Doug's Darkworld

“This year on the 18th of June, when the Moon, a slim crescent, first became visible, a marvellous phenomenon was seen by several men who were watching it. Suddenly, the upper horn of the crescent was split in two. From the mid point of the division, a flaming torch sprang up, spewing out over a considerable distance fire, hot coals and sparks. The body of the Moon which was below, writhed like a wounded snake. This happened a dozen times or more, and when the Moon returned to normal, the whole crescent took on a blackish appearance.”

Well, that’s interesting. It certainly sounds like they witnessed an impact of some sort on the Moon. And when this report came to the attention of modern astronomers in the 1970s, that’s what some of them thought too. In fact they suspected this was the origin of the Giordano Bruno Crater, a 22km wide crater located on the Moon where the impact was witnessed. It’s one of the Moon’s youngest craters, with white rays of ejecta extending for hundreds of kilometres. Cool enough, but it gets a bit alarming after that. This is because the timing and location of the strike are consistent with the impacter being a part of the annual Taurid meteor shower. While the stuff in meteor showers is generally sand grain size, there’s no guarantee it has to be. Uh oh. And as final evidence of the impact, instruments installed on the Moon by the Apollo astronauts detected faint vibrations that could be the Moon still “ringing” from the impact a thousand years earlier.  And for over 20 years that’s where the matter stood. Now mind you, this wasn’t considered a fact in any sense of the word, but it was regarded as an interesting and possibly true hypothesis.

Science moves on though. What was possible in the late 70s looked far less promising by the turn of the century. The first fly in the ointment was the age of Giordano Bruno Crater. Yes, it looks fresh, but by 2,000 astronomers had a much better understanding of (not to mention maps of) the Moon’s surface. And it was clear that while Giordano Bruno was a fresh crater geologically speaking, it as still at least a  million years old. Our ancestors may very well have witnessed the creation of Giordano Bruno, but 12th century monks most certainly did not.

So, it was a other crater, right? Nope, it gets worse. Astronomer’s understanding of impact events had grown by leaps and bounds since the original hypothesis. And by 2,000 it was clear that an impact so large as to create the spectacle described by the monk would have showered the Earth with massive amounts of debris for some times afterwards. While it was conceivable that the monks observations were the only ones recorded, it’s hard to imagine that days or weeks worth of spectacular meteor showers,  with almost certainly some major impacts on Earth, would have gone unremarked in the historical record. In medieval Europe, they would have thought it was the end of the world.

 OK then, if the monks didn’t see an impact on the Moon, what did they see? Current thinking is that they saw a meteorite enter the atmosphere and explode, a fireball in the popular vernacular. And by a fluke of chance, from the monks perspective, it was juxtaposed on the new Moon. Atmospheric distortion caused by the event being near the horizon (the above illustration isn’t meant to be taken literally,  the Moon was just above the horizon) explains the Moon “writing like a snake.” It was a spectacular sight no doubt, but not all that unusual, and it had nothing to do with the Moon. This is also why there are no other reports of the incident, most people simply saw a fireball, a remarkable but not terribly unusual event. I also suspect, and this is pure speculation on my part, that a major lunar impact would appear as a bright flash (or no flash) followed by a puff of “smoke” on the Moon, the expanding cloud of debris, that dissipated very quickly.

Mystery solved? Probably. Is it a certainty that the monks witnessed a fireball? No, but pretty close to one. If other accounts of the event ever show up, maybe this explanation will get more definitive. Or be ruled out entirely if people some distance away also saw what appeared to be an event on the Moon. If that’s the case, who knows what they saw. That’s the wonderful thing about science, if new evidence rules out the theory, time for a new theory.

(The above image is claimed as Fair Use under US copyright law. It’s not being used for profit, and it’s the only image illustrating the 1178 event I could find. Credit and copyright: Peter Grego As for the mysterious vibrations detected on the Moon, the so called “ringing” from the impact, they were eventually explained as a result of processes occurring inside the Moon)