Friday, October 28, 2011

Ptolemy was an instrumentalist

It is common to say that Ptolemy's Almagest was wrong because it said that the Sun went around the Earth. I say this opinion is wrong on several levels. The 20th century view is that motion is relativity, so it is legitimate to say that the Earth goes around the Sun or the Sun goes around the Earth.

It is also wrong because, as I explain in my book, geocentrism plays no role in the Almagest model.

I just discovered The cosmos: a historical perspective, by Craig G. Fraser. While he does not agree, at least he acknowledges that Ptolemy can be seen as an instrumentalist:
In the case of Ptolemy, evidence for his instrumentalism is found in his presentation of different geometrical models to explain the same motion, models that are clearly incompatible if they are regarded as material mechanisms to preduce the motion in question. [p.24]
On the other hand, the first book of the Almagest has a discussion of Aristotelian arguments for geocentrism, and book five has a method from Aristarchus for estimating the distance from the Earth to the Moon.

Fraser goes on:
Underlying the notion of a rift between astrcrtorny and cosmology is the view, expresscrl by Ashoe (2001, Tl), that "the role of a geometrical model of the motion of, say, a planet is that of serving as a basis for computing the planet°s position at a certain time in some relevant coordinate system" and that (116) "the principal aint of the Almagest is to enable you to answer the question: Given your location on the Earth, and given the time, in precisely which direction should you look in order to see a given celestial body?" Ancient Greek astronomy is devoted to the calculations of positions of planets as functions of time and includes eclipse theory; it is mathematical and is concerned with prediction. Cosmology attempts to identify the physical arrangement of the heavens; it is qualitative and is concerned with explanation.
I think that is correct. By this distinction, the Almagest was astronomy and not cosmology. It is unfair to say that its cosmology is wrong when it is not a cosmology book.
Ptolemy, in the Alfmagest, accepts as fundamental the concept of planetary order with respect to the Earth. The planet Mercury is always closer to the Earth than the Sun is to Earth, and thc Sun is always eluser to the Earth than Mars is to Earth. Although Ptolemy seems to suggest in the ninth book that the actual ordcr of planets may logically be arbitrary. the existence of an order ltself is never questioned, either implicitly or explicitly; each planet has a fixed zone within which it alone moves, defined by its maximun and minimum distances frum the Earth.
No, Fraser is going off the rails here. I had not noticed that Ptolemy suggested that the order of the planets is arbitrary, but it seems obvious from his model anyway. He puts all of the planets at a nominal distance of 60 units from the Earth, without paying any attention to the possibility that they might collide if they were really all on the same celestial orb. He is just modeling how the planets are seen from Earth. There is no fixed zone for each planet.