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 link to the catastrophic eruption of Indonesia’s Krakatoa volcano last year, which ejected an immense amount of dust into the sky.
This link has been demonstrated more recently after major eruptions like that of Mount Pinatubo in the Philippines in 1991, explains Sébastien Guillet of the University of Geneva, lead author of the study published in Nature on Wednesday ( new window).
This paleoclimatologist, passionate about medieval archives, knew that the monks recorded celestial phenomena, including lunar eclipses: they paid particular attention to their coloring, thinking of the apocalypse, which speaks of a blood-red moon.
I was listening to Pink Floyd’s Dark Side of the Moon album when I realized that the darkest lunar eclipses all happened about a year after major volcanic eruptions, says Sébastien Guillet.
There is evidence of intense volcanic activity with strong and nearby eruptions in the 12th and 13th centuries – including that of Samalas in 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 really 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 as they were too far away.
This innovative approach manages to pinpoint the year, sometimes even the month, of the outbreak, says Anne Lawrence-Mathers, a historian at Britain’s University of Reading, in a comment accompanying the study.
When the monks saw a dark moon, it was because it was blocked by aerosols being thrown into the stratosphere, more than 10 km above sea level, the paleoclimatologist says. Only the strongest eruptions hurl their ash clouds so high – converted into aerosols upon entering the atmosphere.
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 helped start the Little Ice Age, which affected parts 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, according to the 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, says 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 that it’s better to start with their exact date to understand if they did have done or not had an impact on climate and society.