About an hour after sunset on June 18, 1178, the Moon exploded. That’s what it looked like to five terrified, awestruck monks watching the skies over the abbey at Canterbury, in southeastern England, anyway. They hurried (or made haste, as people did during the Middle Ages) inside and told the abbey’s chronicler Gervase of Canterbury what they had just seen, and all five stunned skywatchers swore that they were “prepared to stake their honor on an oath that they have made no addition or falsification in the […] narrative.”

The Moon was a pale, shining crescent in the skies over England that night, and the monks said it looked like the upper horn of the crescent split in half, and “from the midpoint of the division a flaming torch sprang up, spewing out, over a considerable distance, fire, hot coals and sparks.”

Whatever terrible cosmic explosion had just happened, it seemed to shake the whole Moon. “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,” Gervase wrote. “Afterwards it resumed its proper state.”

All five monks dramatically swore that they weren’t lying or even exaggerating a little, no one else mentioned seeing anything unusual, though “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.”

Nearly a thousand years later, astronomers, planetary scientists, and historians still can’t say for certain exactly what the Canterbury monks saw that night in 1178. If we give them the benefit of the doubt and assume they were true to their oaths, the most likely explanations are that they watched an enormous asteroid slam into the Moon’s northern hemisphere, or that they experienced a truly spectacular optical illusion.

A Close Call For Life On Earth

An asteroid crashing into the Moon could explain what the monks saw in 1178. The collision would blast a plume of molten rock – heated by the force of the impact – out into space. A hard enough impact might even spin the Moon slightly on its axis. Those things seem to fit the description Gervase wrote down; conveniently, they also create evidence that we can look for to figure out whether a giant chunk of space rock really did collide with the Moon during the Middle Ages.

To blast debris into space from the upper horn of a crescent Moon, the asteroid would need to hit the Moon’s northern hemisphere, close to the imaginary line that divides the familiar Earth-facing side of the Moon from the far side, where only robotic spacecraft have ever landed. In 1959, the Soviet Luna-3 spacecraft took the first photographs of the previously uncharted terrain on the far side of the Moon, and those photos revealed a crater in just about the right spot.

In 1976, geologist Jack Hartung suggested that the 22 km (14 mile) wide crater, now called Giordano Bruno, marked the site of the impact the medieval monks witnessed. Fine-grained rock and dust radiate out from the crater in bright rays, stretching 150 km (93 miles) across the lunar surface and still mostly undisturbed by micrometeorite impacts. That means Giordano Bruno is probably the youngest of the Moon’s large craters, and Hartung suggested that it could be very young – about 842 years.

And if Hartung is right, Earth had a close brush with Armageddon (the apocalypse, not the movie) in 1178. The asteroid that dug Giordano Bruno crater must have been 1 to 3 km (0.5 to 2 miles) wide. “If this hypothesis is correct, the Earth and human civilization narrowly avoided disaster in 1178,” wrote planetary scientist Adam Withers in a 2001 presentation.

How Old Is The Crater?

In 1978, exactly 800 years after the event, a laser rangefinding study measured the distance between Earth and various points on the Moon’s surface. After an impact big enough to create a crater like Giordano Bruno, the Moon should still have a slight, slow longitudinal wobble, and that’s exactly what the study observed. The amount of wobbling even seemed to match what planetary scientists had predicted as a result of a recent large asteroid impact.

But 3 years later, another study concluded that the Moon actually has a bigger longitudinal wobble than a recent impact could actually explain. That study’s authors suggested that the wobble was actually the result of turbulent friction between the Moon’s core and its mantle. So that line of evidence wavers a bit (not sorry).

We’re back to the crater, then. Without rock samples, it’s hard to say precisely how old any particular geological feature on the Moon is. One of the best ways to estimate from a distance is count how many small craters are scattered across the feature. Small objects hit the Moon fairly regularly, and planetary scientists have a good idea how frequent such these minor impacts are, so the number of small craters in – for example – the floor of a much larger crater, or a basalt lava flow, can suggest how long the feature has been on the surface, collecting impact scars.

In 2010, the Japanese spacecraft SELENE sent home high-resolution images of the area around Giordano Bruno crater, and researchers counted the small craters superimposed on its ejecta rays and in the floor of the crater. It turned out that Giordano Bruno crater is probably between 1 million and 10 million years old – most likely around 4 million. The odds of it having formed in the Middle Ages are around 0.1%.                                                                                                                                                                  

A 1-In-510-Million Optical Illusion

There’s another problem with the meteor impact idea. In 2001, Withers calculated that a 1-3 km (0.5 to 2 mile) wide asteroid hitting the Moon would have blasted about 10 million tons of rock and dust, mostly in tiny 0.1-10 cm-wide fragments, far enough into space to patter against Earth’s atmosphere. For at least a week after the impact, all those tiny lunar meteors would have lit up the night sky in a dazzling meteor shower, with about 50,000 meteors streaking across the sky every hour.

That’s the kind of thing people tend to notice, and medieval astronomers across the Middle East, Europe, China, Korea, and Japan all kept detailed records of their observations. But no one mentioned a huge meteor shower in the summer of 1178. According to Withers, no meteor shower means no big impact on the Moon.

Instead, he suggests that the monks actually saw a much smaller meteor burning and breaking up as it streaked through Earth’s upper atmosphere, directly along their line of sight and in front of the Moon.

“And it was a pretty spectacular meteor that burst into flames in the Earth’s atmosphere – fizzling, bubbling, and spluttering,” he said in a 2001 press release. And while a meteor hitting the Moon just after sunset would have been hard to miss for observers across most of western Europe, a meteor burning up in Earth’s atmosphere would seem to be in front of the Moon only from about a 1 or 2 square kilometer patch of ground. “That would explain why only 5 people are recorded to have seen it,” said Withers. The monks at Canterbury just got extremely lucky.

The Human Impact

Without evidence we can’t currently get our hands on, it’s hard to say for sure exactly what the Canterbury monks told Gervase about in 1178. But they clearly saw something remarkable, and that’s the kind of experience you don’t forget.

“Imagine being in Canterbury on that June evening and seeing the Moon convulse and spray hot, molten rock into space,” said Withers in 2001. “The memories of it would live with you for the rest of your life.”