Warning, spoiler ahead
A somewhat more mathematical version: assuming small number of radians n from either the pole or the equator.
For pins, the area of the (unit) sphere is pi*n^2 for the pole and 2pi*n for the equator, with the ratio n/2 for pole/equator area, which goes to zero as n goes to zero.
And as you said, for a disk "near the pole" means its normal is near the equator, so the relevant area is 2pi*n, "near the equator" means its normal is near the pole, so the result is the opposite.
An interesting aside is that disks and pins in 3D are actually Hodge duals (that's the reason torque or magnetic field can be treated as vectors sometimes).
Yes, that's how I visualized i, although it wasn't completely obvious to me why it was correct to do so. I suppose a more obvious, but unwieldy, visualization is marking two special spots (not colinear) along the perimeter of the disk and imagining how they could be animated within a sphere as the disk rotates.
Has anyone read "How We Reason" by Philip Johnson-Laird? He and others in his field (the "model theory" of psychology/cognitive science) claim that their studies refute the naive claim that human brains often operate in terms of logic or Bayesian reasoning (probablistic logic). I gather they'd say that we are not Jaynes' perfect Bayesian reasoning robot or even something resembling a computationally bounded approximation to it.
I'm intrigued by this recommendation:
It seems like an interesting read, but I'd like to know if the research field is a scientific one, i.e. that their stories aren't just pleasing, but can predict, or at least explain real phenomena.
In the Google books preview, I see the author spends some time claiming that we build iconic visual/spatial representations and that a lot of our thinking isn't verbal or available to verbal introspection (fairly uncontroversial to me).
I liked the two related imagination-puzzles:
1. I have thousands and thousands of very thin needles, which I hold in a bundle in my hands. I throw them up into the air, imparting a random force to each of them. They fall to the ground, but, before any of them hits the ground, I stop them by magic in mid-air. Many of the needles are horizontal or nearly so, and many of them are vertical or nearly so. Are there likely to be more needles in the first category, more needles in the second category, or do the two categories have roughly equal numbers?
[and the same thing but for very thin circular disks - let's assume they're also dense, so the air isn't a factor]
2. I have thousands and thousands of very thin circular disks, which I hold in a bundle in my hands. I throw them up into the air, imparting a random force to each of them. They fall to the ground, but, before any of them hits the ground, I stop them by magic in mid-air. Many of the disks are horizontal or nearly so, and many of them are vertical or nearly so. Are there likely to be more disks in the first category, more disks in the second category, or do the two categories have roughly equal numbers?
He claims that for spatial propositions where we can imagine a picture that's more or less equivalent ("the cabinet is behind the piano" [as we face the keys]), the negation of that proposition can't be so pictured (in direct correspondence) because ... where would you put the cabinet? You could imagine all the alternative places it could be (presupposing that there is a specific piano and specific cabinet). You could imagine something "not cabinet" behind the piano (a cabinet with a red x, a cabinet repelling field?). He suggests an or(p1,p2,....pn) of images where we imagine the cabinet to be (that aren't behind the piano). I'm not sure what we can conclude from this. We already know that negation is tricky - linguistically, and mentally. Maybe I like to imagine someone telling me "no, you're wrong to say X" - to use a non-visual system.
He explains that inferences about (written) non-spatial visual relations (light/dark clean/dirty) take longer to process than spatial ones, that the spatial and visual word inference word problems had different fMRI hot spots, that congenitally blind people weren't faster on spatial queries (i.e. were slower on average than non-blind, but didn't suffer any additional penalty on the visual ones). I suppose this could be taken as weak evidence that we can perform "logical" inferences with some sort of spatial-relationship processing, and that perhaps non-spatial attributes take longer to translate (even though they refer to visual qualities like light/dark).
I'm leaning toward buying the book, since the writing is pleasant. But I thought first I'd ask if anyone could recommend for the quality of research in this field.