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NASA Space Science News
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.
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.
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.)
"I calculate
that this would cause a week-long meteor storm comparable to the
peak of the 1966 Leonids," he sad. 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 witnessess 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."
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