Kevin's Watch

Is there a thread anywhere that serves like a 'noticeboard' where you can just make observations or indeed ask questions that could be answered in one post by someone who knows the answer.

I want to ask you (erudite and learned )guys if anyone knows where I could acess an image of Ptolemys geocentric model of the cosmos with all it's cycles, epicycles and defferents etc (there were over 60 of them I believe); I have googled it and only ever seem to come up with these old medieval drawings with the earth at its center and lots of concentric circles around it. What I'm after is a diagram of the whole arrangement such that if it were a clockwork model the apparent motions of the planets would be seen to correspond with the observable motions (with their retrogressions, back flips etc) in the sky. I believe the Ptolomeic system did pretty much achieve this feat, massivly complicated though it was - but I just can't seem to find a diagram of how it would have worked.

It seemed a nonsense to start a new thread to ask this one answer question so I started a thread to ask where I should ask it instead

people.sc.fsu.edu/~dduke/ptolemy.html

The route: Wikipedia Ptolemy->Almagest->related links->Ancient planetary model animations->Ptolemy's

I'd suggest just asknig in the Loresraat, I don't think people are concerned about creating brief one-off topics much.

And anyway, if we decide to start talking, or disagreeing, about it, it might not stay "one answer."

I don't think it matters at all. A topic in the forum that is appropriate to the question should be fine. Also, you'd probably get a quicker answer if the topic specifies what the question is.

(And any general thread soon gets filled with people posting random pictures and the like...)

--A

Avatar wrote:And anyway, if we decide to start talking, or disagreeing, about it, it might not stay "one answer."

I don't think it matters at all. A topic in the forum that is appropriate to the question should be fine. Also, you'd probably get a quicker answer if the topic specifies what the question is.

(And any general thread soon gets filled with sarge posting random pictures and the like...)

--A

FTFY.

The answer to your question is: No, there are no single answer questions. Or there are.

Avatar wrote:
(And any general thread soon gets filled with people posting random pictures and the like...)

--A

I really wish people would stay on topic. That is why we have topic headers. Makes it easier to sort the stuff from the fluff. If you get my transcontinental drift.

sgt.null wrote:

Avatar wrote:
(And any general thread soon gets filled with people posting random pictures and the like...)

--A

I really wish people would stay on topic. That is why we have topic headers. Makes it easier to sort the stuff from the fluff. If you get my transcontinental drift.

Thread drift is inevitable.

Vraith wrote:The answer to your question is: No, there are no single answer questions. Or there are.

Really?

Nice one Murrin. The cycles and epicycles etc were not as I expected but it was a good working model! Do you know if it actually worked in terms of agreeing with the observed positions of the planets and acurately predicting their future ones?

I don't really know anything about it, just did some quick searching. It looks like the smaller circles within the orbits are there to compensate for the Earth's rotation around the sun, I wouldn't be surprised if it fits pretty well with what was observable then.

It actually worked very well at predicting where the bodies would be in the sky.
[I think significantly less than 1% margin of error on ordinary time scales...of course that adds up when we start talking decades, centuries, millenia]

edited to add the words margin of error cuz the sentence makes no sense without them. Duh me.

Well, at least some people stuck to the topic. I mean the question.

--A

If I remember correctly, the model had to be constructed with it's 'watch like' mechanism to account for retrograde (ie backward) movements seen in the paths of the planets when their motions were observed night after night. If the Earth centered model of the cosmos were to be maintained (remembering these retrograde motions would not be seen in a model with perfectly circular orbits of the planets with the earth at it's center) the eccentric cycles and epicycles had to be added to account for these observations.

What interests me here is the degree of accuracy (<1%) this model was able to produce re the observed data and still be totally wrong at it's very center (excuse the weak pun ). I wonder how accurate our current calculations are re their fit. From an 'insrumentalists' point of view it matters little which of the models (heliocentric or geocentric) are correct - what matters to them is merely the predictive accuracy of the respective models. For him there is no requirement for a scientific model to correspond to 'the truth'. it is exactly that - a model for use in prediction,no more. (Hasten to add this is *not* leading to an argument in favor of geocentrism).

Well really if you think of it as modelling motion around an arbitrary reference point and not around a fixed point then the system isn't wrong at all. All motion is relative, it's just simpler to model around the largest local centre of gravity than an arbitrary point in the system (earth).

peter wrote:
What interests me here is the degree of accuracy (<1%) this model was able to produce re the observed data and still be totally wrong at it's very center (excuse the weak pun ). I wonder how accurate our current calculations are re their fit. From an 'insrumentalists' point of view it matters little which of the models (heliocentric or geocentric) are correct - what matters to them is merely the predictive accuracy of the respective models. For him there is no requirement for a scientific model to correspond to 'the truth'. it is exactly that - a model for use in prediction,no more. (Hasten to add this is *not* leading to an argument in favor of geocentrism).

From last to first...IIRC, there were at least a couple peeps in the 200BCE era that knew the sun-center was correct [at least in more manageable sense], but they said basically [paraphrasing of course] "But that's just a fact, the truth is a matter for religion/philosophy."

I would think it matters more to the instrumentalist which is correct...because the instruments will tell you [when they're good enough] that the other planets literally do not do little reverse circles at various points in their orbits. The math is the part that is equally accurate for both models...one kind is just simpler to calculate...though if [as I now suspect you are] you're speaking of the philosophical school "instrumentalism", I concede the point.

On the accuracy, it's pretty amazing they could do it so well...and if they knew there were more planets and that orbits were ellipses not circles they'd have done even better. On our current fit of predictions...barring something like a planetary-mass or larger thing sneaking up on us, we're talking a time scale of at least 10's of millions of years before the error is up to 1%, I suspect even much longer. For instance, in the late 1980's [the reason I know about this is a random long story itself, almost a surreal...but I won't digress further] a refinement of an earlier calculation method allowed for the prediction of everything from Mars inwards on a scale of 1 INCH of error total over the span of January 2000 Plus or minus 4thousand years. I'm betting there are much better methods and observations now.

So much for single answers...

(Ok, sorry, I'll shut up.)

--A

Avatar wrote:So much for single answers...

(Ok, sorry, I'll shut up.)

--A

Hey, I said there weren't [or were] so don't blame me! [or do].

Don't mind me, just carry on.

--A

Yeah The single answer thing got a bit lost along the way - we could call it 'thread shift'.

Vraith - thats too much Man - that story has to be told! (if you've got time or the inclination of course ). I think I'm talking about the 'philosophical school' of Instrumentalism but can't be 100% on it. I first came across the idea of science as 'model building' in a book by Horace Freeland Judson called The Search for Solutions many years ago, and I'm pretty sure he introduced the idea that it was the 'predictive value' of the models that was important rather than their exact correspondance to reality. This idea is treated pretty scathingly by David Deutsch in The Beginning of Infinity, which I am currently reading - I'm guessing that he is of the 'Each model gets succesively closer and closer to the truth' school of physisists, but in fairness the book crosses the boarders between science and philosophy fairly freely, so I can't be too sure in what direction his opprobrium of instrumentalism is pointed.

Murrin - that was a point I was close to making but lost confidence in the idea as I was typing it. Do the 'bang up to date' models of the universe/existence/reality really fix anywhere as the reference point - and on this idea as you say, any point can be taken as such and a (albeit highly complicated) mathematical model constructed to fit the observations. In the light then of our latest Cosmologies/Quantum Multiverses etc can any one of the models be truly said to be 'more real' than the other. ie will our latest theories actually force our hand to take a more instrumentalist view of the nature of scientific models (Deutsch's obvious distain of them notwithstanding).

You guys do realize that some of us check the Watch first thing in the morning when our eyes are barely open and we haven't had any coffee yet, right?