May 18, 1952: Carbon-14 Sets Stonehenge Date at 1848 B.C., More or Less

1952: An analysis of the carbon-14 radioisotope in a piece of charred oak from an excavated pit at Stonehenge estimates that the mysterious structure on England’s Salisbury Plain is 3,800 years old, plus or minus 275 years. The carbon-dating process that dated Stonehenge to about 1848 B.C. was conducted by the technique’s godfather, Willard Libby. […]

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1952: An analysis of the carbon-14 radioisotope in a piece of charred oak from an excavated pit at Stonehenge estimates that the mysterious structure on England's Salisbury Plain is 3,800 years old, plus or minus 275 years.

The carbon-dating process that dated Stonehenge to about 1848 B.C. was conducted by the technique's godfather, Willard Libby. The University of Chicago professor developed radiocarbon dating in the late 1940s and won the 1960 Nobel Prize in chemistry for it.

When plants or animals die, they no longer exchange their carbon with fresh atoms from their environment. Thus, as the radioactive carbon-14 in dead matter decays to the more plentiful isotope carbon-12, the proportion of C-14 to C-12 declines. Carbon-14 has a half-life of about 5,600 years, so measuring the proportion of C-14 that's still present in dead organic matter, and comparing it to the known proportion of C-14 in living matter, will indicate the age of the sample.

To be sure, carbon dating has its limitations. Libby assumed the ratio of C-14 to C-12 was constant, but the enormous amount of old carbon (from coal, petroleum and other fossil fuels) unearthed since the Industrial Revolution has changed the ratio. Improved techniques now date the earliest stone structures at Stonehenge to about 2600 B.C.

Whatever its exact age, as Time magazine noted when reporting the 1952 dating, Stonehenge has been "credited, at one time or another, to the Phoenicians, Celts, Romans, Sumerians, Druids and early Christians."

Sir Joseph Norman Lockyer, who co-discovered helium and founded the journal, Nature, wrote in 1901 that the Heel Stone section of Stonehenge "had been originally aligned with the summer solstice" and calculated that it was built in 1800 B.C.

Further investigations have suggested that Stonehenge was an astronomical observatory, a place of worship and healing or perhaps a cemetery. Whatever its exact history, origins or age, thousands each year flock to Stonehenge to welcome the sun on the summer solstice.

Correction: The original version of this post misstated the cause of inaccuracy in first-generation radiocarbon dating. It also linked to, and partially relied on, a website of dubious validity. See the comments below for more details. This Day in Tech regrets the error.

Source: Various

Photo: technowannabe/Flickr

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