Scientist dies in 1673 in French Guiana

On a rainy day in May 1673, in a tropical forest in French Guiana, a scientist named Meurisse died. His death could be attributed to a disease or a fatal accident, but no complete description of it has been found. The only person who was with him was his travel companion, the astronomer Jean Richer, who was then seriously ill and struggling to stay alive.

Mission to measure Earth-Sun distance

The previous year, they had been sent from Paris to Cayenne, on the northeast coast of South America, 4,400 km away. The French Academy of Sciences, at the request of the astronomer Giovanni Cassini, had entrusted them with a mission to perform a measurement that would reveal the distance between the Earth and the Sun, a value that was not yet known at the time.

Cassini's dedication to astronomy

In the 1670s, new astronomical instruments had been developed and Cassini was determined to use them to answer this question once and for all. He lived on the second floor of the Paris Observatory and worked tirelessly on the problem. Gabriella Bernardi, author of Giovanni Domenico Cassini: A Modern Astronomer in the 17th Century, describes Cassini as a man entirely devoted to his profession.

Cassini's arrival in Paris

In April 1669, two years after Adrien Auzout's speech, Cassini arrived in Paris. He had been personally invited by King Louis XIV and would soon become one of the most illustrious figures of the Academy. Cassini became aware that precise information about Mars could be obtained at a time when it would be at its closest point to Earth in fifteen years, which could be used to calculate its parallax, a key measurement for calculating the distance between the Earth and the Sun.

Preparation for the journey

Richer and Meurisse spent days and nights working alongside Cassini to prepare for the common observations they would have to make when they would be thousands of kilometers away. The two apprentices knew they were embarking on a dangerous journey. All those who had been sent on these ships at that time had an inferior status, according to historian Nicholas Dew.

Testing instruments in La Rochelle

Richer and Meurisse first went to the French port of La Rochelle, where they spent three months testing and meticulously calibrating their instruments, including an octant, a quadrant, several telescopes of different sizes and a few pendulum clocks. On February 8, 1672, they set sail for Cayenne aboard a merchant ship, potentially a slave ship that had been emptied and whose final destination was Senegal.

Arrival in Cayenne

On April 22, 1672, the duo arrived in Cayenne. The small desolate colony was visited by only two or three ships a year and was separated from the rest of Guiana by the narrow Mahury estuary and the thin Cayenne river. The end of April was almost the peak of the monsoon season in the Amazon, a climate of oppressive humidity where mosquitoes swarm.

Building an observatory in the jungle

Richer began exploring the jungle to find the best place to build an observatory. After a few weeks, he found a suitable spot and the two men recruited indigenous workers who built a structure made of branches, tree bark and palm leaves, with a large hole in the roof for their telescopes. The observatory was completed before mid-May. Richer's first observation took place on May 14, and he calculated the height of the North Star, a promising beginning for what would be a very difficult mission.

Challenges of the rainy season

The rain was merciless and Richer wrote to Cassini that the bad weather had prevented him from making observations for several days. "Since our arrival, there has practically not been a day without rain." At one point, the ants entered in such large numbers into the pendulum clocks of the scientists that they blocked the delicate machinery of the gears, causing at least one of them to stop completely.

Observing Mars in October 1672

In October 1672, the rainy season finally stopped, just in time to observe Mars. Over several weeks, Richer devoted himself to measuring the planet and the surrounding stars. On the other side of the Atlantic, 7,000 kilometers away, Cassini and the Danish astronomer Ole Rømer also took measurements at the predetermined times, looking out the window of the Paris Observatory.

Other astronomers contributing to the mission

Meanwhile, in London, the astronomer John Flamsteed, of the Royal Society, also measured the parallax of Mars to determine the distance from the Sun, cleverly observing Mars once at the beginning of the evening, before waiting several hours for the Earth to rotate and making a new measurement. His final calculation was close to Cassini's, but not as precise.

Meurisse's mysterious death

Meurisse's death in 1673 may have been due to yellow fever, malaria, pneumonia, or severe malnutrition. "When someone left the European coasts to go to the other side of the ocean, others assumed that person would die," says Dew. "We would like to know more about Meurisse's death, but it is not unusual for the documentation to be so sparse."

Richer's return to Europe

Richer then found himself alone and too ill to continue. He searched for specimens to bring back to the Academy, such as a live crocodile that he chained in the ship's hold. Devastated by illness, he set sail and left Cayenne on May 25, 1673, with the draft of his mission report. During his long return journey, the crocodile died of hunger, but Richer recovered.

Publishing the findings

In 1679, Richer published his official report, Observations astronomiques et physiques faites en l’isle de Caienne. Thanks to the information in this report, Cassini was finally able to perform his calculations and announced in a publication in 1684 that the Sun, which appeared so close, was in fact 140 million kilometers away from us, a result remarkably close to the true distance between the Sun and the Earth, approximately 150 million kilometers apart.

Impact on public interest in astronomy

News of the expedition and the revelation of the true size of the solar system spread quickly, especially thanks to the writings of Bernard le Bovier de Fontenelle, whose scientific writings were written in a unique romantic style. For the first time, astronomy became a topic to discuss at dinner.

Discovering the Earth's shape

The calculation of our distance from the sun is not the only legacy left by Richer's expedition to Cayenne. During his stay in South America, the astronomer also measured the length of a pendulum and compared the results to his clocks, calibrated with precision. Something was wrong. The seconds indicated by the pendulum seemed shorter in Cayenne than in Paris.

Gravity and the Earth's shape

Although Richer did not realize it at the time, this was due to the fact that there is a little less gravity near the equator, where the Earth swells as it rotates, which explains this shorter measurement of seconds. Isaac Newton discovered the reason fifteen years later, using Richer's measurements to support his new theories on gravity.

Legacy of the Cayenne expedition

"Think, my lord," wrote Voltaire in 1740, in a letter to Lord Hervey, "that without the journey and the experiments of those sent to Cayenne in 1672, [...], Newton would never have made his discoveries on attraction." According to Bernardi, the success of the journey is due to the modern approach taken by Cassini. "At the time, it was a total innovation," she says. "Cassini was the first to understand that a regular observation plan, in collaboration with many other colleagues, allowed tackling more difficult problems and obtaining important results, as is the case today in the great sciences."

Return journey through the monsoon

When Richer's ship set sail, the monsoon season was again at its peak. The heat was probably unshakable and the river close to overflowing, the incessant rain falling on the land he was leaving behind.