More than 800 years ago, shortly after sunset on the evening of June 18, 1178, five monks at the abbey of the Christ Church of Canterbury in England looked up at the moon and saw something incredible. Gervase, the official Chronicler of Canterbury, recorded it as such:

The upper horn of the Moon 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.

What did the monks really see that night? In 1976, a geologist, Jack B. Hartung, hypothesized that the Canterbury monks witnessed a massive meteor impact on the moon, which caused molten matter to rise up from the interior and extrude onto the surface. Obviously this would leave a visible mark. Hartung claimed there was a crater that was consistent with this sort of impact: it’s called the Giordano Bruno crater, and it is very young, geologically speaking, which we can tell from the rays of dust (or “ejecta”) radiating out from it. Only very recent impacts show this pattern, so the Bruno crater is very young. It could have been formed in the 12th century.

bruno crater

The Giordano Bruno crater on the surface of the moon. Was it formed in 1178?

Others dispute this. While the Bruno crater hypothesis was widely accepted for years, in 2001 a graduate student at the University of Arizona, Paul Withers, argued that an impact large enough to result in the Bruno crater would have caused havoc on Earth. Such a collision would have sent showers of debris headed toward our own planet, and they would have burned up in the atmosphere giving us one of the most spectacular meteor showers in history, likely visible from all over the world. At its peak there would have been a staggering 50,000 meteors an hour. No such shower was recorded in 1178. Therefore, the Canterbury event must have been something else.

Withers’s hypothesis is that the explosion did not happen on the moon at all. If a meteor happened to come down to Earth at precisely the right place and time–say, right in front of the moon as it rose that night–it would have seemed to observers that the moon was literally exploding. In fact such a meteor would have burned up in our atmosphere and not caused any significant damage (unlike the Tunguska event, or a similar event in Russia in 2002). It’s a bit far-fetched, but it could account for what the monks say they saw.

The truth is, we don’t know what happened the night the moon exploded back in 1178. This is one of those curious unknowns that straddles the line between a scientific and historical mystery. Perhaps when human beings return to the moon, which hopefully will be soon, we will know more. Until then, the account of the Canterbury monks remains in an old illuminated manuscript, waiting for modern scientists or astronomers to explain its secrets.