LogFAQs > #967061168

A meter was originally the measure of length equal to the ten-millionth part of Earths meridional arc between the north pole and the equator:

In La Loi Du 18 Germinal An 3 (the law of 17950407) pertaining to weights and measures, the meter was defined in Article 5 as:
Mtre, la mesure de longueur gale la dix-millionime partie de l'arc du mridien terrestre compris entre le ple boral et l'quateur. (Meter, the measure of length equal to the ten-millionth part of Earths meridional arc between the north pole and the equator.)
This decree of 1795 occurred during the middle of the effort of two surveyors to determine how long this meridian quadrant actually was in order for people to be able to put this definition into practice in a useful manner. This definition for the meter (as well as the definitions of the other metric units) did not take effect until 1799, when a platinum bar was selected as most representative of the length the surveyors determined, and that bar became known as le mtre des Archives (the meter of the archives). (Note: Many people think there were two etch marks on the bar indicating the meter; this first bar, in 1799, was actually an end standard, so that the two ends were used instead of marks in the interior, and the length of the bar became the practical definition of the meter.) This practice of having an official definition and a distinct practical definition went on for some decades. In the meantime, further surveys covering a little over 10 in latitude instead of the original less than 7 out of the 90 constituting a quadrant and done more carefully soon indicated the that the meter of the archives was a bit too short.
When the convention of the meter was signed off in 1875, the metric system transitioned from being under control of the French government to becoming internationalized and controlled by the International Bureau of Weights and Measures, who directed that an improved, platinum-iridium bar be produced to replace the meter of the archive (actually a multiplicity of such bars to be sent to various countries for the basis of their units of measurement). Because it had become very well known at this time that the meter of the archive was very close to 0.2 mm too short based on improved measurements of the meridian quadrant in accordance with the legal definition, so the question became which should be used for the length of the new barthe legal definition of 1/10,000,000th of a meridian quadrant or the practical definition actually used by people based on the meter of the archive? The latter was chosen as making more sensewhat people had been actually using for decades in a time of discrepancy with the legal definition. When measurements are involved, two distinct definitions will always ultimately end up with a conflict and a choice to be made. This is the point at which the 17950407 definition of the meter in terms of Earths size was ultimately abrogated.
Since then, geodesists have learned even more reasons that the original definition was not good, so it was good that definition had already been abandoned. Among the newer reasons involve Earth not being an oblate spheroid but having bumps of some tens of meters, so the length of the desired quadrant depends on the meridian selected to measure. Some time-dependent variations of Earths structure and rotation (a precessing wobble) cause further variations. According to the latest analysis of measurement data indicate that the meridian quadrant depends on longitude but is close to 10,001,965.729 3 m.