On June 18, 1178, five monks at Canterbury reported witnessing a catastrophe in the sky:
There was a bright new moon, and as usual in that phase its horns were tilted toward the east; and suddenly the upper horn split in two. From the midpoint of this 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.
Stony Brook earth scientist Jack Hartung speculates that this may have been an impact event that created the 20-kilometer crater Giordano Bruno.
More discussion and an alternative explanation at the NASA website:
and, of course, at Wiki: http://en.wikipedia.org/wiki/Giordano_Bruno_%28crater%29