
Background and travel preparations – “lemon or tangerine?”
The Earth has been known to be spherical since at least the time of Pythagoras in the 500s BCE. Although the knowledge only became common later, in the circle of scholars the Earth has been a sphere for a long time. In the 17th century, observations related to gravity were made, which started a discussion about the exact shape of the Earth. In France, the debate about the shape of the Earth became active in the early 1730s, when the country relaunched the accurate mapping of the empire.
According to the Cassinis, the Earth resembled a lemon
In France, the Cassini dynasty was at the forefront of surveying. The director of the Paris Observatory, the Italian-born Jean-Dominique Cassini (1625–1712) and his son, the astronomer Jacques Cassini (1677–1756), concluded from 30 years of fieldwork that the Earth’s poles were tapered like a lemon.

Newton turned the Earth into a tangerine
It had been known since the 1660s that gravity varies from one latitude to another.
At that time, the French astronomer Jean Richer (1630–1696) noticed that his pendulum clock was not keeping time in Cayenne, French Guiana. Richer had to shorten the pendulum of the clock to make it show the correct time. The location at different latitudes affected the movement of the pendulum.
This is how Richer first observed the varying strength of gravity on the Earth’s surface: gravity is weaker at the Equator than in, say, France.
Later, the English physicist, mathematician and astronomer Isaac Newton (1642–1727) proposed, based on his theory of gravitation, that Richer's observation proves that the centre of the Earth is farther from the Equator in Cayenne than further north in Paris.
The Earth must be bulging out from the Equator and flattened like a tangerine on its poles.
Newton's theoretical calculations contradicted the empirical results of the Cassinis.
In general, France was wary of Newtonian thinking. Newton's theory was about gravitational forces, which were difficult to detect with the senses. At the beginning of the 18th century, a mechanistic natural philosophy based on the thinking of the mathematician and philosopher René Descartes (1596–1650) prevailed in French scientific circles.

In 1732, Pierre Louis Moreau de Maupertuis publishes Discours sur les différentes figures des astres avec une exposition des systèmes de MM. Descartes et Newton, in which he compares Cartesian and Newtonian physics, vortex theory and the theory of gravitation.
In addition to Maupertuis, the so-called Newtonian school in France included the philosopher and writer Voltaire (1694–1778) and the natural scientist Émilie du Châtelet (1706–1749) as its most important figures. The latter would later on translate Newton's Principia (Philosophiæ naturalis principia mathematica, 1687) into French.


The mapping of France launched a discussion about measurement methods
In the early 1730s, the French Minister of Finance, Philibert Orry (1689–1747), relaunched the accurate mapping of the empire that had been first started in the 1680s but had not been completed. The project brought geodesy to the heart of discussions at the Academy as well.
At first, the discussion was mainly about different measurement and calculation techniques, and only later was the question of Newtonian gravity and its effect on the shape of the Earth added.
The Cassinis mentioned above began work on the east-west baseline in 1733.
Around the same time, the criticisms of the Cassinis’ methods made a decade earlier by the Italian mathematician Giovanni Polini (1683–1761) attracted attention in France. According to him, the Cassinis’ results were within the margin of error and were therefore not reliable.

Empirical study as a solution to the dispute
Several academics presented their views on measurement methods. The Cassinis’ theory of a tapering Earth began to attract competing views.
The field of geodesy produced a wealth of theoretical papers and empirical observations.
In late 1733, astronomer Louis Godin (1704–1760) proposed that the Academy equip a degree-measuring expedition to the Equator. The following year, King Louis XV (1710–1774) ordered the expedition.

The South American expedition set sail in May 1735. Immediately after the departure of the first expedition, Maupertuis proposed that France also equip a second expedition. He said that one expedition would not be enough to solve the problem with certainty.
At the end of 1735, the King issued a decree to send a geodetic expedition to the Arctic Circle.
To the Crown, the Académie des sciences had justified the importance of the matter by its impact on the accuracy of navigation and thus on trade. The study of the shape of the Earth was thus, at least ostensibly, for economic gain.
An equally important – if not more important – reason was to show other powers the power of France. Scientific ambitions also underlined the power of France in the scientific arena.

Theoreticians prepare for fieldwork
The expedition was the first chance for both Maupertuis and Alexis Clairaut, where they could try their theoretical knowledge in practice. Maupertuis had no experience in fieldwork, but that did not prevent him from presenting himself as the leader of the expedition.
Initially, either Iceland or Norway was planned as the destination for the expedition. However, under the influence of the Swedish astronomer Anders Celsius, the measurements were decided to take place in Sweden, at the bottom of the Bay of Bothnia.
The expedition required a lot of preparation: acquiring of measuring instruments, equipping the ships and liaising with local authorities for help.
The expedition set off from Paris on 20 April 1736.


In the port of Dunkirk on the northern coast of France, a presumably eager group of young men boarded the ship. In addition to 37-year-old Maupertuis, 23-year-old Clairaut and 35-year-old Celsius, astronomers Charles Étienne Louis Camus (36), Pierre-Charles Le Monnier (21) and the oldest member of the expedition, abbé Réginald Outhier (42), travelled north.

Journey from Paris to Tornio in spring and early summer 1736
Sources:
Filosofia.fi https://filosofia.fi/fi/ensyklopedia/newton-isaac ja https://filosofia.fi/fi/ensyklopedia/varhaismoderni-luonnonfilosofia
Markkanen, Tapio. “Maantieteellinen pituus – paikanmäärityksen vuosituhantinen haaste.” Maan muoto. Ed. Poutanen, Markku. Ursan julkaisuja 86. Jyväskylä, 2003.
Pekonen, Osmo. “Johdanto: Maan muotoa mittaamassa”. Maan muoto ynnä muita kirjoituksia Lapista. Ed. Osmo Pekonen. Väyläkirjat, 2019.
Pekonen, Osmo. La rencontre des religions autour du voyage de l’abbé Réginald Outhier en Suède en 1736–1737. Lapin yliopistokustannus, Rovaniemi, 2010.
Terrall, Mary. Maupertuis. Maapallon muodon mittaaja. Trans. Osmo Pekonen. Väyläkirjat, Tornio, 2015 (orig. 2002).
Turunen, Tauno ja Kultima, Johannes. “Kreivi Pierre-Louis Moreau de Maupertuis, Lapin tieteellisten mittausten aloittaja”. Maan muoto. Ed. Poutanen, Markku. Ursan julkaisuja 86. Jyväskylä, 2003.