Many theories have arisen, but we may never know for sure.
In the late 12th century there was a chronicler at Christ Church in Canterbury named Gervase. Aside from matters related to his office, relatively little is known of this monk’s life; he is often confused with one of several other of Gervases of the same era. It is in brother Gervase of Canterbury’s records, however, that can be found one of the most curious passages in the annals of the old English Cathedral city.
The records states that five monks reported a strange sight to Gervase shortly after sunset on the night of June 18, 1178:
They saw “the upper horn [of the moon] split in two.” Furthermore, Gervase writes, “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.”
It is unclear if the monks believed this to be a mystical vision, or if there was any deeper meaning attributed to what they had seen. There is no further mention of these events in Canterbury records. The story, however, has baffled astronomers and scientists who have many theories as to what caused this unique and seemingly isolated sighting.
In 1976, geologist Jack B. Hartung proprosed that the description closely resembled what would have been seen during the formation of the crater known as Giordano Bruno, a relatively new crater as far as the moon’s timeline goes.
We can tell that Giordano Bruno is new from the vivid ray system — lines formed by debris upon meteorite impact — that is still apparent today.