Lunar eclipses occur when the moon passes through the earth’s shadow, which blocks the sun’s rays. The star loses its white luster and turns reddish.
In 1884, the French astronomer Camille Flammarion noticed that this red coloration had darkened. He then suggested a connection to the catastrophic eruption of the Krakatoa volcano (Indonesia) the previous year, which had ejected an immense amount of dust into the sky.
This link has been demonstrated more recently after major eruptions like that of Mount Pinatubo (Philippines) in 1991, Sébastien Guillet of the University of Geneva, lead author of the study published in Nature on April 5, tells AFP.
This paleoclimatologist, passionate about medieval archives, knew that the monks recorded celestial phenomena, including lunar eclipses: they “payed particular attention to their coloring, thinking of the apocalypse, which speaks of a blood-red moon”.
“I was listening to Pink Floyd’s album + Dark Side of the Moon + when I realized that the darkest lunar eclipses all happened about a year after major volcanic eruptions,” says Sébastien Guillet.
“What to be afraid of”
It has been proven that intense volcanic activity with strong and nearby eruptions took place in the 12th and 13th centuries – including that of Samalas (Indonesia) in 1257.
These eruptions have left traces in ice cores containing fallen volcanic particles. But aside from Samalas, the chronology remained approximate.
The thorough reading of the texts of the monks of the 12th and 13th centuries, mainly in Europe but also in the Middle East and Asia, allowed the refinement of the calendar.
Of the 51 total lunar eclipses reported between 1100 and 1300, chroniclers have observed at least five in which the star appeared unusually dim. “There was indeed something to be afraid of,” wrote a Japanese scribe on December 2, 1229.
The scientists compared the exact days of these events with information from ice cores and compared these results to contemporary data. They deduced the date of the explosions that had occurred a few months earlier – which the monks were unaware of because they were too far away.
“This innovative approach manages to point to the year, sometimes even the month of the outbreak,” said Anne Lawrence-Mathers, a historian at the UK University of Reading, in a commentary accompanying the study.
“If the monks saw a dark moon, it was because it was blocked by aerosols thrown into the stratosphere,” at an altitude of more than 10 km, according to the paleoclimatologist. Only the strongest eruptions hurl their ash clouds so high – converted into aerosols upon entering the atmosphere.
Little Ice Age
Within 200 years there have been six gigantic eruptions, which is extraordinary, the scientist emphasizes.
Recent research has speculated that this intense volcanic activity may have contributed to the emergence of the “Little Ice Age,” which affected part of the northern hemisphere from the 13th to the 19th centuries.
Volcanic aerosols were thus able to limit the sun’s radiation and cool temperatures on earth. “Strong tropical eruptions can cause a global cooling of about 1 degree in a few years,” says geomorphologist Markus Stoffel, one of the authors of the study.
Analysis of tree rings, indicators of temperature changes, confirmed this cold snap, which particularly affected crops.
“The monks hadn’t experienced such a cold summer for a long time, with the impression of seeing a constant fog,” describes Sébastien Guillet.
“However, there is still no consensus among scientists” about the causes of this ice age, “and we still have a lot to learn from these eruptions,” he argues, adding, “it is better to use their exact date to understand whether or not they had an impact on climate and society”.