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18 June 2018

The Mystery of the Moon that was Split in Two

Astrophysics | History | Research | Space
Estimated reading time Time 4 to read

One hour after sunset on 18 June 1178, at least five men in southern England reported having witnessed an unusual phenomenon in the sky. According to the monk Gervasio, chronicler of the Abbey of Christ Church in Canterbury, the upper horn of the crescent Moon was split in two. “From the midpoint of the division a flaming torch sprang up, spewing out, over a considerable distance, fire, hot coals and sparks,” wrote Gervasio, adding that the Moon “writhed, as it were in anxiety” and that “it throbbed like a wounded snake.” After all this, the celestial body turned blackish.

Giordano Bruno is a 22-kilometre crater. Credit: LROC

What did these men observe? The narrative of Gervasio of Canterbury remained almost forgotten for centuries until Jack B. Hartung, geophysicist at the State University of New York at Stony Brook, rediscovered it in 1976. The chronicler’s assertion that the witnesses had sworn the truth of the story on their honour aroused Hartung’s interest. This led him to publish a study that seemed to solve the historical enigma with an explanation as fabulous as the spectacle that Gervasio related.

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Hartung favoured the possibility that the origin was an enormous impact on the lunar surface. In the region where Gervasio had located the phenomenon, a 22-kilometre crater is located, named after the Italian philosopher and astronomer Giordano Bruno. Studying the high-resolution images taken in the 1970s by the Apollo missions, Hartung could see that the long, bright radial marks produced during the formation of the crater had not yet been erased by the lunar dust spread by micrometeorites, indicating that its origin was recent and, therefore, could correspond to the phenomenon seen in 1178.

An enormous impact or a meteorite

 Hartung recognized that the probability was “extremely small,” since there was only one chance in a thousand that such a great impact would have occurred throughout the recorded history of mankind. However, his theory received an accolade two years later from astronomers Odile Calame and John Derral Mulholland, who analysed new images collected by the Soviet probe Luna 24 to conclude that, although their study could not prove Hartung’s interpretation, at least the data were “consistent with the hypothesis.”

However, this does not imply that Hartung’s theory was widely accepted. As early as 1977, meteorite specialists Harvey Harlow Nininger and Glenn I. Huss questioned the hypothesis, claiming that the formation of Giordano Bruno’s crater would not have caused the phenomena reported by the monk. Instead, the two experts proposed another more plausible explanation that Hartung himself had mentioned in his study: the entrance of a meteorite into Earth’s atmosphere that coincidentally remained in the Moon’s visual line for observers from southern England.

A crater over one million years old

In fact, many experts have been sceptical, since the youth of the crater must be understood in geological time. According to the astronomer Tomokatsu Morota, who in 2009 calculated an age for Giordano Bruno of between one and ten million years, the density of small craters formed on the ejected material leads to a conclusion: “there is no possibility that it was formed 800 years ago.” The cosmogeologist Jörg Fritz, who in 2012 estimated the age of the crater at more than one million years, adds yet another factor: “We can exclude that the crater formed 800 years ago as we would still be exposed to a rain of lunar rocks from that rare and large event,” he tells OpenMind. Likewise and due to the impact, “parts of the crater would still show elevated temperatures that are, however, not observed by lunar orbiter missions.”

A fireball of the 1966 Leonids. Credit: J. W. Young (TMO,JPL,NASA)

In 2001, then-graduate student Paul Withers of the University of Arizona put numbers on the lunar meteor shower that would have fallen on Earth if Giordano Bruno’s crater had been created in 1178. In his study, the astronomer detailed that the crater was created by the impact of an asteroid between one and three kilometres, which according to previous estimates would have released ten million tons of lunar material in the direction of Earth.

Withers estimated that this should have triggered an intense meteor shower during the week after the collision, similar to that of the Leonids in 1966, which produced up to 100,000 shooting stars per hour. “It would have been a spectacular sight!” Withers said after the publication of his study. “Everyone around the world would have had the opportunity to see the best fireworks show in history.”

¿A real enigma or a fantasy?

No chronicle of the time recorded such phenomenon, which led Withers to conclude that the most plausible explanation is the one advocated by Nininger and Huss. “I think [the witnesses] 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,” Withers said. “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.”

The Giordano Bruno crater photographed from the Apollo 13 spacecraft. Credit: NASA

For Withers, this would explain why only a few people witnessed the alleged moon anomaly, which was, in his view, nothing of the sort. Unlike the asteroid whose impact more than one million years ago scarred the lunar surface with Giordano Bruno’s crater, the meteorite that could have caused the optical illusion of 1178 would have been of a small enough size to have burned up in the Earth’s atmosphere without major consequences.

So, enigma solved? Perhaps there never even was an enigma: in his study, Withers outlined another even greater problem, which is that on 18 June 1178 the crescent Moon was not yet visible from Canterbury. The astronomer suggested that perhaps the date was incorrect, but in 2002 the historian of astronomy Peter Nockolds went further by suggesting that Gervasio’s story could be a complete fantasy: the monk, Nockolds argued, had the habit of associating strange celestial apparitions to Christian victories in the Crusades. The vision of the half moon breaking in two could merely be a propagandistic symbology about the triumph of the Crusaders against Islam. We will probably never know the truth.

Javier Yanes

@yanes68

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