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Old 05-22-2009, 06:06 PM
zut zut is offline
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Join Date: Apr 2000
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Originally Posted by Sage Rat View Post
Those appear to both be about double the thickness. A hair is about a tenth of a millimeter and aluminum foil about 2 tenths. So it probably would be detectable as a scratch or whatever.
Aluminum foil is not, in general two tenths of a millimeter. Aluminum foil, in general, has a thickness less than two tenths of a millimeter. Household aluminum foil is in the range of .02mm, a full order of magnitude less.

As to whether you could feel features of this size on a pool ball, I think it would depend more on the steepness of the feature than the absolute height. Something like the Grand Canyon, with its steep walls, you could probably feel. Mount Everest, I don't know.

Originally Posted by The Controvert View Post
According to this post, the smoothness of the Earth is compared to ball bearings and billiard balls.
Due respect to the Bad Astronomer, but he's wrong on this one. "Smoothness" is a measurement of surface finish, not a measurement of sphericity or deviation from a standard diameter. The specification quoted (diametral tolerance of +/- 0.005 inches) covers only the deviation from standard size. It does not, necessarily, cover how spherical the ball may be, and it *certainly* says nothing whatsoever about surface roughness.

So the Earth isn't *more* smooth than a billiard ball or a reasonably-sized ball-bearing. Whether it's close is a matter of opinion, but I'd say not really.