Geoscience Reference
In-Depth Information
When the Academy discussed the standard of length that was to be used by
France, the length of a seconds pendulum was at first the main contender. The
Academicians understood better than most the variation in gravity that there was
between equator and pole because of the departure of the Earth from an ideal
spherical shape and its rotation. They stipulated that the pendulum would have to
be located at a latitude halfway between the equator and the pole and Cassini IV
supported this proposal.
There was however an alternative school of thought dating back a hundred years;
the French astronomer Gabriel Mouton (1618-1694) had the idea in 1670 of using
part of the circumference of the Earth as the standard. He proposed the use of the
length on the Earth's circumference of a minute of latitude as a standard, which he
called a
mille
.
15
The nautical mile was originally defined in a similar way as the length of 1 arc
minute of latitude although it is now defined as exactly 1852 meters (the speed of
1 knot is 1 nautical mile per hour). Mouton proposed that the
mille
should be
divided decimally in tenths, hundredths, etc. which he called the
centuria, decuria,
virga, virgula, decima, centesima,
and the
millesima.
This decimal scheme, which
greatly facilitated calculations, had been proposed in 1585 by the Flemish scientist
Simon Stevin (1548-1621). Eventually it was decided to use Tito Burattini's name
for a meter, but based on Gabriel Mouton's idea of using the Earth as a basis of its
length, and to divide it decimally as suggested by Simon Stevin.
The National Assembly decided in 1791 that the measurements of the Paris
Meridian from the equator to the pole should serve as the basis for the meter. Short
of making the measurement by going to the equator and the North Pole, the meter
was defined as the ten millionth part of 90° of latitude as pro-rated from a measure-
ment of the extent of latitude along the Paris Meridian from Dunkerque to
Barcelona. This meridian was centered on 45 degrees of latitude, neither at the
equatorial bulge nor at the polar flattening, and it was already well reconnoitered.
It was also international (running across the Franco-Spanish border) and the French
Government invited international participation. This initiative to internationalize
the metric system backfired. No country was happy to cooperate with the
Revolutionary Government, seeing the project as one of French nationalism initi-
ated by a government that had seized power by revolution and had executed thou-
sands of the French relatives of people who held political power in other European
countries. Spain participated, but only half-heatedly. Spain passively allowed the
meridian to be surveyed in its own territory by the French scientists.
THE ACADEMY DECIDED, however, that the earlier surveys were not accurate
enough to define the meter (Alder 2002). A new measuring device had become avail-
able in France making it possible to measure the circumference of the Earth to
unprecedented accuracy and define the meter as a modern standard. This was the
repeating circle first invented by the German mathematician and astronomer Johann
15
Mille
is
mile
, but the same word in French is
thousand
. The length of one mile originated as 1000
paces marched by the Roman legions.
