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.

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.

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

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.

“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.

“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.”