Edit made on February 20, 2009 by RiderOfGiraffes at 11:41:52
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Broadly speaking, the original definition of the metre was
"one ten-millionth of the distance _
from the North Pole to the Equator _
on the meridian running through Paris."
This was adopted in competition with the definition of
length of a pendulum with length giving _
one second half the period" period of one second"
The definitions are remarkably
similar, but similar because of the coincidence that EQN:\pi^2 is very nearly equal to /g,/ acceleration due to gravity.
However, both definitions have problems. You can
read more about them in the Wikipedia article quoted below.
More recently, the metre has been redefined as:
"the distance travelled by light in vacuum during _
a time interval of 1/299 792 458 of a second."
Thus the speed of light is now, by definition, 299 792 458 m/s.