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miscellaneous musings
cesium133:
The meter was defined long before the light-second. The original definition back in the 1780s was 1/10,000th the length of a line from the North Pole to the equator passing through Paris. When they realized how crappy of a definition that was (they didn't have the ability at the time to measure that distance accurately), they redefined it in terms of an arbitrary metal rod in Paris.
edit -- and in between the arbitrary metal rod and the speed of light definition, they defined it in terms of an emission line in Krypton-86.
edit 2 -- Akima beat me to linking to the Wikipedia article.
Akima:
Because the speed-of-light thing was back-filled to provide a high-precision standard long after the metre had been defined by other criteria. To define it as you suggest would change its size by a small but significant (for high-precision work) amount.
Method of Madness:
Ok, cool. I was just thrown by it being so close to 1/300 million that it almost had to be intentional, and not just a coincidence.
cesium133:
It's also why the inch is 25.4 millimeters instead of 25 mm. When the U.S. abandoned the old standard for the inch in the late 1800s, the definition was known well enough that changing it by 400 microns would have been a problem, but poorly enough that rounding it off at 25.4 mm (rather than defining it more precisely) was sufficient.
Method of Madness:
Dang, if the inch were 25mm I'd be nearly an inch taller :roll:
(~73.2 inches instead of ~72.4)
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