Science & Technology

These 12th Century Monks May Have Seen an Asteroid Strike the Moon

There's something both quaint and inspiring about seeing scholars from long ago attempting to wrap their minds around cosmic events such as eclipses, meteors, and the motion of the planets. Sometimes they saw great truths in the stars, as when Copernicus realized the Earth moved around the sun, and other times, ancient astronomical events got inextricably tied to religious miracles. But sometimes, the mystery isn't easily solved — like when a large group of monks from Canterbury all apparently witnessed an explosion on the moon in the year 1178 C.E. We're still not entirely sure what they saw, but modern astronomers have some ideas.

Oblique view from Apollo 16 showing the Giordano Bruno crater.

What the Monks Saw

According to the 12th-century monk and chronicler Gervase of Canterbury, "five or more" monks at the monastery witnessed an incredible sight the night of June 18, 1178. What they saw was ... well, just read it for yourself:

"Now, there was a clear new moon, as was usual at that phase, its horns extended to the east; and behold suddenly the upper horn was divided in two. Out of the middle of its division a burning torch sprang, throwing out a long way, flames, coals and sparks. As well, the moon's body which was lower, twisted as though anxious, and in the words of those who told me and had seen it with their own eyes, the moon palpitated like a pummelled snake. After this it returned to its proper state."

The "horns" of the moon are the narrow tips of the crescent as the rest is cast in shadow, so what these monks described to Gervase sounds like a serious impact. For them to have seen a "burning torch" throwing "flames, coals, and sparks" out a long way from the moon, it must have been dozens of miles long, if not hundreds. Gervase notes that these monks were each willing to swear an oath that they had seen this event, and swearing an oath is a pretty big deal if you take the Ten Commandments as seriously as those monks probably did. But the mystery of what, exactly, could have caused such a sight was largely a mystery for 800 years. That was until geologist Jack B. Hartung proposed a solution.

Craters Gonna Crate

Hartung first encountered the event as a passing reference in a work by Isaac Newton and felt driven to explore the subject more. Once he'd tracked down the original text and compared the monks' description of the event to the known geography of the moon, he came to an astonishing conclusion: what they'd seen was the formation of the Giordano Bruno crater, the youngest crater known to exist on the lunar surface. He made a compelling case, and the hypothesis was generally accepted for many years. But some doubts have remained.

The thing is, astronomers don't really know how young the crater is. While estimates at the time suggested that it might have been less than a thousand years old, more modern figures place it between one and 10 million years ago, most likely about four million. The other problem is that Giordano Bruno is pretty enormous. At about 14 miles (22 kilometers) across, the asteroid that caused it would have to be between a half-mile and 2 miles (1-3 kilometers) across — big enough to be a threat to human civilization if it had struck our home turf instead. That's a pretty close call, and it happened only a half-second ago if you think about it on a geological scale. It would be pretty weird if only five people on the entire planet witnessed an Armageddon-level event like that.

Doubty with No Chance of Meteor Showers

In 2001, a grad student at the University of Arizona's Lunar and Planetary Laboratory hammered in what might be the final nail in Hartung's hypothesis. Like any astronomer would be, Paul Withers was intrigued by this medieval tale of explosions in the sky, but something seemed off about the moon-impact theory. After all, it takes a lot of rocks to fill a 14-mile-wide crater. All those moon rocks would have had to go somewhere, and that somewhere would be the atmosphere of our home planet. "I calculate that this would cause a week-long meteor storm," he told the University of Arizona. "Everyone around the world would have had the opportunity to see the best fireworks show in history." But there's no record of any such event. In other words, it didn't happen. But something certainly did.

So it probably wasn't an enormous moon-splosion that those monks saw all those years ago. Instead, many modern astronomers believe it's much more likely that they spotted a meteor burning up on entry into the Earth's atmosphere — one that just so happened to line up with where the moon was in the sky. According to this theory, the meteor would have basically been coming straight for them, giving them an astonishing light show but shielding that view from the rest of the world. We'll probably never have an answer for certain, but we're betting it was a minor meteor and not a medieval prequel to "Deep Impact."

While some monks were staring up at the stars, others were carving rude messages into the pews. Check out "Medieval Graffiti: The Lost Voices of English Churches" to discover the strange, bawdy, and sometimes esoteric writing found in discreet places. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

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Written by Reuben Westmaas June 15, 2018

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