From: Ron Kean (ronkean@juno.com)
Date: Sun Jul 18 1999 - 19:26:38 MDT
On Sat, 17 Jul 1999 15:23:56 -0400 "Michael S. Lorrey" <mike@lorrey.com>
writes:
>Ron Kean wrote:
>> In the 1980s, the meter was redefined in terms of the speed of
>light,>> whereas earlier it had been defined as a certain number of
>wavelengths of>> a certain spectral line of Krypton 86. The meter is
now defined as >the>> distance light travels in 1/299,792,458 second.
That means we no >longer>> have to wonder what is the exact speed of
light. It is 299,792,458
>> meters per second, exactly, by definition.
>
>Wasn't it originally based on the distance from the north pole to the
>equator along the surface of the earth?
>
>Mike Lorrey
>
Yes, but not along the earth's surface exactly, since the earth's surface
is not the same as sea level. At first it was conceptually defined as
one 10,000,000th of the horizontal sea level distance from the equator to
the north pole, along the meridian of Paris, France. For convenience and
practicality, it was re-defined as the distance between two marks on a
platinum-iridium bar kept in a vault at the BIPM (Bureau Internationale
des Poids et Mesures) in Sevres, France, near Paris. Due to the
relatively primitive surveying technology and geodesy of the time, the
marks on the bar ended being not quite exactly the meridional distance
intended, but once the bar standard was established, it became the
definition. Then it was again re-defined in terms of light wavelengths.
Finally in the 1980s it was re-defined in terms of the speed of light.
At one point Michelson (famous for the Michelson-Morley experiment) went
to France to carefully measure the length of the meter (the reference
bar) using his invention: the optical interferometer. But I think he
used sodium light, not krypton 86.
The fact that the meridional circumference of the earth is nominally
40,000,000 meters makes it useful for some purposes to express latitude
in grads, as each grad of latitude is nominally 100 km. A grad is 1/100
of a right angle, or 9/10 degree. The earth's equatorial circumference
is about 1/3 per cent larger than the polar, or meridional,
circumference.
Ron Kean
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