What the Monks Saw: A Medieval Mystery

Just to show that life is full of unexpected turns, on June 18, 1178, a group of medieval English monks threw a fine puzzle in the laps of astronomers that lingers to this day.

On that day, shortly after sunset, five monks at the abbey of the Christ Church of Canterbury (on the site of Canterbury Cathedral,) noticed that something very odd was happening in the sky.  Gervase, who was Canterbury’s chronicler, provided the eyewitness record:

“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.  Then, after these transformations, the Moon from horn to horn, that is along its whole length, took on a blackish appearance.”

I am sure you are about to bring up the obvious question:  What the hell happened?  Go ahead and ask, but the answer you are likely to get from scientists is “Dunno.”  In 1976, geologist Jack Hartung suggested that the monks observed a meteor impact on the Moon so massive that it caused molten matter (the “flaming torch”) to rise into space.  Hartung pointed out that the Moon has a crater (named after Giordano Bruno,) that is possibly relatively young--perhaps young enough to date from June 18, 1178.  

Alas for this delightfully brief and tidy answer, it is far from universally accepted.  Other scientists have pointed out that an impact that huge would have had dramatic and impossible to ignore effects on Earth.  It likely would have caused one of the biggest meteor showers in human history, observable all over the world for days afterwards.  As far as we know, such spectacular showers were not recorded by the Canterbury monks, or anyone else on our planet for that matter.  Additionally, in 2008 high-resolution images obtained by a Japanese satellite were used to give the Giordano Bruno crater an age of about 4 to 6 million years, which would obviously negate Hartung’s theory.

In 2001, a University of Arizona student named Paul Withers came up with a more elaborate scenario.  He proposed that if, on that June night in 1178, a meteor came down to Earth and exploded right in front of the rising Moon, it could conceivably have appeared to the Canterbury witnesses that the Moon itself was erupting.  This would be a rather remarkable astronomical coincidence, but not an impossible one.  Unfortunately, this hypothesis can neither be proved or disproved.

So, what did those Canterbury monks observe?  We’ll likely never know for sure, which is what charms me about our little historical footnote.