Cosmology and Astronomy
- Q: What is the role of contraries (opposites) in pre-Socratic cosmology?
A: Early Greek natural scientists had various perplexing questions to grapple with. These included: How did things begin, what are they made of and how did the dispositions of the elements (including the celestial bodies) come to exist as we now perceive them? Three assumptions were made in order to attempt an explanation. These assumptions were: that the world began from a primary unity, that the first things formed from that unity were pairs of opposites and that these opposites interacted in different degrees to form the makeup of all things. Some examples of these opposites and the differing interpretation various philosophers used include Anaximander's indefinite which he says splits into pairs. Anaximenes had air as his elemental stuff, and all formed because of stratification due to differing densities and rarities of the air. Lastly, Heraclitas believed love and strife to be the prime motivators of dissociation in the universe.
5) Q: Explain the difference between appearance and reality in at least one Pre-Socratic or Greek (Plato or Aristotle) cosmology.
A: Sweeny, in Infinity in the Presocratics explains that Parmenides believed in a universe without difference and multiplicity, without motion and change or separation and contradiction, indefiniteness, imperfection and absence1. Therefore, the universe has neither temporal origin nor termination, it is a place of absolute identity and unity, of total stability and permanence, of continuity and sameness, of determinateness, perfection and completeness. To Parmenides, reality or being is unity, is originless and possesses temporally endless immutability. The difference then between appearance and reality is indefinable. As Tarán states in Infinity in the Pre-Socratics: "There can be no difference in what exists, so nothing can be distinguished inside Being... Being is all alike because it can only be without restriction... Since it exists it does so without restriction because there can be nothing other than Being, for other than Being would be non-Being and this is impossible"2. This complex statement essentially explains the inherent futility in attempting to perceive a difference between Being (reality) and appearance.
Q: In what sense, according to Plato, was the true utility of the study of astronomy and cosmology the saving of the soul?
A: Plato believed in the divinity of the heavens, and that everything had a life principle. This life force was the underlying motivator to the entire universe. This belief allowed Plato to deal with the universe as having a purposive order and rational decision making abilities. He believed that the individual characteristics of things came from their own choices. Plato illustrated this belief well in respect to the heavenly bodies in his treatise Timaeus. He postulated that the stars are gods and all motions thereof are psychokinetic. Also, the souls of the star gods are perfectly rational, and that all perfectly rational motion is circular. Therefore, he deduced, all stellar motion is circular, and perfect. This example of saving the phenomena at the expense of observational evidence also serves to save the soul principle of Plato's cosmology.
Q: In what sense is Plato's cosmology Pythagorean?
A: In his book the Republic, Plato states that the eyes are made for astronomy, the ears are made for harmony. This is an important statement because it illustrates how Plato saw the relationship between number and harmony. This is a Pythagorean ideal, they believed that numbers were crucial to understanding nature. Plato believed that the true source of understanding in cosmology is to be found in the first four numbers. He was not concerned with observation, he thought that the senses were deceptive and that geometry was the only pure enough tool to use. This Platonic rapprochement toward Pythagorean thought may be said to be the beginning of the debate between observational and theoretical sciences.
Q: Explain the significance of observation in Aristotle's cosmology.
A: The previous question showed Plato's interpretation of the cosmology of the age. Aristotle was at odds with the way that Plato used science to his own ends, going so far as to call Plato's way of thinking primitive and confused. Whereas Plato was attempting to ‘save the phenomena', Aristotle wanted to explain the phenomena. Aristotle believed that observations were more important than ratios. Of the Pythagoreans, Aristotle held the opinion that they were not seeking observation, they were simply accommodating their own theories.
Q: Why, according to Plato and Aristotle, do the heavenly bodies move in uniform circular motion?
A: Aristotle believes that all natural bodies are capable of moving in space and that all motion in space is either straight or circular. The reason is that these are the only simple motions, because the straight and circular lines are the only simple magnitudes. Plato is slightly more doxological, thinking that the Demiurge created the world soul by cutting up into strips the soul-stuff he has produced, and joining the ends to make ‘circles' that is, mobile circular bands. These bands are circular because another Platonic doctrine states that rotary motion is the one most appropriate for reason and intelligence.
Q: What is the problem with retrograde motion?
A: One of the most puzzling questions plaguing pre-Copernician astronomers was that of retrograde motion. Retrograde motion was observed to happen by the early astronomers, and was believed to be actual backward motion. Copernicus showed that this backward motion of the planets was an illusion. It occurs because the planets revolve around the sun at varying distances. As a result, the earth appears to overtake a more distant planet, which then appears to be traveling backward. Another possible source of apparent retrograde motion is that of an inner planet such as Mercury which has a smaller orbit than the earth. During a single earth year, Mercury orbits the sun several times. Thus its direction seems to change repeatedly as it overtakes the Earth and then it moves away from us. In fact, its direction is constant.
Q: What role does the equant play in Ptolemy's theory of the planets?
A: The equant was invented by Ptolemy in order to make his cosmological theories work. It is situated the same distance from the center of the deferent as the earth, but in the opposite direction. The equant balances the discrepancies encountered in trying to conserve the idea of uniform circular motion in the heavenly bodies. In respect to the equant point, the planets move in uniform circular motion. The criticism of the equant point was raised because the actual motion (with respect to the earth) of the planet on the epicycle is non-uniform. Ptolemy constructed a mathematical device in order to conserve the proper motion of the epicycle, rather than relying on observation and deduction, he wanted to ‘save the phenomena'.
Q: Which Aristotelian principles does Copernicus overturn?
A: Aristotle's original principles included the notion that the earth was the central point of the universe (geocentric). Aristotle also did not make a distinction between the motions of terrestrial bodies and those of celestial bodies. Neither did he believe in the notion of empty space or void, stating instead that it was filled with ether. Lastly, Aristotle believed that all celestial bodies exhibited the same uniform circular motion in their travels. Copernicus, on the other hand said that: there is no one single center for all celestial orbits (therefore, no geocentrism), that earth is the center only of the lunar orbit, that all orbits encompass the sun which is the center of our solar system(heliocentric), and that the ratios between sun/earth and the fixed stars are insensible. The last point of Copernicus' is a rejection of Aristotle's views on the void, saying that man cannot grasp the distances, and therefore they are unmeasurable.
Texts Consulted
Sweeny, Leo Infinity in the Presocratics: A Bibliographical and Philosophocal Study
Martinus Nijhoff, The Hague, Netherlands. 1972
Sambursky, Shmuel Physical Thought from the Presocratics to the Quantum Physicists
Pica Press, New York. 1975
Hathaway, Nancy The Friendly Guide to the Universe
Penguin USA.1994
Munitz, Milton K. Theories of the Universe
The Library of Scientific Thought, New York University.
Ehrhardt, Arnold The Beginning
Manchester University Press. 1968
1 Sweeny, Leo Infinity in the Presocratics: A Bibliographical and Philosophical Study
Martinus Nijhoff, The Hague, Netherlands. 1972
2 Ibid.
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