How did the meter acquire its definitive length?

Abstract

AbstractThe French Academy of Science decided in 1791 that the new unit of length, the meter, is to be as one ten millionth part of the meridian quarter from the north pole to the equator. In order to find out the numerical value for this length, the Academy equipped two expeditions for measuring the meridian arc between Dunkirk and Barcelona. Then by mathematical calculations one could derive the length of the meridian quarter. After the expeditions had returned to Paris and their data had become available, Pierre-Simon Laplace (1749–1827) estimated the flattening of the earth. He obtained the value 1/150.6, which cannot stand as regards known facts. This is a story about how Laplace managed to save the meter project. Combining the new French arc measurements with the older measurements made in Peru 1730–40 he estimated the flattening anew and found the plausible value 1/334. With the help of this new flattening he calculated the definitive length of the meter to be 443.296 Parisian lines. The final section of the article tells the refinements of the meter’s definitions from the year 1799 to the present.