'Locked to the Sun and Moon': The startling 1960s theory that Stonehenge was a prehistoric computer

In August 1966, a US astronomy professor shared his astonishing findings with the BBC. He believed that the Neolithic stone circle was built to predict eclipses.

Experts have long debated the meaning of Stonehenge, the prehistoric stone circle in the south-west of England. They have speculated that the mysterious monument might have been a place to commemorate the dead, a sacrificial altar, or a site of other religious ceremonies. But in August 1966, a Boston astronomy professor, Gerald Hawkins, told the BBC's Chronicle programme that he'd solved the puzzle thanks to a peculiar configuration of numbers – and an early IBM computer. 

Hawkins' theory formed the basis of his popular and influential 1965 book Stonehenge Decoded. The British-born scientist suggested to the BBC that alignments of the huge slabs and archways meant that "circles at Stonehenge could have been used as a computer to follow the Moon, and predict eclipses". Eclipses, such as those witnessed around the world this month, could have been accurately foretold by ancient Britons – more than 4,500 years ago.

In the BBC footage, Hawkins gestured to an exact scale model of Stonehenge, showing how in the biggest circle, marking the circumference of the site, there are 56 holes. "Fifty-six was the only number that would fit a certain pattern of the Moon," he claimed, suggesting that the holes "make a simple counting device for predicting eclipses".

The meaning for those who built Stonehenge was, he believed, significant. "Eclipses are spectacular events that surely aroused the deepest emotions of our primitive ancestors," said Hawkins. "The light of the Moon, which is, of course, borrowed from the Sun, is cut off. It passes into the Earth's shadow. The Moon turns from silvery white to red. The ancients regarded it as blood red and attributed ominous meanings to it."

Stonehenge Decoded had "the surprising effect of silencing speculation for several decades", said a 2007 article in the British magazine Current Archaeology.

According to Hawkins, it was "the first theory concerning the purpose of Stonehenge that has been academically accepted and published in the scientific journals". Although the connection between the site and summer solstice had been recognised since the 18th Century, the professor took the idea further, proposing that what had been seen as a temple of Sun-worship could have been a sophisticated "astronomical observatory". 

In 1961, he had visited Stonehenge to try and understand "how the monument was supposed to mark the sunrise". As he told the BBC five years later, "I looked around in the dawn light at Stonehenge… between the great stone archways. I felt that my view was controlled, confined. The archways seemed to force me to look at certain points on the horizon. What was I supposed to see?"

Struck by the way ancient architects had framed his sightline, Hawkins returned to the US with charts of the site, plotting the positions of its centre point and each stone, archway, hole and mound.

Attempting to determine "exactly what would have been seen at the time of Stonehenge… would be long and tedious and had probably deterred others before me", Hawkins told the BBC. "Fortunately, I had the use of a calculating machine, the electronic computer." 

At the Harvard-Smithsonian Observatory in Massachusetts, Hawkins and his team instructed the computer to calculate what would have been seen along each alignment at the time Stonehenge was built. He used the calculations as the basis for Stonehenge Decoded.

As the BBC presenter said in 1966, "The computer yielded some tantalising results." Many of the Stonehenge alignments seemed to point to positions of the rising and setting Sun and Moon. According to Hawkins, "Stonehenge was locked to the Sun and Moon as tightly as the tides. It was an astronomical observatory. And a good one, too."

While the marking of solar positions at midwinter and the equinoxes was "natural to expect", he told the BBC, "the Moon at Stonehenge was a surprise. The Moon is a very difficult thing to observe because it changes its position so much more complicatedly than the Sun." 

Hawkins concluded that the purpose of Stonehenge was clear. "There can be no argument about whether it was a temple or an observatory. It was both, because the Sun and Moon were gods in those days." 

If this were true, Stonehenge was "a device that priest-rulers could have used to enhance their power", argued a 1965 article in Time magazine. "On the day or night that their stone computer predicted an eclipse, they might well have summoned their subjects to Salisbury Plain to observe a spectacle that terrorised most ancient peoples. When the eclipse started, the priests probably intoned the prayers that enabled the Sun or Moon to escape the blackness." 

Even at the time, there were critics of his theory, with some pointing out that three of the 56 holes intrinsic to it might have simply been caused by the former growth of trees. As the British archaeologist Richard Atkinson wrote in the New York Review of Books in 1966, "That the layout of Stonehenge has some astronomical significance is not in doubt… The interpretation of that alignment, however, and of any other astronomical characteristics of the monument, requires the exercise of scholarly caution."

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And new investigations since the 1960s have discredited Hawkins' ideas. "It's a dead theory today, in the dustbin filled with so many discounted theories about Stonehenge," says Prof Michael Parker Pearson, who led a landmark study transforming modern understanding of the monument. The eclipse computer was, he says, "a trendy metaphor for the time… but now wildly inappropriate for understanding Stonehenge".

While Hawkins thought the construction of the monument was a more integrated assembly, "we now know that Stonehenge was built in a very different, multi-stage sequence over a period of more than 1,500 years," Parker Pearson, a professor at University College of London's Institute of Archaeology, tells the BBC. "We also know that the solstice lunar standstill orientations are not actually precise – they were to within one or two degrees only – so it was never an instrument for measuring time, but rather a monument celebrating particular solar and lunar movements.

"We do have evidence from as far back as 1000BC in Britain that people tracked and probably predicted lunar eclipses," adds Parker Pearson, "but that's 1,500-2,000 years after Stonehenge's heyday." 

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