ASTRONOMY 9: HISTORY OF COSMOLOGY

Assignment #6

2000 February 7

Reading Assignment

Course Reader, vol 1.

Note: Page numbers in the course reader refer to the typeset numbers (but the table of contents at the front refers to the handwritten ones). Also, check out the star charts for February and March, which are at the back of the reader.

Readings for Wednesday 2/9:

• pp. 80-89: Plato, excerpts from The Republic
• pp. 89-100: Plato, excerpts from Timaeus

Readings for Friday 2/11:

• pp. 101-107: Aristotle, excerpts from On the Heavens
• pp. 107-108: Aristotle, excerpts from Metaphysics
• pp. 109-113: Aristotle, excerpts from Physics
• pp. 115-117: Aristarchus, excerpts from On the Sizes and Distances of the Sun and Moon

Optional Readings

Note: These books have been placed on reserve at Moffitt Library.

• Gleiser, Ch. 2
• Koestler, Ch. 1-5

Notes on the Readings

1.

Plato's Republic, Book VII: Socrates, Plato, and Aristotle form the great trio of ancient Greek philosophers, who laid the foundation for much of Western thought. Although these philosophers were mainly concerned with ethical questions (unlike the earlier Ionian and Eleatic cosmologists), here we will explore the significant implications of their work for cosmology and astronomy. Plato (about 428-348 BC, Athens) was a student of Socrates (470-399 BC), who was famous for his unyielding questioning of others' knowledge. Plato built his system of thought around the ideas of Socrates, who doesn't seem to have recorded any of his own dialogs in writing. The Republic is thought to have come from the middle part of Plato's writings (about 375 BC). On pp. 80-83 you will find Plato's famous simile of the caves. The main speaker is Socrates, who is questioning Glaucon (actually Plato's brother). The physical world, in Plato's mind, is a mere shadow, an imperfect replica of the true world of Forms. What would Plato have thought of modern science? On pp. 84-87, we pick up where Socrates and Glaucon are discussing the curriculum (perhaps followed at the Academy) for the education of the philosopher-rulers of Plato's ideal aristocracy. Would you want to live in such a society? Where does astronomy sit on the curriculum? What is Plato's opinion of astronomical knowledge obtained through observation?

2.

Plato's Timaeus: This is Plato's work on cosmology. Plato explains his reasons for believing in his own vision of the cosmos. Some questions to think about as you are reading: Is Plato's cosmos eternal, or did it have a beginning? Why? What is the role of the gods? How confident is Plato about his cosmological ideas? How many worlds are there? How many primary ``elements'' are there? What is the shape of the cosmos?

3.

Aristotle's On the Heavens: Aristotle (384-322 BC) surveyed the whole of human knowledge of his time, and wrote a lot of works on philosophy and science. Most of the surviving works are not from the many volumes Aristotle wrote personally but are thought to be lecture notes, perhaps recorded by a student at the Lyceum. Thus it may not be too surprising that many of them are difficult to follow and sometimes even self-contradictory! Aristotle's cosmos is divided into the perfect, unchanging heavens, and the imperfect ``sublunary sphere'' (everything on Earth, within the orbit of the Moon). Everything was thought to have a natural place within this scheme, and natural motion was the result of objects' natural tendency to attain their places. The passage quoted here begins with Aristotle's reasons for believing in a fifth element, sometimes called ``quintessence'' (watch for this to show up much later in the course!) or ``ether'', apart from the usual four (earth, fire, air, and water). He argues that bodies have natural motions (as opposed to unnatural ones). He thinks fire and air naturally move upwards in a line, while earth and water move down. But he argues that there is a simple body (or element) for each simple motion, and circular motion is also a simple motion, so there must be a new element whose motion is naturally circular.

On p. 103, he turns to the question of the spatial extent of the universe. He decides that it must be finite because we see the heavens moving in circular motion daily, which they could not do if they were infinite in extent. Then he argues that there can only be one world like Earth, because the elements naturally move towards or away from the cosmic center (Earth's center), but if there were a second Earth, the direction of this natural motion would have to be different. Next, the heavens must be spherical, since the sphere is the most ``perfect'' solid: it has only ``one surface''. The Earth, as the place to which heavy objects fall, must be at the natural center of the universe and cannot move. Finally, the Earth must be spherical (what are Aristotle's arguments?), and it cannot be very large relative to the distances to the stars.

4.

Aristotle's Metaphysics: What does Aristotle think of the Pythagoreans (note the appearance of their sacred number, 10)? In this selection, Aristotle is concerned with the motion of the planets. He is aware of the Earth-centered model of Eudoxus and Callippus, which postulated 55 interconnected spheres to explain the complicated observed motions of the planets (eg., retrograde motion). Note Aristotle's opinion of cosmological mythology and religion at the end.

5.

Aristotle's Physics: In this work, Aristotle outlines the general principles which are applied to cosmology in On the Heavens. Note that although many of Aristotle's ideas about physics are horribly wrong (as we will see later on), they nevertheless have the ring of ``common sense'' to them. Such is Aristotle's profound influence on Western scientific thought.

This passage in particular is concerned with the importance of circular motion, an idea which will dominate all astronomical thought until Kepler in the seventeenth century! Note Aristotle's refutation of Zeno's dichotomy paradox, and his jab at the Ionians (p. 113). The number of new ``elements'' is thought to be equal to the number of spheres, but Aristotle doesn't seem very sure of himself on this point.

6.

Aristarchus On the Sizes and Distances of the Sun and Moon: This passage looks rather complicated in part due to the fact that the trigonometric functions were not yet used. Aristarchus (about 310-230 BC) seems mainly to have been concerned with solving geometrical problems; his astronomical accuracy was rather poor, even by the standards of his time. But he gets close enough to realize that the Sun was much bigger than the Earth, providing some motivation perhaps for placing it, not the Earth, at the cosmic center.

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