It was a row that divided scientists, launched global jogging expeditions and ended in murder for one man: was the Earth shaped like an orange or a lemon?
The 18th century debate – and the efforts that settled it – can now be relived by visitors to this year’s Royal Society Summer Science Exhibitionin a show called “Figuring the Earth”.
The exhibition, which opens to the public on Tuesday and will be on view in London until October, celebrates the importance of international competition and cooperation.
It turns out that the citrus fruit puzzle is a case in point.
“The debate comes from ways to measure and ways to think about the Earth,” said Dr Louisiane Ferlier, curator of the exhibition.
Some members of the French Academy of Sciences interpreted measurements taken in Paris by scientists including Jacques Cassini as supporting the idea that the Earth is elongated at the poles and looks like a lemon or a melon.
In contrast, Isaac Newton had suggested that the centrifugal force caused by the Earth’s rotation would result in the planet being flattened at its poles and thus having a similar shape to an orange.
As Voltaire wrote in 1724: “In Paris you see the earth shaped like a melon; in London it was flattened on both sides.”
With scientists divided over which view was correct, the French Academy of Sciences launched two international expeditions, funded by Louis XV, to settle the question of whether the circumference of the Earth was greater around the equator or around the poles.
In celebration of the debate, and its resolution, Figuring the Earth will showcase a host of objects. Among them is Isaac Newton’s Principia Mathematica, which contains his theory about the shape of the earth.
Next to it is the translation from Latin into French by Marquise Émilie du Châtelet – an 18th-century French noblewoman who became a mathematician and physicist – on loan from the French Academy of Sciences.
“They have never been shown side by side,” Ferlier said.
Other objects include divisors, a two-legged measuring instrument used during the French geodetic mission to the equator – one of two research efforts supported by the academy.
The other expedition, Ferlier noted, went to Lapland and involved a team that included the French mathematician and philosopher Pierre Louis Maupertuis and the Swedish scientist Anders Celsius.
In both cases, the teams used an approach known as triangulation to determine the shape of the Earth.
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Unfortunately for those who supported the idea of a lemon-shaped Earth, including Cassini himself, the results supported Newton’s theory.
But the work didn’t just shed light on the Earth’s shape: the scientists who went to the equator also documented how to tap rubber from trees, studied the bark of the tree from which quinine is derived, and had numerous adventures – including the murder of the expedition’s surgeon by a mob in Ecuador, apparently after becoming romantically entangled with a local woman.
“It’s a lot of drama,” Ferlier said.
The exhibition also considers more recent efforts to map and explore the Earth. Among the items on display are instruments and results related to the great trigonometric survey of India, begun in 1802, as well as a model of the first artificial satellite, Sputnik 1.
“[In the 20th century] satellites have completely revolutionized the way the shape of the Earth is calculated and mapped,” said Ferlier.
But the spirit of international competition and cooperation epitomized by the historic debate continues today: among the research showcased in the wider summer exhibition is an international effort known as the Deep Underground Neutrino Experiment, as well as results from the James Webb Space Telescopewhich involved teams from all over the world.
Ferlier said Figuring the Earth highlighted fundamental aspects of science.
“Science has to be collaborative, and it has to be responsive,” she said. “And you have to get things wrong to get things right.”
