How We Know The Earth Is Ancient

**SPEAKER_1** (0:00)
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**Matt O'Dowd** (1:00)
In astronomy, we talk about billions of years, like it's no big deal. But how can we be sure about timescales so far beyond the capacity for human intuition? Our discovery of what we now call deep time is very recent. As recent as our discovery of the true spatial vastness of our universe. And it came as scientists tried to measure the age of the Earth.
What they found was as shocking and humbling as anything ever seen through the telescope.
How long has the world been here? The question is surely as old as human civilization, and perhaps older. The earliest written record we have of a cosmogony, an origin story for the cosmos, came from the Sumerians, followed soon by the Babylonians. There were floods, dismemberments of gods, all that good stuff that was later adopted by other traditions. But the main thing was this notion that the world had a beginning. The early Jews adopted this idea, and in turn it was picked up by the early Christians, and the idea of a single unique creation event became a cornerstone of some of the earliest religious texts, like the book of Genesis. The Genesis story was so influential that many people used it to try to figure out the age of the world. Probably the most famous guess came from the Irish bishop James Usher in the early part of the 17th century. Usher looked not only at the Bible, but at hundreds of other ancient texts, trying to align the different and often conflicting histories that they presented. Usher eventually concluded that the world began at 6pm on Saturday, October 22nd, 4004 BC. An oddly precise prediction given the source material, and obviously a bit lower than the value given to us by science. The French naturalist, Georges-Louis Leclerc, Comte de Buffon, was among the first to use what we call scientific methods to calculate the age of the Earth, publishing his result in 1778
It was ingenious, really. He assumed the Earth started out as a ball of molten rock, which subsequently cooled down to its current temperature. Buffon spent six years measuring the cooling rates of materials in his lab, and then calculated that the Earth was 74,832 years old. And yes, that's also a weirdly precise number. Both Usher and Buffon are docked half a point for excessive significant figures.
Buffon's premise was clever if flawed. His 75,000-ish years was too small, and we now know why. And I'll come back to that. At around the same time as Buffon was staring at warm lumps of iron, the Scottish geologist James Hutton wandered Britain, pondering the ages of its rock formations. He became convinced that they were formed when molten rock pushed through the crust from Earth's interior. This directly gave us igneous rocks like granite and basalt, while sedimentary layers...

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