[time-nuts] Exceptions...

Lux, James P james.p.lux at jpl.nasa.gov
Mon Oct 27 04:05:03 UTC 2008

On 10/26/08 5:34 PM, "Jim Palfreyman" <jim77742 at gmail.com> wrote:

> If the Earth were a snooker ball you would not be able to tell it is not a
> sphere...
Perhaps after consuming an ale or two..

Let's see.. It's out of round by 1 part in 300.  Human visual acuity is
about 1 minute of arc, 1/60th of a degree, or about 0.3 milliradian. Holding
that snooker ball at arms length (call it a meter), and it's roughly 52 mm
in diameter (per Wikipedia), so it subtends 52 mrad, and the 1/300 out of
round would be 1/6 mrad.  Oooh.. Pretty close to human visual resolution.

Hold that ball a bit closer (which, if you suspected it was not spherical,
you might be tempted to do..), and you had a real circle to compare to?
Would it look visibily non spherical? Probably depends on how much
astigmatism you have.
Certainly, one could easily measure the difference (160 micron)

Here's a real question, though.  Would a ball that's out of round by one
part in 300 not roll straight?

While we're speaking of polishing cannon balls...
Could this valuable piece of information be used to win a bar bet?  I'm
going to a conference soon, and such things are useful to know during poster


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