In my summer teaching I often start the day with some examples of interesting things that happened that day in history. It is a fun way to start the day, and I seek to find examples that connect with things/issues we are covering in class, often related to technology, psychology and so on. A few years ago, when researching historical events that occurred on the 25th of June I came across an interesting problem. There was an event that some sources reported as having occurred on the 25th of June 1178 and other sources that reported it as occurring on the 18th of June of the same year. Clearly both of these dates could not be right… so what was going on?

Particularly beguiling to me was that this event was something that I had learned about many years ago, when in high school in New Delhi, from Carl Sagan’s TV series Cosmos. I was a huge fan of this series and had seen it more than once and had read the book cover to cover many times. So this mystery of the two conflicting dates was doubly interesting. As it turns out my attempts to figure out this inconsistency led to some wonderful insights about history, calendars and the very nature of science itself.

First to provide some context about the event itself. In one of the episodes of Cosmos, Sagan describes an even that took place in 1178. You can watch it for yourself in the clip below (start around 2 minutes in)


Here is a transcript (archived here)

On the evening of June 25, 1178, five British monks reported something extraordinary, which was later recorded in the chronicle of Gervase of Canterbury, generally considered a reliable reporter on the political and cultural events of his time, after he had interviewed the eyewitnesses who asserted, under oath, the truth of their story.The chronicle reads: There was a bright New Moon, and as usual in that phase its horns were tilted towards the east. Suddenly, the upper horn split in two. From the midpoint of the division, a flaming torch sprang up, spewing out fire, hot coals, and sparks.

The astronomers Derral Mulholland and Odile Calame have calculated that a lunar impact would produce a dust cloud rising off the surface of the Moon with an appearance corresponding rather closely to the report of the Canterbury monks. If such an impact were made only 800 years ago, the crater should still be visible. Erosion on the Moon is so inefficient, because of the absence of air and water, that even small craters a few billion years old are still comparatively well preserved. From the description recorded by Gervase, it is possible to pinpoint the sector of the Moon to which the observations refer. Impacts produce rays, linear trails of fine powder spewed out during the explosion. Such rays are associated with the very youngest craters on the Moon-for example, those named after Aristarchus and Copernicus and Kepler. But while the craters may withstand erosion on the Moon, the rays, being exceptionally thin, do not. As time goes on, even the arrival of micrometeorites-fine dust from space-stirs up and covers over the rays, and they gradually disappear. Thus rays are a signature of a recent impact.

As it turns out the records of the monks are now available online and this is what the Chronicle of Gervaise a 12 century manuscript says (from here):

“In this year, on the Sunday before the feast of St. John the Baptist, after sunset when the moon has first become visible, a marvellous phenomenon was witnessed by some five or more men who were sitting there facing the moon. Now there was a bright new moon, and as usual in that phase, its horns were tilted towards 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… Then after these transformations the moon from horn to horn, that is along its whole length, took on a blackish appearance.”

As Sagan says, the generally accepted interpretation of this narrative, first suggested by Dr Jack B Hartung some 800 years later, is that it is a description of a crater impact. The “upper horn split in two” is the apparent effect of a plume of dark dust or vapour, the “flaming torch [of] hot coals and sparks” describes the molten ejecta, and the way in which the rest of the Moon “writhed”, “throbbed” and eventually “took on a blackish appearance” could be the effects of a temporary lunar atmosphere of gas and vapour created by the impact. As Sagan says in his documentary:

The meteoriticist Jack Hartung has pointed out that a very recent, very fresh-looking small crater with a prominent ray system lies exactly in the region of the Moon referred to by the Canterbury monks. It is called Giordano Bruno after the sixteenth century Roman Catholic scholar who held that there are an infinity of worlds and that many are inhabited. For this and other crimes he was burned at the stake in the year 1600. Another line of evidence consistent with this interpretation has been provided by Calame and Mulholland. When an object impacts the Moon at high speed, it sets the Moon slightly wobbling. Eventually the vibrations die down but not in so short a period as eight hundred years. Such a quivering can be studied by laser reflection techniques. The Apollo astronauts scattered in several locales on the Moon special mirrors called laser retro reflectors. When a laser beam from Earth strikes the mirror and bounces back, the round-trip travel time can be measured with remarkable precision. This time multiplied by the speed of light gives us the distance to the Moon at that moment to equally remarkable precision. Such measurements, performed over a period of years, reveal the Moon to be quivering with a period (about three years) and amplitude (about three meters), consistent with the idea that the crater Giordano Bruno was gouged out less than a thousand years ago.

How very cool is that. I remember this episode well because it is a powerful one, connecting an event that took place in the 12th century with today’s science. It appealed to the rationalist, nascent scientist in me in a powerful way.

So imagine my surprise to find these two different dates being referenced for the event. Notice the date that Sagan references in Cosmos is the 25th of June. Here are two others that cite June 25.

… and here are two that reference the June 18 date.

So what is going on here? Why two different dates?

It turns out that this problem was not all that hard to figure out. A bit of research showed that this dual-date conundrum was really a function of the kind of calendar system being used to set the date, i.e. the specific date depended on whether one used the Gregorian or the Julian calendar. You can test this out yourself by visiting Tarek’s Hijri/Gregorian/Julian Converter ( which contains a Gregorian to Julian (and vice versa) converter. 18th June 1178 in the Julian Calendar turns out to be 25th June, 1178 on the Gregorian calendar. So in some sense, both dates are right (or wrong, for that matter). As date-problems go this one of the less complicated ones. For instance, consider the following:

… an article appeared in the Edinburgh Courant of February 19, 1706. The article was an abridgment of one published in the London Gazette of February 13, 1705, which, in turn, was a translation from the Amsterdam Gazette of February 22, 1706. All three were published in the same week. The discrepancy in year was caused by the fact that Scotland and the Low Countries began the year on January 1, while England, until 1752, began it on March 25. The discrepancy in days was caused by the use of the Gregorian calendar in the Low Countries, at a time when England and Scotland still adhered to the Julian calendar. (

Anyway, coming back to the story of the Canterbury Monks, the successful resolution one paradox (of the dueling dates) led to another one, more interesting by far. As a fallout of my web searches I came across some recent evidence that questions the existence of the very event itself – and all the scientific conjecture regarding collisions and wobbling moons that it engendered. Here’s what I found out:

In 2001, Paul Withers, then a graduate student in astronomy at the University of Arizona (here is his current website) in an article published in the journal “Meteoritics & Planetary Science” debunked the moon collision theory. He argued that the consequences of such a collision would have been quite significant and the fact that no astronomers (anywhere in the world) reported anything like this happening seems to indicate that this was a more local phenomena – one that could be observed just at Canterbury. Withers suggests that the monks just happened to be at

… the right place at the right time to look up in the sky and see a meteor that was directly in front of the moon, coming straight towards them…. 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 kilometer patch on Earth’s surface, you’d get the perfect geometry,… That would explain why only five people are recorded to have seen it.

I won’t claim that I wasn’t a bit disappointed at reading this. This story has been such a part of me, for so long, to find it debunked was slightly bothersome. That said, this entire episode, represents for me the self-correcting nature of science, where truths are determined by data (not by opinion). Also important to highlight here in the role that Internet based technologies play revealing this aspect of science to us. This makes science more accessible and more true to itself. This is a great lesson to learn.

The complete reference to the Wither’s article is as follows:

Withers, P. (2001). Meteor storm evidence against the recent formation of lunar crater Giordano Bruno. Meteoritics & Planetary Science Volume 36, Issue 4. p. 525-530. (Table of contents available at

This was also reported in the scientific press. For instance check out these two news stories.