Writing a decade later, Gervase of Canterbury recounted an extraordinary astronomical event that had been witnessed by a group of his fellow monks on the 18th June, 1178:
Gervase, born c.1141, was a monk of Christ Church, Canterbury, and was amongst those who served St Thomas Becket and helped bury him. He was also a chronicler, writing a number of regnal and ecclesiastical histories.
Whilst Gervase did recount stories and visions for rhetorical and political purposes (the most egregious example being a vision he claimed to have of the sainted Thomas Becket driving out an Archbishop who had angered Gervase), the account of this lunar event is not ascribed any particular meaning, nor does it appear within the context of larger events of disputes. Gervase was very interested in observing heavenly bodies, often describing eclipses. So as fanciful as his description may appear, it should not be discounted as fancifulness or rhetoric.
Over the years, historians and astronomers have put forward a number of theories about what phenomenon Gervase is describing. The most prominent theory, put forth by geologist Jack Hartung in 1976, proposes that the monks witnessed an asteroid or comet hitting the moon, causing a plume of molten matter to rise up from the Moon’s surface in a manner consistent with Gervase’s account. By narrowing down the physical location on the Moon from the description, Hartung believed that the monk’s could have been watching the formation of the Giordano Bruno crater, one of the youngest craters on the surface of the Moon.
This theory has been disputed, with one astronomer suggesting that the formation of such a crater would have created 10 billion kilograms of debris, causing at least a week of blizzard-like meteor storms on Earth. No accounts of such conditions have been found for this time period anywhere in the world.
Paul Withers, at the University of Arizona Lunar and Planetary Laboratory, believes the answer is much simpler: