How far away is the sun? They went on a perilous journey to find out. – National Geographic

On a drizzling day in May 1673, deep in the dense rainforest of French Guiana, a scientist died. Known to historians only by his first name, Meurisse, he may have been cut down by disease or perhaps a fatal accident, but a full description of his death was never properly recorded. The only person with him was his partner, an astronomer named Jean Richer, who was stricken ill and fighting for his own life.

The pair had been dispatched to Cayenne, on the northeastern coast of South America, the year before from Paris, 4,400 miles away. Sent by the French Academy of Sciences at the behest of astronomer Giovanni Cassini, part of their mission was to take a measurement that would reveal the distance between Earth and the suna value that was not yet known.

As long as humankind has gazed up at the sky, there have been attempts to determine the distance to the sun. Scientists in antiquity such as Eratosthenes and Ptolmey produced estimates that varied significantly, often greatly underestimating the true value.

By the 1670s, aided by newly developed astronomical instruments, Cassini was determined to find the answer once and for all. Inhabiting the second floor of the Paris Observatory, he worked on the problem unrelentingly. He had no hobbies, says Gabriella Bernardi, author of Giovanni Domenico Cassini: A Modern Astronomer in the 17th Century. From his diary emerges a man completely devoted to his profession.

"When people left European shores to go across the ocean, they assumed that you might die."

ByNicholas DewMcGill University

In many ways, the late 17th-century journey to French Guiana was routine, part of a series of scientific expeditions dispatched by Cassini. Richer and Meurisse had voyaged to northeastern North America two years earlier to measure latitudes and the heights of the tides, and French scientific expeditions would follow to destinations such as Senegal and Ecuador.

But it was the voyage to Cayenne that collected the key data that, united with Cassinis mathematical prowess, produced the first precise measurement of the vast distance between Earth and the sun.

On January 11, 1667, five years before the mission to French Guiana, astronomer Adrien Auzout stood in the meeting room of the sumptuous Bibliothque du Roi in Paris. Before a small assembly of men in long, thick wigs, he laid out a bold program of scientific research.

Right at the time the Academy is being conceived, they're already thinking about astronomical expeditions, says Nicholas Dew, a historian at McGill University. Auzout was the planner of this [He had] the vision of using colonial trade networks to send observers to points around the globe to conduct observations in astronomy.

Auzout's plan was wide-ranging and visionary. He recognized that certain astronomical questions, including the distances to the planets and the sun, would require taking observations simultaneously in two different locationssuch as in Paris and a far-flung locale. Auzout argued for a voyage all the way to Madagascar, where the East India Company was expected to establish operations, and the proximity to the equator would allow astronomers to take key observations.

As the men listened, the sounds and smells of a squalid, congested city may have wafted through the windows. In the late 17th century, Paris was known for raucous church processions, drunken merrymaking, and open gun violence. At the strike of seven each morning, city officials marched down the wide boulevards, ringing large bells to wake residents, directing them to clean the filth that had accumulated in front of their homes or risk a fine.

The bustling city was a hotbed of both intellectual activity and commerce, where a large, affluent population mixed freely with members of a forward-thinking scientific community. Many of the most skilled scientific instrument makers were in Paris at the time, and on the outskirts of the city, construction was beginning on a major new astronomical observatory.

Two years after Auzouts speech, in April 1669, Cassini arrived in Paris. He had been personally invited by King Louis XIV and would swiftly become one of the Academys modest illustrious figures.

Cassini was 44 years old when he set off for Parisa bachelor with a carriage full of astronomical instruments, says Bernardi.

As the Academy continued to prepare for an astronomical expedition to the equator, the scientists shifted their focus from Madagascar to Cayenne. This French settlement was a shorter distance away, and the Academy had to act quickly to catch a noteworthy event: In the fall of 1672, Mars and Earth would be at their closest points to each other in 15 years.

Cassini realized that precise observations of Mars during this time could be used to calculate the parallax of the planeta measurement of Marss apparent difference in position as seen from the two observing sites. This key measurement could then be used to work out the distance from Earth to the sun, making the close approach of Mars an opportunity that mustnt be missed.

Richer and Meurisse spent several days and nights working alongside Cassini to prepare for the joint observations they'd have to make while thousands of miles away. The pair of apprentices knew they were embarking on a perilous journey.

Anyone who's been sent on these ships in this period, they're all on a lower status level, Dew explains. The dangerous, scary, long-distance travel is done by the lower-grade, lower-paid people.

Traveling first to the French port of La Rochelle, Richer and Meurisse spent three months methodically testing and calibrating their instruments, including an octant, a quadrant, several telescopes of various sizes, and a few pendulum clocks.

They set sail for Cayenne on February 8, 1672, on a merchant vesselpossibly an empty slave ship on its way to Senegal. Gazing up from the ship's deck one evening during the passage, Richer made detailed observations of a comet with two bright tails streaking across the inky-black sky.

Cassini had given Richer several objectives: He was to measure the positions of the southern stars, the heights of the tides, and the duration of twilight. He was to make observations of Jupiters moons and take detailed notes on the movements of Venus, Mars, and Mercury. He and Meurisse were also expected to take barometric measurements and keep an eye out for unusual flora and fauna.

The pair arrived in Cayenne on April 22, 1672.

Fert aurum industris: Work brings wealth. Whoever coined Cayennes official motto must have had a grim sense of humor.

The tiny, desolate settlement could not have been an encouraging sight to Richer and Meurisse. Visited by only two or three ships a year, the island of Cayenne was separated from the rest of Guiana by the narrow 11-mile Mahury estuary on one side and the slender Cayenne River on the other.

As they stepped off the boat, the pair may have realized that they had chosen the most unpleasant time of year to arrive. In the Amazon, late April is near the height of monsoon season, oppressively humid and thick with mosquitoes. The sheets of rain fell on them mercilessly, flooding the river yet providing no relief from the sweltering heat.

At the center of the settlement stood Fort Cprou, a bleak, lonely structure, rebuilt in stone from wood after the most recent attack by the Indigenous population, signifying the French colonists determination to stay. A short walk from the fort was the Kings Store, a general store that served the settlement and often had little on the shelves.

There was also a modest Jesuit church and mission house. A 1685 account, noted in Catherine Losier's Supplying Cayenne Under the Old Regime: Archeology and History of Commercial Networks, describes it as a dwelling occupied by four fathers and a brother, along with 82 enslaved African people32 men, 23 women, and 27 childrento work the Jesuits crops and tend to their livestock. Enslaved Africans made up roughly 85 percent of the settlement.

And then there were the Kalina. The Indigenous people, also called the Galibi, had resided in the Cayenne region for over two thousand years before Europeans arrived. As one settler, Paul Boyer, would write after a visit around 1654: All the Galibi could think about was how to be rid of the French.

Past interactions between the two groups had been troubled. Less than 30 years before Richers arrival, in 1644, French officer Charles Poncet de Brtigny arrived in Cayenne with a few hundred men. He used an iron cattle brand bearing his name on the Kalina who displeased him, tried to force them to wear clothes, and kidnapped Indigenous women, confining them to his quarters. Within a year, a tribesman had sunk an ax into Bretignys skull, the opening of a blood-soaked ambush that left only a handful of Frenchmen alive in a settlement that had been burned to the ground.

The French didnt only have the Kalina to worry about. The Dutch managed to capture the colony a decade after Bretignys reign, only to be forced out by fresh French troops in a surprise attack. French settlers were then driven out by the British in 1667, wrenching back control of the colony a year laterjust four years before Richer arrived.

For Louis XIV, Guiana provided a strategic position for France to gain a foothold on the South American continent. But there was another reason the nations of Europe were enticed by the region, often divulged in a whisper: El Dorado. The Europeans fighting to control Cayenne believed that the fabled city of gold was hidden somewhere within Guiana, and whoever controlled Cayenne would have a direct route to the riches.

Richer and Meurisse, though, had embarked on a quest for scientific treasure.

Away from the settlement, across the thin, narrow river that gave Cayenne its name, lay the rest of Guianaa dense primeval rainforest, containing plants and animals not found anywhere else in the world. The environment would have been so alien to Richer and Meurisse, so different from the cobbled streets of Paris, that its hard to imagine which animals might have caught their eyes firstthe anteaters, iguanas, or spider monkeys? Were they astonished by glimpses of speckled jaguars or bright green parrots?

Academy records indicate that Richer and Meurisse took detailed notes on flora and fauna, but almost all of these have been lost to time. At one point, Richer came face to face with an electric eel, later writing that a simple touch with a finger or the tip of a stick, so numbs the arm and that part of the body closest to it that one remains about 15 minutes without being able to move.

Immediately upon arrival, Richer began scouting the jungle for the best place to build an observatory. Locating a spot after a couple of weeks, the two men recruited Indigenous workers and built a structure composed of branches, tree bark, and palm tree leaves, with a sizeable hole in the roof for their telescopes.

Sometime before mid-May the observatory was finished. Richers first observation was on May 14, when he calculated the height of the North Star. It was a promising start to what would be a very challenging mission.

The rain was merciless, and Richer wrote to Cassini of not being able to take observations for several days at a time because of the inclement weather. Almost not a day has passed without rain since our arrival.

At one point, so many ants crawled into the scientists pendulum clocks that the insects jammed the delicate machinery of cogs and wheels, causing at least one to stop completely.

Richer and Meurisse relied heavily on supplies from home, even though local food was available in the form of game, fish, and edible plants such as bananas, avocados, and mangos. The two Frenchmen preferred to eat familiar foods, including packets of cured meat, flour, Bordeaux wine, coffee, and cheesesupplies that were seldom replenished by the passing ships.

Sending food to the colonies was a constant issue, Dew says. The Europeans want to eat what they're used to eating . . . They're thinking: we have to have bread, we have to have wine.

The slowness of mail and the rareness of passing ships meant that Richer and Meurisse were effectively on their own.

Finally in October 1672, the rainy season stoppedjust in time to observe Mars. Richer measured the planet and nearby stars over the course of multiple weeks.

Across the Atlantic, 4,400 miles away, Cassini and Danish astronomer Ole Rmer also made measurements at the agreed upon times, peering out the window of the Paris Observatory.

Meanwhile, in London, astronomer John Flamsteed of the Royal Society was also measuring the parallax of Mars to determine the distance to the sun, cleverly observing Mars once early in the evening, waiting several hours for Earth to rotate, then measuring again. His final calculation would be close, but not quite as precise as Cassinis.

In the spring of 1673, Meurisse perished, possibly from yellow fever, malaria, pneumonia, or even severe malnutrition. When people left European shores to go across the ocean, they assumed that you might die, Dew says. When Meurisse dies, it would be nice if we knew more about it, but it isn't unusual for the documentation to be so sparse.

Richer, now alone, felt too sick to carry on. He sought out specimens to carry back to the Academy, capturing a live crocodile and chaining it up in the hold of the ship. Wracked with illness, he boarded the vessel with the draft of his mission report and departed Cayenne on May 25, 1673. On the long journey home, the crocodile died of starvation, but Richer recovered.

In 1679 Richers official mission report, Observations astronomiques et physiques faites en l'isle de Caienne, was released. Aided by Richers data, Cassini could finally make his calculations, announcing in a 1684 publication that our sun, which looked so close, was actually 87 million miles awayremarkably close to the true distance of about 93 million miles.

Word of the expedition and revelation of the sheer size of the solar system spread rapidly, thanks in large part to the popular writings of Bernard le Bovier de Fontenelle, who wrote about science in a unique, novelistic style. For the first time, astronomy was a subject for the dinner table.

Determining the distance to the sun was not the only legacy left by Richers expedition to Cayenne. While in South America, the astronomer also measured the length of a pendulum and compared the results to his precisely calibrated clocks. Something was off. The swinging pendulum seemed to produce a shorter second in Cayenne than it would in Paris.

Though Richer didn't realize it at the time, this was due to the fact that there is slightly less gravity near the equator, where Earth bulges as it spins, causing the pendulum to measure a shorter second. Isaac Newton would puzzle out the reason some 15 years later, using Richers measurements as evidence for his new theories of gravity.

Just think, Milord,Voltaire wrote in a letter to his friend Lord Hervey in 1740, without the voyage and experiments of those sent by Louis XIV to Cayenne in 1672 never would Newton have made his discoveries concerning attraction.

Bernardi believes that the success of the voyage was due to Cassini's modern approach. At the time, it was a complete innovation, she says. Cassini was the first to understand that a regular plan of observation, in collaboration with many other colleagues, made it possible to tackle more difficult problems and achieve important results, just like big science does today.

Just as the trading vessel that carried him home slipped away from the green shores of French Guiana, Richer would slip into relative obscurity, the accurate calculation of the Earth-sun distance becoming almost wholly Cassinis triumph. Once safely back in France, Richer broke from the academy and took a position as a military engineers assistant.

It was, once again, the height of monsoon season when Richer's ship sailed away. The heat would have been unwavering and the river close to overflowingthe relentless rain beating down upon the land he was leaving behind.

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How far away is the sun? They went on a perilous journey to find out. - National Geographic