Lunar eclipses occur when the Moon passes through the Earth's shadow, which shields the Sun's rays. Losing its white luster, the star becomes reddish.

In 1884, the French astronomer Camille Flammarion noticed that this red coloration had darkened. He then suggested a link with the cataclysmic eruption of the Krakatoa volcano in Indonesia the previous year, which had ejected an immense amount of dust into the sky.

This link has been demonstrated more recently, after large eruptions such as that of Mount Pinatubo (Philippines) in 1991, explains to AFP Sébastien Guillet of the University of Geneva, lead author of the study published on April 5 in Nature.

This paleoclimatologist passionate about medieval archives knew that monks recorded celestial phenomena, including lunar eclipses: they were "particularly attentive to its coloration, having in mind the Apocalypse which speaks of a blood-red moon".

Eruption of Mount Pinatubo in June 1991 in the Philippines © ARLAN NAEG / AFP/Archives

"I was listening to Pink Floyd's album +Dark Side of the Moon+ when I realized that the darkest lunar eclipses had all occurred about a year after major volcanic eruptions," says Sébastien Guillet.

"What to be afraid of"

It is established that the twelfth and thirteenth centuries experienced intense volcanic activity, with powerful and close eruptions - including that of Samalas (Indonesia) in 1257.

These eruptions have left traces in ice cores, which contain volcanic particles that have fallen back to Earth. But outside Samalas, the chronology remained approximate.

The exhaustive reading of the texts written by the monks of the twelfth and thirteenth centuries, mainly in Europe but also in the Middle East and Asia, made it possible to refine the calendar.

Of the 51 total lunar eclipses reported between 1100 and 1300, chroniclers observed at least five where the star appeared abnormally dark. "There was really something to be afraid of," described a Japanese scribe on December 2, 1229.

The scientists cross-checked the exact days of these events with ice core information, and compared these results with contemporary data. They deduced the date of the explosions that occurred a few months earlier - of which the monks were not aware because they were too far away.

"This innovative approach succeeds in pointing to the year, sometimes even the month, of the eruption," says Anne Lawrence-Mathers, a historian at the British University of Reading, in a commentary attached to the study.

"If the monks saw a dark moon, it was because it was obstructed by aerosols propelled into the stratosphere," at more than 10 km altitude, according to the paleoclimatologist. Only the most powerful eruptions eject their ash plumes so high – converted into aerosols as they enter the atmosphere.

Little Ice Age

Six gigantic eruptions have occurred in the space of 200 years, which is exceptional, says the scientist.

Recent research has hypothesized that this intense volcanic activity may have contributed to the establishment of the "Little Ice Age", which affected part of the northern hemisphere from the thirteenth to the nineteenth century.

Volcanic aerosols have thus been able to limit the Sun's radiation and cool temperatures on Earth. "Strong tropical eruptions can cause global cooling of about 1 degree in a few years," says geomorphologist Markus Stoffel, one of the authors of the study.

The analysis of tree rings, indicators of temperature changes, confirmed this cold snap that affected crops in particular.

"The monks had not seen 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" on the causes of this ice age, "and we still have a lot to learn from these eruptions," he said, adding, "it is better to start from their exact date to understand whether or not they have had an impact on the climate and societies."

© 2023 AFP