“The scholars of the Enlightenment faced many dangers”

You are an academician, just like the hero of the summer series of the “pilgrim” on invisible borders, the astronomer Jean-Baptiste Delambre who measured the meridian of Paris between 1792 and 1799 …

Indeed, he was my predecessor, perpetual secretary for the mathematical sciences of the Academy of Sciences from 1800 and until his death in 1822.

Part of its archives is also kept here. Besides, did you know that the Paris meridian crosses our walls directly? I calculated it: he passes exactly through our bookstore at the entrance to the Institut de France, Quai de Conti, in Paris!

How do you look at your predecessors who participated in the measurement of the earth?

I have a lot of admiration for all these scholars: not only Delambre, but also his colleague Joseph de Lalande, or the Pierre-Simon de Laplace surveyor, the mathematician Jean-Charles de Borda, etc. In 1795, when the Convention created the Office of Longitudes, these academicians were all part of the first team! If sailors knew since the Middle Ages how to determine their latitude thanks to the position of the stars, it was very difficult for them to calculate their position on the terrestrial globe, east or west, compared to an original meridian. The longitudic office was given the task of improving these measures in order to restore France an advance on the English navy.

So the measure of the zero meridian, then determined from Paris, was an important question …

Already, between 1683 and 1748, the three Cassini (father, son and grandson) had measured it. What motivated this new Dunkirk expedition to Barcelona is above all the desire to define a stalker meter. Because in the meantime, Jean-Charles de Borda had developed a new, more precise instrument: the rehearsal circle. Delambre and Méchain were therefore sent to refine this measure.

Who were they?

Let us not forget for example the work of the mathematician Adrien-Marie Legendre who will correct the calculation of their angles. Because these were not all on the same plane since the bell towers they took for summits of their triangles were not all at the same level!

Neither François Arago who extended the measure of the meridian in 1806. During this operation, he will be taken prisoner in Spain and will escape to join Paris! We thus honored him in the year 2000 by placing medallions in his name, along the meridian, in Paris itself.

These scholars did not hesitate to give their person!

Indeed. Moreover, for me, the most remarkable expeditions were those sent by the Academy, both in Lapland under the direction of Pierre-Louis de Maupertuis, and in Ecuador, with Pierre Bouguer, in 1736-1737 to resolve a controversy between the English physicist Isaac Newton and the French Jacques Cassini.

It was necessary to determine whether the earth was flattened with the poles as supported by Newton or the Ecuador as Cassini said. Imagine the difficulties of the field in both cases! The cold, the heat, the altitude, the misunderstanding of the populations … The scholars of that time faced many dangers. And they managed to measure a precise meridian arch each time. For the whole world it was very important because it confirmed Newton’s hypothesis on gravity.