Medieval “Dark Eclipse” Helps Date Ice Cores — and Time Volcanic Eruptions

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A mosaic from the 2010 December solstice total lunar eclipse.
Dave Dickinson

You never know where an astronomical event might turn up in old historical records, and how it might link up with evidence from modern science. Researchers at the University of Geneva recently uncovered such an astronomical tale from the archives, using contemporary accounts of a curiously dark lunar eclipse to give insight into volcanic eruptions and their effect medieval climate.

The researchers, who published their study April 21st in Scientific Reports, were examining ice cores from Greenland and Antarctica over the past couple millennia, looking for sulfate spikes that would indicate volcanic eruptions.

“Ice core records from Greenland reveal a major sulfate deposition signal in the first decade of the 12th century,” says study lead Sébastien Guillet (University of Geneva). “But the exact dating and the source of this volcanic horizon has been debated for many years.”

These spikes had usually been ascribed to the 1104 AD eruption of the Hekla volcano in Iceland, but a 2015 study by climate researcher Michael Sigl (University of Bern, Switzerland) suggested that the timing was off, and that the real culprit was a tropical eruption around 1108-9 AD.

“We decided to examine whether the re-dating of this volcanic horizon was supported by independent evidence,” says Guillet.

To do that, the researchers looked to another unique event that transpired around the same time: a total eclipse of the Moon on May 5, 1110.

During this time, the Anglo Saxon Peterborough Chronicle noted that “on the fifth night in the month of May, appeared the Moon shining bright in the evening, and afterwards by little and little its light diminished, so that as soon as night came, it was so completely extinguished withal, that neither light nor orb, nor anything of it was seen.”

The circumstances for the May 5, 1110, total lunar eclipse.
NASA / GSFC / Fred Espenak

Astronomers have long known of the bizarre “dark eclipse” of 1110 AD. Astronomer Georges Frederick Chambers (1841-1915) cited the eclipse as “an instance of a ‘black’ eclipse when the Moon becomes quite invisible, instead of shining with the familiar coppery hue.” But no one had linked its darkness to a volcanic eruption.

To obtain a baseline, the researchers looked at 17 manuscripts referencing lunar eclipse appearances and hues between 1100 and 1120.

“Surveying medieval annals and chronicles for lunar observations is fascinating work, but it’s not always an easy task,” says Guillet. “For instance, most of the texts were written in Latin. Lunar eclipses were not systematically recorded by chroniclers, which means you can read many sources without finding any relevant information.”

(By all accounts, the early 12th century was a tough time: French manuscripts cited in the study also note that 1109 marked the start of a famine that lasted three years.)

Non-sea salt sulfur sample amounts taken from ice cores from the early 12th century, versus the color of lunar totality noted in manuscripts for five eclipses.
Scientific Reports / Sébastien Guillet

The researchers used the medieval sources’ descriptions of the lunar eclipses to place each one on the Danjon scale, which systematically describes the appearance of the Moon during a total eclipse, from 0 (dark) to 4 (bright). The eclipsed Moon typically appears a bright, copper-red at totality, as it reflects the “light of a thousand sunsets.” But the Moon can take on a dark cast and even appear to vanish completely during mid-eclipse if the air contains aerosols or dust particles, such as in the wake of a large volcanic eruption.

Not all lunar eclipses appear the same . . . here is a tale of two totalities from 2003.
Tom Ruen / public domain
A chromolithograph from the German magazine Sirius, comparing the appearance of the 1884 lunar eclipse shortly after the Krakatoa eruption with the relatively clear eclipse of 1888.
Sirius / public domain

Modern examples of this phenomenon include the October 4, 1884, eclipse after the 1883 eruption of Krakatoa and, more recently, the eruption of Mt. Pinatubo in the Philippines and the dark eclipse that followed in December 1992.

Two contrasting views of the Earth’s atmosphere show the effects of a volcanic eruption. At top, a relatively clear view appears in an image taken from Space Shuttle mission STS-41D, while at bottom, mission STS-43 astronauts take an image two months after the eruption of Mount Pinatubo, showing dark lines of dust and aerosols in the upper atmosphere.
NASA

“This testimony (of the 1110 total lunar eclipse) sharply contrasts with other observations of total lunar eclipses that were retrieved,” says Guillet. “Our hypothesis is that the unusual darkness of the 1110 total lunar eclipse could be explained by the presence of volcanic aerosol at that time in the stratosphere.”

The 1108 eruption of Mount Asama in Japan is a prime suspect, the team says, but they note that its volcanic ash would have to be transported across the tropics to be recorded in the ice of Greenland and Antarctica. So a more likely alternative is that a second eruption or series of eruptions occurred at this time.

Watching the Moon turn red is a fun and easy celestial spectacle to see: You just have to be on the correct moonward-facing hemisphere of Earth during a total lunar eclipse. None of the lunar eclipses in 2020 are total, so the next opportunity to see lunar totality occurs about a year from now on May 26, 2021, favoring the Pacific Rim region.

It’s always intriguing to see old accounts of astronomical phenomena linked to new science. What other curious events are lurking in the pages of old manuscripts, awaiting discovery?

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