Reading Assignment

Course Reader, vol 1.

Readings for Monday 2/14:

pp. 118-133: Ptolemy, excerpts from the Almagest

Optional Readings

pp. 134-145: Ptolemy, excerpts from Tetrabiblos
pp. 201-206: Gingerich, Astronomy in the Age of Columbus

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

Gleiser, Ch. 2
Koestler, Ch. 1-5

Journal Assignment

Due: Friday, 2/11, 5:00 pm, on the internet
This is a short (a few sentences will suffice) ``undirected'' entry: write about whatever aspect of the class comes to mind. Unlike the other entries, it is not to be turned in on paper; I want you to post your work on the class discussion board, which you can access through the web page. If you don't have other ideas, it will be useful if you tell me what you like and dislike about the course, or how you think it might be improved. Note that you can post anonymously, but for this assignment you need to at least tell me who you are (by e-mail, if you want to post anonymously so that your fellow students won't know who you are), so that I can give you credit.

Notes on the Readings

1.

Ptolemy's Almagest: Very little is known of Ptolemy's life (approx. 85-165 AD, full name Claudius Ptolemaeus, not to be confused with the Egyptian kings who were also called Ptolemy!). However, his great synthesis of Greek geocentric cosmology, now known by the Arabic name Almagest (``The Greatest''), survived as a text which was to dominate astronomical thought for 1500 years! It is essentially an encyclopedia of astronomy and mathematics, and it was so influential that the idea of the Earth-centered cosmos became known as the Ptolemaic system. The first eight sections of Book I, which gives a broad outline of the geocentric cosmology, are found on pp. 118-124. In the preface (p. 119), Ptolemy first discusses the relative merits of theology, physics, and mathematics. To which does he assign the greatest importance? What implications does this have for his cosmology? The last paragraph of the preface reveals Ptolemy's intention to summarize the state of astronomical knowledge, but note that he also made important contributions of his own (including some original astronomical observations). Ptolemy then tries to justify the basic tenets of his Earth-centered system. He argues that the heavens are spherical, based partly on the observation that the stars seem to move in circles around the pole, but also notice the Aristotelian ``ether'' showing up here. The arguments given for a spherical, immovable Earth located at the cosmic center are also largely derived from Aristotle. Although Ptolemy shows that he understands that the Earth is relatively small, his entire cosmos would have occupied a radius of only about 50 million miles, vastly smaller than the universe as we know it today (the Sun is almost twice as far away)!

We pick up again in Book III; notice what Ptolemy has to say about simplicity in explanations of natural phenomena, despite the fact that he is about to construct a monstrously complex explanation for retrograde motion!

Book V has a short but complicated section on the construction of an astrolabe; just notice how difficult it is to devise instruments which can measure the positions of stars and planets accurately enough to construct an accurate model of the solar system. Section 13 of this book, determining the distance to the moon, gives you a taste of Ptolemy's geometrical arguments. In Book IX, Ptolemy turns to the detailed construction of his geocentric system. Note the last paragraph included, which returns to the idea of simplicity in science and the Greeks' desire to ``save the phenomena''. That is to say, the Greeks did not actually believe in the physical reality of the many epicyclic motions necessary to explain the apparent motions of the planets; rather, they thought of themselves as devising mathematical tricks to model the physical world, which was merely a pale shadow of some simpler underlying reality.

2.

Ptolemy's Tetrabiblos: Last week we discussed the fact that until about 1700, there was no sharp division between astronomy and astrology. Ptolemy was no exception, and his Tetrabiblos (``Four Books'') systematized many astrological ideas which are still very much with us today! Check out the very amusing and bizarre section on ``Monstrous or Defective Births'' (p. 145).

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jonathan baker
2000-02-09