Among his other writing duties and pleasures, Gervase also kept the monastery's chronicles, and in 1178 he recorded something interesting:
This year on the 18th of June,[*] when the Moon, a slim crescent, first became visible, a marvelous phenomenon was seen by several men who were watching it. Suddenly, the upper horn of the crescent was split in two. From the mid point of the division, a flaming torch sprang up, spewing out over a considerable distance fire, hot coals and sparks. The body of the Moon which was below, writhed like a wounded snake. This happened a dozen times or more, and when the Moon returned to normal, the whole crescent took on a blackish appearance.Scholars and astronomers have puzzled over this. Astronomer Jack Hartung in the September 1976 issue of the journal Meteoritics proposed that they had witnessed a meteor strike that created the lunar crater "Giordano Bruno." Recent photos by Apollo made it, in his opinion, a candidate for a young crater only 800 years old. Hartung had his critics, but without new data on the crater, the verdict could rightly claim to be "up in the air." Until years later a student named Paul Withers in Weekly Scientist pointed out an obvious flaw. An impact great enough to create the 14-mile wide Giordano Bruno would have thrown "10 million tons" of debris off the lunar surface; the ejecta in short order would have created unbelievably spectacular meteor showers in Earth's atmosphere. These meteor showers were not noted by monks in England or any other of the sky-gazing cultures that dotted the globe at the time.
So what did the monks see? His guess is that they were looking in the right place at the right time to observe a meteor entering Earth's upper atmosphere between their line of sight and the horns of the crescent moon.
For a modern astrophysicist's view on Gervase' chronicle, see Brian Koberlein's Google + post for today, June 25th, called "Mining for Science."
*N.B. The Proleptic Gregorian Calendar (used for dates prior to 1582 because of a necessary correction) turns this date into June 25th, meaning today is the 834th anniversary of the monks' observation.