The meter : a republican measurement

The meter : a republican measurement
Extrait du Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la Recherche.
https://www.obspm.fr/the-meter-a-republican.html
The meter : a republican
measurement
Date de mise en ligne : vendredi 10 octobre 2014
Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la
Recherche.
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The meter : a republican measurement
Delambre and Méchain were appointed to do the measurements used to define the meter.
The precise triangulation from Dunkirk to Barcelona mobilized the two astronomers 7 long
years.
« Double standards ! » the very symbol of inequality. In answer to the 1789 lists of grievances, but also to those of
the 1576 estates general, the Revolution required the Académie des Sciences in 1790 to standardize the system of
weights and measures. The new, and unified, system of measurements should be egalitarian, a means to facilitate
exchange and to ensure the integrity of commercial operations.
The ten millionth fraction of a quarter of the terrestrial
meridian
A requirement for equality, but also for universality, as proclaimed in Condorcet's famous slogan, the metric system
must be « pour tous les hommes, pour tous les temps » (« for all, for ever »). For this, the meter would be eternal
since based on the Earth, itself eternal. It was defined by the law of 18th germinal year III (7th of April 1795) : the
meter was thus defined as the ten millionth fraction of a quarter of the terrestrial meridian.
Two members of the Academy of Sciences were assigned the task of measuring the length of a specific meridian arc,
namely that which crosses France from end to end, from Dunkirk to Barcelona, and traversing the meridian axis of
the Paris Observatory. This arc subtends slightly more than 8° in latitude. And so Jean-Baptiste Joseph Delambre
(1749-1822) and Pierre François André Méchain (1744-1804) set off in the summer of 1792.
Triangulation au nord de Paris Observatoire de Paris
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The meter : a republican measurement
Their trip would take all of 7 years. The adventure of the meter, which began with the capture of the Tuileries by the «
sans-culottes » on the 10th of August 1792 and the birth of the first République on the 21st of September 1792,
would end with Napoleon Bonaparte's coup d'état on the 18th Brumaire year VIII (9th of November 1799). The meter
is thus wholly a « republican measurement ».
An accuracy of several seconds of arc
Over 90 geodetic triangles were set up along the meridian, like the veritable backbone of France. These triangles
were measured to an accuracy of a few seconds of arc, thanks to a new instrument created by the chevalier Borda
(1733-1799), the repeating circle. In the meantime, a preliminary meter was defined on August 1st 1793 using the
measurements of the meridian made in 1740 by Nicolas Lacaille (1713-1762) : its value was provisionally fixed at
443,44 "lignes" in the old system.
Finally, the value of the meter was officially defined as 443,296 lignes by the law of December 10th 1799 (19 frimaire
year VIII). However, it was only in 1837 that he French government imposed the metric system, making it obligatory
in the whole of France and its colonies from January 1st 1840.
Cercle répétiteur de Borda Observatoire de Paris
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