According to one of the history websites to which I subscribe today is the anniversary of a lunar event in 1178 that still draws the interest of astronomers and cosmologists. I have adapted what follows, with some historical additions of my own, from the Wikipedia account of the lunar crater now named after the extraordinary late Renaissance Italian intellectual Giordano Bruno (1548-1600).
Five monks from Christ Church cathedral priory in Canterbury reported to the conventual chronicler, Gervase - whose linked biography provides a good historical context for the life of the cathedral community at the time of the sighting - that about an hour after sunset on 18 June 1178, (25 June on the proleptic Gregorian calendar) 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 occurs to me that according the the Rule of St Benedict these five monks should have been tucked up in bed getting their sleep ready to rise for the coming day’s Mattins. As they were not I assume they were skywatching with permission in what was a well regulated house rather than that five of them just happened to be up and about, and looking up at the Moon at that time.
Eleventh and twelfth century England had a significant interest amongst Benedictine monks in observing the heavens and recording what they saw: for example monks at Worcester cathedral priory noted and drew sun spots earlier in the twelfth century. Nor was this interest confined to monastics - a favourite historical non sequitur of mine is Sir Richard Southern’s reference in his biography of Robert Grosseteste to Dinah the lady astronomer who calculated by trigonometry the height of the cathedral tower at Hereford in the reign of King Henry I .... just pause and think about that, and then return to the present era....
In 1976, the geologist Jack B. Hartung proposed that this account by Gervase described the formation of the crater now named after Giordano Bruno.
Modern theories predict that a (conjectural) asteroid or comet impact on the Moon would create a plume of ejecta rising up from the surface, which is consistent with the monks' description. The impact would be expected to perturb the Moon's motions, and laser rangefinding measurements of its libration in longitude were judged to be of the expected magnitude for such an event. In addition, the location recorded fits in well with the crater's location. Additional evidence of Giordano Bruno's youth is its spectacular ray system. The ratio of the length of these rays to the diameter of the crater is the largest for a large crater on the moon, suggesting it is the youngest such crater. Because micrometeorites constantly rain down, they kick up enough dust to quickly (in geological terms) erode a ray system.
However, these observations do not resolve the question of the crater's age. The expected odds of formation of a lunar crater of that size in the last 3000 years are on the order of 0.1%. The impact creating the 22-km-wide crater would have kicked up 10 million tons of debris, triggering a week-long, blizzard-like meteor storm on Earth – yet no accounts of such a noteworthy storm of unprecedented intensity are found in any known historical records, including the European, Chinese, Arabic, Japanese and Korean astronomical archives. This discrepancy is a major objection to the theory that Giordano Bruno was formed at that time. Also, much older craters, e.g., Tycho at 108 million years and Copernicus at an estimated 800 million years, still have prominent ray systems.
These points are discussed at a BBC online article from 2001 at Historic lunar impact questioned
High-resolution images obtained by the Japanese satellite SELENE in 2008 were used to date the crater by counting the smaller craters within it and its ejecta deposits. This gave an age of 4+6−3 million years, which is obviously much too old for the 1178 hypothesis.
This raises the question of what exactly it was that the Canterbury monks saw that June night. An alternative theory holds that they just happened to be in the right place at the right time to see an exploding meteor coming at them and aligned with the Moon. This would explain why the monks were the only people known to have witnessed the event; such an alignment would only be observable from a specific spot on the Earth's surface.
This is discussed in another interesting piece from 2001which discusses the potential phenomena and which can be seen at "What Medieval Witnesses Saw Was Not Big Lunar Impact, Grad Student Says"
In it the researcher, Paul Withers, is quoted as follows:
"And it was a pretty spectacular meteor that burst into flames in the Earth's atmosphere - fizzling, bubbling, and spluttering. If you were in the right one-to-two kilometre patch on Earth's surface, you'd get the perfect geometry....That would explain why only five people are recorded to have seen it... Imagine being in Canterbury on that June evening and seeing the moon convulse and spray hot, molten rock into space...The memories of it would live with you for the rest of your life."
Thanks to those five monks sharing their observation with their confrere Gervase it is an event we can share in almost eight and a half centuries later.