The body of the Moon, which was below writhed... throbbed like a wounded snake

Canterbury chronicles

The most widely accepted theory is that the five men witnessed a huge meteoroid hitting the surface of the Moon; an impact so big it could have wiped out civilisation, had it occurred on our planet. It is said that this impact could have created the 22-kilometre- (14-mile-) diameter lunar crater Giordano Bruno.

But, according to a US researcher, this theory does not quite add up. Paul Withers, a graduate student at the University of Arizona, suggests in this month's issue of Meteoritics and Planetary Science that a collision this big would have resulted in millions of tonnes of moon rock showering down to Earth. There is no record of such a shower ever taking place.

'Flaming torch'

It was about an hour after sunset on June 18, AD 1178, that the group of five eyewitnesses saw the upper horn of the bright, new crescent Moon "suddenly split in two. From the midpoint of this division a flaming torch sprang up, spewing out... fire, hot coals and sparks... The body of the Moon, which was below writhed... throbbed like a wounded snake".

A geologist suggested in 1978 that this dramatic passage from the chronicles of Gervase of Canterbury might be an account of the formation of the Giordano Bruno lunar crater. Giordano Bruno is in a position that could be consistent with this description; it is also the youngest crater of its size or larger on the Moon, meaning 1178 could conceivably be its birth date.

An impact this size on the Moon is predicted to happen once every 15 million years or so

Paul Withers, University of Arizona

Giordano Bruno was made when an asteroid almost three kilometres (two miles) wide slammed into the surface of the Moon. Had it hit Earth, it is likely that none of us would be here today.

"An impact this size on the Moon is predicted to happen once every 15 million years or so," Paul Withers told BBC News Online. "Having one happen in the past 1,000 years would suggest that the predictions might be dangerously incorrect and the Earth might be in more danger from colliding space rocks than is currently thought."

So a sigh of relief should greet Withers' claim that, whatever those men saw that night, it was not the creation of Giordano Bruno.

Meteor storm

An asteroid impact that size on the Moon would have catapulted 10 million tonnes of debris into the Earth's atmosphere, Withers said, causing an impressive meteor storm.

"I calculate that this would cause a week-long meteor storm comparable to the peak of the 1966 Leonids," he said. "They would be very bright, very easy to see. It would have been a spectacular sight! Everyone around the world would have had the opportunity to see the best fireworks show in history."

But no-one ever reported this firework extravaganza. Historical records show nothing, including the European, Chinese, Arabic, Japanese and Korean astronomical archives.

Paul Withers thinks the witnesses might have seen a small meteor that was directly in front of the Moon coming straight towards them through the Earth's atmosphere, as they looked up at the sky that night. Perhaps they did, but we cannot be sure. The events of June 18, 1178, remain a tantalising mystery.