Sunday, April 25, 2010

About the Metre

Probably the most common distance measure today worldwide bar none is the metre however  it isn't commonly adopted in the USA, where the mile, the foot and the inch still reign - domestically at least. In Canada we adopted the SI or System International spelling of the metre while in America it remains the Meter.

By the end of the 18th. century, England had pretty much established standards for weights and measures in the pound, foot and quart and profited well in trade as a result. France meanwhile as a country had no national standards yet to speak of and suffered for it. Each municipality or locality pretty well had its own standard for a loaf of bread or measure of length or a sack of grain. For reference, these standards were frequently engraved into the sides of structures such as bridges, or fortress walls or buildings. The problem with each town having its own standards was that it severely deterred trade, due to the mistrust or understanding of each other's measures. Also many towns had their own keeper of weights and measures and I can imagine that he was not too interested in having his standards go obsolete by trusting that of another town.

The French Academy of Science became quite aware of this problem and came up with a very ingenious concept. The Academy wanted to determine a measure that would be "universal" for all mankind and that could not be argued. So they chose the size of the earth as a reference, since the earth is indeed common to all. After much deliberation, they decided that the measure shall be a metre and the metre will be of such a length that there shall be 10,000,000 or ten million metres between the Equator and the North Pole. Using the best astronomical instruments of the day and mathematics the Academy came up with its metre.

Today's metre is based on the same original metre that the Academy calculated in the late 1700s. So how well did the Academy do? Using Google Earth to determine an estimate of the distance from the north pole to the equator I get  around 9,987,601 metres.  That is an error of about 12.4 kilometres which to us doesn't seem that good, but it is an error of 0.12% which considering the technology and mathematical challenges and unknowns of the day, it is still very impressive. Over the length of a metre the error is 1.2 millimetres.

For a longer and possibly more boring version of this story see

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