Splitting the Moon

What did some monks in the twelfth century actually witness?

Gervase of Canterbury was a twelfth-century English monk who lived from about 1141 to 1210. He is best known as a historical chronicler, and wrote accounts of both the secular and ecclesiastical history of Britain, as well as producing quantities of maps showing the landholdings and bishoprics at the time. Both of these have been of considerable value to scholars, and his writings are lucid, fact-based, and clear-eyed.

Which makes the other event he wrote about even more curious.

In June of the year 1178, Gervase says, some of the monks of the abbey were out on the lawn at twilight, enjoying a bit of leisure time in the pleasant warmth of early evening. That was when they saw something astonishing:

[On the evening of June 18, 1178] after sunset when the Moon had first become visible, a marvelous phenomenon was witnessed by some five or more men... Now there was a bright new Moon... its horns were tilted toward the east; and suddenly the upper horn split in two. From the midpoint of the division a flaming torch sprang up, spewing out, over a considerable distance, fire, hot coals, and sparks. Meanwhile the body of the Moon which was below writhed, as it were, in anxiety and to put it in the words of those who reported it to me and saw it with their own eyes, the Moon throbbed like a wounded snake. Afterwards it resumed its proper state. This phenomenon was repeated a dozen times or more, the flame assuming various twisting shapes at random and then returning to normal. Then after these transformations the Moon from horn to horn, that is along its whole length, took on a blackish appearance. The present writer was given this report by men who saw it with their own eyes, and are prepared to stake their honor on an oath that they have made no addition or falsification in the above narrative.

I first heard about this peculiar account almost exactly eight hundred years after it happened, on the episode of Carl Sagan’s Cosmos called “Heaven and Hell.” Sagan’s take on the story is that what Gervase wrote is substantially true; that despite the superstition of the time, he transcribed an unembellished record of what the other monks had seen. Further, Sagan said, the survey work done on the Moon since that time found what may account for the odd event—a 22-kilometer-wide recent crater just barely over the edge of the near-Earth side on the northeastern quadrant, named Giordano Bruno after the martyred sixteenth century astronomer. What the monks witnessed was the meteorite impact that produced the crater, first creating a plume of molten rock and then scattering dark ash across the Moon’s surface. Interestingly, Giordano Bruno has rays of debris surrounding it, suggesting its recent origins:

[Image is in the Public Domain courtesy of NASA/JPL]

Further evidence supporting this conjecture is that laser rangefinding data shows that the Moon is oscillating slightly—in Sagan’s words, “ringing like a bell”—at a frequency consistent with a meteor impact eight hundred years earlier.

Not everyone agrees with this interpretation, however. Paul Withers, of the University of Arizona’s Lunar and Planetary Laboratory, points out that such an impact would have accelerated much of the debris to escape velocity, and a significant quantity of it would have been pulled in by the Earth’s more powerful gravitational field, triggering “blizzard-like meteor storms” with as many as fifty thousand meteors per hour for several days, perhaps up to a week. No one recorded any such event. Surely the meticulous Chinese and Korean astronomers of the time would have seen and written about such an unprecedented phenomenon. In fact, nobody else on Earth we know of who was keeping records at the time even recorded witnessing the initial impact—if impact it was.

Withers suggests a much more local, and prosaic, solution; what the monks of Canterbury saw was a bolide, a meteor that explodes in midair. The most famous bolide is the Chelyabinsk meteor of February 2013, when an estimated eighteen meter long, nine thousand metric tonne chunk of rock exploded over the Russian town of Chelyabinsk, creating a tremendous fireball and shattering windows throughout the region. The Canterbury event, Withers said, was a bolide over southeastern England that just happened to create its fireworks in front of the crescent Moon, which would explain why it wasn’t seen elsewhere.

I’m not entirely happy with this explanation, either. As Chelyabinsk illustrates, bolides are loud. There is nothing in Gervase’s account indicating that the Canterbury event made any sound at all. Plus—if you’ll look at videos of the Chelyabinsk meteor (you can see a short clip at the page linked above)—they move fast, leaving behind a bright streak. Surely the monks of Canterbury had seen “shooting stars” many times before, and would have reported this not as a phenomenon on the Moon, but simply a humongous shooting star that exploded.

And finally, if it was a bolide, how could this account for the monks’ statement that the paroxysms on the Moon were “repeated a dozen times or more”?

I’m still leaning toward the lunar impact explanation, myself, but I’m aware that it leaves plenty of unanswered questions. It’s a curious account, however you look at it. We may never know for certain what happened, but even so, we’re lucky that someone as clear-headed as Gervase of Canterbury was around during those dark and superstitious times to record an event that surely must have scared the absolute hell out of everyone who witnessed it.