Two astronomers spent seven years measuring France's meridian arc. And their bitter rivalry almost derailed the creation of the metric system.
Serge Ottaviani, CC BY-SA 3.0
A Mission Born of Revolution
In 1792, with the French Revolution in full swing, France's National Assembly sought to replace the country's chaotic system of weights and measures, a remnant of the feudal era with 250,000 different units. The goal was to create a rational, universal system "for all people, for all time." The French Academy of Sciences was tasked with this, deciding the new unit of length, the "mètre," was one ten-millionth of the distance from the North Pole to the Equator, measured along the meridian that passes through Paris.
To determine this length, two of the nation's leading astronomers, Jean-Baptiste Joseph Delambre and Pierre-François André Méchain, were commissioned to undertake an immense geodetic survey. Their seven-year mission (1792-1798) was to precisely measure the meridian arc between Dunkirk on the English Channel and Barcelona, Spain. The mountain peak of Pic du Midi de Bigorre was an important triangulation point for the southern portion of this survey. The astronomers used a new, high-precision instrument called a Borda repeating circle to measure the angles between established landmarks, a painstaking process of triangulation that would allow them to calculate the arc's length.
A Rivalry Measured in Toises
The mission was perilous from the start. Taking place during the Reign of Terror, the astronomers and their strange instruments were often viewed with suspicion. They were arrested multiple times as suspected spies or counter-revolutionaries, and their white triangulation flags were thought to be royalist symbols. The war that broke out between France and Spain in 1793 added another layer of danger to Méchain's work in the Pyrenees.
The immense pressure fractured the relationship between the two scientists. Delambre, methodical and patient, handled the northern section. Méchain, a perfectionist, was responsible for the more difficult, mountainous southern section. While working near Barcelona, Méchain made a tiny error in his measurements. He was unable to replicate his initial results for the latitude of the city but, pride and fear of failure, he concealed the discrepancy.
This secret tormented Méchain for the rest of his life. He delayed submitting his data, frustrating Delambre and the scientific committees in Paris. The project fell years behind schedule. Méchain eventually died of yellow fever in 1804 while in Spain attempting to re-measure and correct his flawed data. After Méchain's death, Delambre took possession of his records, discovered the cover-up, and published the final results of the survey between 1806 and 1810. The original meter, officially adopted in 1799 and embodied in a platinum bar, was based on this slightly inaccurate survey. Méchain's error made the platinum meter about 0.2 millimeters shorter than the intended definition.
💡Fun Facts
The mission's goal was to measure the Earth, but the final, official length of the meter was defined by a physical platinum artifact, the *Mètre des Archives*, in 1799.
Méchain's agonizing error was minuscule, affecting the final length of the meter by less than the thickness of two sheets of paper.
The survey of the meridian arc also helped scientists determine the Earth's flattening (its bulge at the equator) with a value of 1/334.
The Pic du Midi's value for observation was established, but the permanent astronomical observatory on its summit was not officially founded until the 1870s.
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