md wrote:
I think Mach 1 is defined as the sound of speed at sea level at 0 deg C.
Implicitly we mean "on earth". Mach 1 is therefore an absolute number,
and the same everywhere in the universe.

I wondered about that, so I looked it up. As far as I can tell, Mach 1
is when an object travels as fast as sound *in that medium*. Presumably,
this is useful because interesting turbulence phenomena show up at that
speed, whatever it might be in that medium under those conditions. If
Mach 1 were defined absolutely, as you suggest, Mach 1 would no longer
be a convenient way of speaking of the speed at which those phenomena
appear.

Brian Tung
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