According to the medieval chronicles of a Canterbury monk, 12th-century sky watchers got an eyeful one June evening, as the new crescent moon appeared to ignite and convulse, spewing molten rock into space. Nearly 800 years later, a geologist suggested that this event fit with the location and age of one of the moons youngest larger craters, Giordano Bruno. What those Canterbury folks witnessed, he proposed in 1976, was a meteor impact that created a lunar crater 22 kilometers across10 times as wide as northern Arizonas Meteor Crater. Findings reported in this month's issue of Meteoritics and Planetary Science support a different explanation, however.

Previous studies indicated that such an impact would have sent 10 million tons of space rubble into the earth's atmosphere in the days that followed. Graduate student Paul Withers of the University of Arizona decided to further probe the properties of the resulting meteor storm. "I calculate that this would cause a week-long meteor storm potentially comparable to the peak of the 1966 Leonids storm," Withers notes, adding that the shower of inch-long fragments would have been highly visible. "It would have been a spectacular sight to see," he remarks. "Everyone around the world would have had the opportunity to see the best fireworks show in history."

Yet reports of this sort of blinding storm from 12th-century witnesses are unknown. Withers thus favors a theory first put forth in 1977, which holds that an earthbound meteor created the display instead. "I think [the Canterbury witnesses] happened to be at the right place at the right time to look up in the sky and see a meteor that was directly in front of the moon, coming straight towards them," he says.

"It was a pretty spectacular meteor that burst into flames in the earth's atmospherefizzling, bubbling and spluttering. If you were in the right one-to-two-kilometer patch on Earth's surface, you'd get the perfect geometry," Withers observes. "That would explain why only five people are recorded to have seen it."