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5.3 How the Earth Cooled

One summer day in 1741, in a scene worthy of a mad scientist movie, a French aristocrat entered the foundry on his estate to conduct a strange experiment. He ordered the casting of two dozen small, solid iron globes. As the globes were removed from the furnaces at white-hot heat, the curious aristocrat held a watch and observed the glowing spheres carefully while they cooled to the temperature at which they could first be touched with bare fingers. According to one account, he sought out the people in his village with the most sensitive skin to determine when the spheres could be touched. The name of the aristocrat was Georges-Louis Leclerc, the Comte de Buffon. The goal of the naturalist’s odd experiment was to figure out the age of planet Earth.


Georges-Louis Leclerc, comte de Buffon. Click here for original source URL

Buffon carried out a cleverly designed experiment. He measured how long it took the globes to cool and then extrapolated those numbers to Earth-sized globes. By seeing how the time depended on the volume of the sphere, Buffon hoped he could calculate the cooling time of an immense sphere like the Earth. To an observer who didn’t understand what he was trying to do, his actions probably seemed very mysterious. Misunderstood real-life scenes like this may have led to the idea of madmen "experimenting with the forces of nature." Perhaps such early scientists inspired Mary Shelley to create her own character two generations later, Doctor Victor Frankenstein.

We think of science today in terms of complex instruments and precise measurements. But current research is often just an extension of the simple questions that these visionary scientists of the past asked. Buffon pondered the basic question "How old is our planet?" He suspected that the Earth was a sphere of rock and metal, and he wanted to determine its history. Buffon’s experiment also shows how scientists approach simple questions about nature. If you look at the amazing variety of the surface of the Earth - mountains and canyons and plains and volcanoes - it seems impossible that a single experiment could measure the age of all these features. Buffon simplified the question by imagining the Earth as a solid metal sphere and by working with a scaled-down model of the planet.

The Comte de Buffon, whose title means "Count" in French, is better known in history simply as Buffon. He was colorful character with eclectic interests. Almost disinherited after a duel at a French university, he inherited an estate at age 25 and became wealthy enough to pursue his experiments in science. He charmed Paris society and had Thomas Jefferson as a dinner guest. Jefferson reported that Buffon was "a man of extraordinary powers of conversation." One of Buffon’s friends wrote that "Monsieur de Buffon has never spoken to me of the marvels of the Earth without inspiring in me the thought that he himself was one of them" - not a bad testimonial, and nicely appropriate for a scientist whose primary field of study was the Earth.

Comte de Buffon’s wide interests led him to study all aspects of the natural world, and he produced a 36-volume encyclopedia called the Theory of Nature, which was published over the years from 1749 to 1785. Even though few people today have heard of him, he was the Carl Sagan of his day. His books included some of the first bestsellers of popular science, which fed an immense public interest in the new discoveries of the age.

A major question in Buffon’s day was whether the Earth was really only a few thousand years old, as most Biblical scholars claimed. The Biblical theory was based on counting generations back through the Old Testament.  Today we know that the Earth is 4.5 billion years old, from studying meteorites that formed at the very beginning of the solar system’s history.  But back then, many scientists thought the question was unanswerable (meteorites and advanced radiometric dating techniques were unknown at the time).  

Buffon addressed the question directly. He assumed that Earth had formed in conjunction with the much hotter Sun, and that the Earth had therefore started in a molten state. He had a bright idea inspired by earlier remarks of Newton. It was clearly impossible to simulate the cooling of the entire Earth. But if he could measure how long it took his solid globes to cool and also show how the cooling time depended on the size of the globe, he could then figure the cooling time for a much larger globe the size of Earth. Following the scientific method, he repeated his experiment many times to get more reliable numbers. He also measured the cooling time for spheres of different materials, such as glass and stone. 

In 1778, Buffon published his conclusion that the planet was 74,832 years old.  This was radically older than the current thinking, and it directly contradicted the Biblical calculations.  We know today that the real age of the Earth is still greater - Buffon didn’t take into account other factors in the cooling of the Earth, such as internal heat from decaying radioactive elements. However, Buffon was correct in concluding that the Earth is much older than the few-thousand-year estimate popular in those days.