"Missing" Moon Linked to Major 1761 Eruption?

Ker Than
for National Geographic News
January 16, 2009
A "disappearing" moon that preceded an unusually bitter winter in China was most likely the result of a mysterious volcanic eruption in the 1700s, a retired NASA scientist says.

Astronomer Kevin D. Pang collected evidence from the fields of geology, biology, and Chinese history that suggests a major eruption belched out enough dust and gas to completely blot out the moon during a 1761 total lunar eclipse.

A total eclipse occurs when the moon enters completely into Earth's shadow. (Watch video of the February 2008 lunar eclipse—the last total eclipse of the moon until December 2010.)

Lunar eclipses can vary in brightness and color based on the angle of the moon's path and the composition of Earth's atmosphere.

While no sunlight hits the moon directly, some gets filtered by Earth's atmosphere and is bent toward the moon, causing it to shine in hues ranging from bright orange to blood red.

"But when there's a large volcanic eruption," Pang said, "the moon can drop in brightness by a million times, or in some cases disappear altogether."

Pang presented his results last week during the 213th meeting of the American Astronomical Society in Long Beach, California.

Volcanic Winter

Heavy amounts of particles in the air could explain why, in May of 1761, astronomers reported that the moon appeared very dark or disappeared altogether, even with the aid of telescopes.

An atmosphere clogged by a powerful volcanic eruption would also lead to global cooling and trigger extended bouts of strange weather, experts say.

(Related: "Ancient Global Dimming Linked to Volcanic Eruption" [March 19, 2008].)

To test his theory, Pang searched the scientific literature about tree rings and ice cores from Antarctica and Greenland. He found evidence of a "volcanic winter" around the same time as the dark eclipse.

For example, sulfur dioxide gas ejected during a volcanic eruption can react with water vapor in the air to form acid rain, which then leaves chemical fingerprints in polar ice.

Furthermore, bristlecone pine trees high in the Sierra Nevada mountains experienced stunted growth and frost damage in 1761, Pang said.

The researcher also looked through old Chinese weather chronicles from the early 1760s. Those records revealed that large parts of China experienced an unusually bitter winter and heavy snowfall in 1761 and 1762.

Rivers and wells across central China froze, ships could not sail, and innumerable trees, birds, and livestock died due to the cold, the chronicles state.

Finding a Culprit

A good candidate for the cause of the 1761 events is the Makian volcano on the Indonesian island of Halmahera, Pang thinks. Records show that this volcano experienced a series of eruptions beginning in September of 1760 and lasting until spring of the following year.

Makian's equatorial position could explain why evidence of its eruption was found at both poles.

But it's also possible the culprit volcano went unrecorded, Pang added.

Richard Keen is a climatologist at the University of Colorado in Boulder who was not involved in the study.

"[Pang] is absolutely correct in saying that volcanoes can darken a lunar eclipse," Keen said. But for the 1761 event, he noted, historical accounts about the dimness of the moon varied by geographical location.

Most of the reports of the moon disappearing were from astronomers in Sweden, Finland, and northwestern Russia. At more southern latitudes, the moon only appeared dimmer than usual.

The differing accounts could be due to the patchy distribution of dust and sulfur particles that occurs shortly after a volcanic eruption, said Fred Espenak, an eclipse expert at NASA's Goddard Space Flight Center in Maryland.

"But if this dust is up there long enough, it tends to uniformly distribute itself over weeks or months," said Espenak, who also did not participate in the research.

Espenak witnessed a modern dimming of the moon during a total lunar eclipse in 1992. That event was also probably due to a volcano, he said.

"It was here in Maryland, but the volcano was Mount Pinatubo in the Philippines the year before."

Michael Baillie, a paleoecologist at Queen's University Belfast in the U.K., said Pang's claim is "very interesting and indeed plausible," but he questioned the scientific usefulness of using lunar eclipses to pinpoint historic volcanic eruptions.

Pang is "almost intimating that we should be able to look at very black eclipses and assess that volcanoes have gone off," Baillie said.

"But it's always going to be a patchy thing. If someone didn't see an eclipse, is that because it was obscured [by volcanic smog] or because it was cloudy that night?"

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