necessary to make the theory match observations.
How did Copernicus change the Ptolemaic system?
The system introduced by Nicolaus Copernicus (1473-1543) was that Earth and all of the planets revolved around the sun in concentric circles. Copernicus was further able to reduce the number of postulated epicycles to 34, still saving the appearances, or not contradicting what was observed. This shifted the fundamental frame of astronomical reference from Earth to the fixed stars. As he wrote:
First and above all lies the sphere of the fixed stars, containing itself and all things, for that reason immovable; in truth the frame of the Universe, to which the motion and position of all other stars are referred. Though some men think it to move in some way, we assign another reason why it appears to do so in our theory of the movement of the Earth. Of the moving bodies first comes Saturn, who completes his circuit in xxx years. After him, Jupiter, moving in a twelve year revolution. Then Mars, who revolves biennially. Fourth in order an annual cycle takes place, in which we have said is continued the Earth, with the lunar orbit as an epicycle. In the fifth place Venus is carried round in nine months. Then Mercury holds the sixth place, circulating in the space of 80 days.
Copernicus' conclusions were based mainly on mathematics, drawing on the perennial value of simplicity and the doctrine that nature always behaves in the most "commodious" (simple) way. To the objection that objects would fly off a moving earth, he responded that a moving sky, because it was larger, would move even faster and do more damage.
What did Copernicus mean by "The Thrice Greatest"?
Copernicus' phrase "The Thrice Greatest" was a reference to Hermes Trismegistus, the Greek name of the Egyptian god Thoth, who was credited with healing arts and secret knowledge by Neoplatonists.
Was Copernicus' new theory purely scientific?
No, because there was considerable mysticism in his astronomical ideas. consider these two passages from his De Revolutionibus Orbium Coelestium Libri IV.
Finally we shall place the Sun himself at the center of the Universe. All this is suggested by the systematic procession of events and the harmony of the whole Universe, if only we face the facts, as they say, "with both eyes open".
At rest, however, in the middle of everything is the Sun. For, in this most beautiful temple, who would place this lamp in another or better position than that from which it can light up the whole thing at the same time? For, the Sun is not inappropriately called by some people the lantern of the universe, its mind by others, and its ruler by still others. The Thrice Greatest labels it a visible god, and Sophocles' Electra, the all-seeing. Thus indeed, as though seated on a royal throne, the sun governs the family of planets revolving around it.
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