I actually handed out apples one time in discussion section, and we measured the thickness of the skins. Vagaries are introduced by the thickness of the skins of different varieties of apples, exactly how you define the thickeness of the atmosphere, etc. Our appleskins turned out to be a little to thin, IIRC, but they were the right order of magnitude.
Get a globe and do the quick and dirty. An altitude of 50 miles is considered as having “entered space”. Effectively, at that altitude you are outside the earths atmosphere. Find out on the scale of your globe how much 50 miles is then measure. If you haven’t seen the demonstration before you will be suprised at how thin the atmosphere is.
What level of roughness is detectable through touch ?
I’m sure you could feel a human hair on a billiard ball, and something narrower than that too.
The accuracy to which the billiard ball is spherical, is a red herring (imho). The thing could be egg shaped, it doesnt change how smooth its surface is.
The tolerance to which the ball is constructed to be spherical, will not be the same as how smooth its surface is.
Not just erosion–plate tectonics plays an important role. For example, shield volcanoes, which occur over hot spots in the magma, can’t get as big on Earth because the crust slides over the hot spot, creating a chain of mountains rather than just one honkin’ mountain, like Olympus Mons.
I’ve heard that the earth is smoother than your average marble.
Also, I’ve read that somewhere (I think somewhere in the US southwest region), they built a microsopically-accurate 3 foot wide copy of earth out of some material, and running your hand over it, it’s too smooth to detect any bumps.
Why would you bother to build an accurate model where you can’t even feel the relief?
At three foot scale, the tallest peaks of the Andes (which are conveniently quite close to the sea) would be 0.04 inches above sea level. I’ll buy that most of the planet would feel very smooth, but you should be able to feel something–maybe not as much as people would expect, though!
I just checked out the stats on the Grand Canyon, a pretty big defect in the earth’s surface. 1.5km deep, 15km wide, 450km long. Given Pokadyne’s stats above (20km = 89[symbol]m[/symbol]m), that would translate to 6.7[symbol]m[/symbol]m deep by 67[symbol]m[/symbol]m wide by 2000[symbol]m[/symbol]m long (2mm)
That’s a pretty darned small scratch, noticable if you look for it, I’d bet, but not a big deal. Scratches like that are probably common for any but the newest billiard balls. I don’t think too many other features of the earth would be much more noticable than the GC.
I once read this thing in the Smithsonian magazine that said that the average person can detect an inconsistency to the tune of a small fraction of a human hair. So, I think human touch can determine very small rubtures or flaws.
In the argument, when do we begin to consider the felt on the pool table and its ability to absorb the shock of bumps on a poll ball.
I think we’re all grossly underestimating the exquisite sensitivity of the human fingertip. We can easily detect features much smaller and thinner than a human hair.
A quick web search failed to produce any hard data, but here are a few simple tests I’ve just done that illustrate this sensitivity:
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Run your finger across the surface of your monitor. It will feel mostly smooth and flat, but (unless you’re working in a “clean room”), you should be able to feel the particles of dust or dried saliva (from coughing, sneezing, etc) that have attached to the screen. (OK that sounded gross, but it’s true!)
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Rub the tips of your index fingers together. You can feel the texture of your fingerprints.
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I have an HP Deskjet inkjet printer, and there is a definite and detectable texture difference on the parts of a page that have been printed with text, compared to blank parts of the same page.
I have no doubt that parts of the earth’s surface would have a definite texture, and that this texture would not be uniform across the globe. There would also be very large areas that felt extremely smooth.
Could it be that the billiard ball analogy is correct in that the “average” smoothness of the earth would be much smoother than a polished billiard ball. While a sensitive finger may be able to detect the Himalayas, the oceans (2/3 to 3/4 of the surface area) would be far smoother than any surface that anyone could ever hope to attain with the most highly polished billiard ball.
Just a thought! No math to back it up.