The Mysterious Case of Crater Giordano Bruno

A band of 12th century sky watchers saw something big hit the Moon 800 years ago. Or did they? A new study suggests the event was a meteoritic trick of the eye.

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April 26, 2001 -- Imagine the shock and amazement of five people who, in 1178 A.D., spied what appeared to be "fire, hot coals, and sparks" bursting forth from the Moon! Apparently something (and it was big) must have hit Earth's satellite.

What was it they saw? Until recently many astronomers thought that well-chronicled event coincided with the formation of lunar crater Giordano Bruno -- the youngest substantial impact feature on the Moon. But that popular idea doesn't hold up under scientific scrutiny, says Paul Withers of the University of Arizona Lunar and Planetary Laboratory.

Above: Some scientists have suggested that the heavenly spectacle witnessed by five people in 1178 A.D. was the impact that created the lunar crater Giordano Bruno, visible in this picture as the bright white spot in the upper left. Recent analysis of ancient astronomical archives casts doubt on this theory, however. [click on picture for a larger image showing the crater's location on the lunar surface.]

Such an impact would have triggered a blizzard-like, week-long meteor storm on Earth -- yet there are no accounts of such a storm in any known historical record, including the European, Chinese, Arabic, Japanese and Korean astronomical archives. Withers reported his analysis and other tests of the hypothesis in this month's issue of Meteoritics and Planetary Science. 

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The dramatic passage in question appears in the medieval chronicles of Gervase of Canterbury.

About an hour after sunset on June 18, 1178 A.D., a band of five eyewitnesses watched as the upper horn of the bright, new crescent Moon "suddenly split in two. From the midpoint of this division a flaming torch sprang up, spewing out . . . fire, hot coals and sparks. . .The body of the moon, which was below writhed. . .throbbed like a wounded snake." The phenomenon recurred another dozen times or more, the witnesses reported.

A geologist suggested in 1976 that this account is consistent with the location and age of the 22-kilometer (14-mile) lunar crater Giordano Bruno, the youngest crater of its size or larger on the Moon.

Based on the size of the crater, it must have been a one-to-three kilometer wide (a half-mile to almost 2-mile wide) asteroid that blasted Giordano Bruno into the Moon's northeast limb. Such an impact on the Earth would be "civilization threatening" -- so it is important to know if such an event happened on the Moon less than a millennium ago, Withers noted.

The impact would have launched 10 million tons of ejecta into the Earth's atmosphere in the following week, previous studies have shown. In the Meteoritics article, Withers reports his calculations on the properties of the subsequent meteor storm.

Left: The impact of a meteorite large enough to form Giordano Bruno would have unleashed a major meteor storm, Withers calculated, comparable to the 1966 Leonids meteor shower pictured here. (During the '66 storm, as many as 100,000 meteors per hour were recorded in some locations.) [click on the picture for more information on the '66 Leonids meteor storm]

"I calculate that this would cause a week-long meteor storm comparable to the peak of the 1966 Leonids," he said. Ten million tons of rock showering the entire Earth as pieces of ejecta about a centimeter across (inch-sized fragments) for a week is equivalent to 50,000 meteors each hour.

"And they would be very bright, very easy to see, at magnitude 1 or magnitude 2. It would have been a spectacular sight to see! Everyone around the world would have had the opportunity to see the best fireworks show in history," Withers said.

Yet no vigilant 12th century sky watcher reported such a storm.

So what did the witnesses see that the Canterbury monk recorded?

"I think they 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. This idea was strongly suggested by others in a 1977 scientific paper.

Right: Video footage of a meteor streaking through the atmosphere during the 2000 Leonid meteor shower, captured by George Varros of Mount Airy, MD. Withers believes those five ancient sky-watchers might have seen the fiery display of such a meteor traveling along their line of sight rather than an impact on the moon.

"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," he said. "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, " Withers added. "The memories of it would live with you for the rest of your life."

A Leonid on the Moon? -- Science@NASA article: The first recorded impact of a meteorite on the Moon may have been captured on video during the 1999 Leonids meteor storm.

Meteoritics & Planetary Science -- scientific journal where Withers's findings are published. Non-subscribers can only view the abstract.

Giordano Bruno -- Encyclopedia Britannica article about the Italian philosopher, astronomer, and mathematician for whom the lunar crater was named. Bruno lived in the 16th Century, but his beliefs were ahead of his time -- including the notion of an infinite universe with millions of life-bearing planets (at a time when the geocentric worldview prevailed). He was eventually condemned as a heretic and burned at the stake for his beliefs.

Potentially hazardous asteroids -- scientists would like to determine whether the spectacle observed in 1178 was a large asteroid striking the moon or a tiny one entering the Earth's atmosphere because such knowledge can help them better estimate the risk of a catastrophic collision on Earth. This Web site contains information about potentially hazardous asteroids.