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Comment by WillAdams | Hacker News Reader

An accurate steel replica of earth the size of a ball-bearing would be the most precise ball bearing ever.

Great book on this:

https://goodreads.com/book/show/35068671-the-perfectionists

I've heard this a couple times and never believed it. Earth is perceptibly not round. Scott Manley did a quick video showing that in some satellite images the difference equates to more than a couple pixels. Some quick googling shows the earth being about 0.3% not-round, which would match a 1000-pixel image being not-round by a few pixels. But a 0.3% deviation is exponentially greater than is allowed by the standards for ball bearings.

>> the Earth deviates from spherical by only a third of a percent

https://en.wikipedia.org/wiki/Figure_of_the_Earth

>> For instance, the Anti-Friction Bearing Manufacturers Association (AFBMA) has a set of grades for bearing balls. A grade three ball has to be spherical within 3 millionths of an inch and the diameter must be accurate within 30 millionths of an inch. This means that for a grade three quarter-inch ball, the diameter would have to be between 0.24997 and 0.25003 of an inch and the smallest diameter measured on the ball has to be within 3 millionths of the largest diameter.

https://science.howstuffworks.com/transport/engines-equipmen...

To do the last extra bit of math - that seems like the ball bearings would then be within .01%? Or am I missing a zero?

Smoothness vs perfect roundness are two different things. The earth is very smooth, even if it's not perfectly round.

I think you're talking about different things. It's true, the Earth is substantially non-spherical. But things like mountains are tiny irregularities on a smooth (not-perfectly-spherical) surface.

A better way to talk about this is something like: given a globe of Earth, say a 50cm diameter one, finished with plate Bristol paper, the texture of the surface would not be as smooth as a perfect model of Earth at that scale.

If I remember correctly, the term is ‘rugosity’. Hah, yup! [https://en.m.wikipedia.org/wiki/Rugosity]

No the dominant deformation on earth is that it is flattened at the poles and bulges at the equator. This is because of axial momentum and makes the earth an oblate spheroid.

If you stand at sea level at a pole you are much closer to the center of the earth than if you stand at sea level at the equator.

Only tangentially related, but GP's source was The Perfectionists—I've had it on my list of "books to read one day" for a while now, but this makes me reconsider.... If the author didn't bother to verify a basic fact like this, what else might be wrong?

To anyone who's read the book with an existing background in precision engineering (i.e. and can independently vouch for its plausibility), is it still worth picking up?

My mis-representation wasn't from the book, and of course, what I meant was "smoothest" rather than "precise".

Ah, I see—thanks for clarifying.

Doubt it. Earth is not even a sphere.

Looks like you’re right! Earth is within .3% of being a sphere, where ball bearings are within .01%.

[https://en.m.wikipedia.org/wiki/Earth%27s_circumference]

Correct --- I should have said "smoothest".

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