In 1178, a group of monks at Canterbury saw the moon suddenly explode into sparks, "writhe," and "take on a blackish appearance." What the hell did they see?
Throughout history people were always mistaking astronomical events for supernatural signs. Sometimes the signs were interpreted as good omens. More often, strange lights in the sky were considered portents of evil. So in 1178, five monks in Canterbury had what must have been a very trying summer. On June 18th, looking up at the moon, they saw something they described in very religious language:
This year on the Sunday before the Feast of Saint John the Baptist, after sunset when the moon was first seen, a marvellous sign was seen by five or more men sitting facing it. Now, there was a clear new moon, as was usual at that phase, its horns extended to the east; and behold suddenly the upper horn was divided in two. Out of the middle of its division a burning torch sprang, throwing out a long way, flames, coals and sparks. As well, the moon's body which was lower, twisted as though anxious, and in the words of those who told me and had seen it with their own eyes, the moon palpitated like a pummelled snake. After this it returned to its proper state.
Thank goodness it returned to its proper state, but what did they see? Some people think they saw nothing — citing the fact that the moon's body, which the monks claimed "was lower" than the moon's horns, should have been above the horns. But some experts believe that they saw the impact that led to formation of the Giordano Bruno crater on the moon. Others disagree, believing that such an impact would have kicked up debris that would lead to a week-long pelting of the Earth. Such a thing would have caught the attention of more than a few monks.
The most credible theory is the monks just saw a particularly spectacular meteor hit the atmosphere. From their point of view, and their point of view alone, it would have looked like part of the moon exploded. No one else would have seen it as anything more than a bright shooting star.
[Sources: The Giordano Bruno Crater, The Mysterious Case of Crater Giordano Bruno.]