Medieval Eclipse Records Help Scientists Understand Volcanic Eruptions

Descriptions of lunar eclipses from monks and poets can shed light on how volcanoes affect Earth’s climate in a new study

the moon appears orange and red in a lunar eclipse
Records of lunar eclipses that appeared unusually dark are telling scientists when volcanic eruptions might have occurred in the past. YOSHIKAZU TSUNO / Gamma-Rapho via Getty Images

When medieval monks and scholars kept records of lunar eclipses, they likely had no idea that scientists today would use their writings to understand volcanoes. But now, these descriptions are revealing details on the historic eruptions that darkened the sky and cooled the Earth.

Lunar eclipses occur when the moon moves into Earth’s shadow and becomes lit in a red-orange glow. But when a volcano has recently blown its top, sending ash and gas into the stratosphere, the eclipse tends to appear unusually dark. Observing a darkened lunar eclipse on December 2, 1229, Japanese poet Fujiwara no Teika wrote, “the old folk had never seen [the moon] like this… as if it had disappeared during the eclipse.”

In a new study, published last week in the journal Nature, scientists turned to medieval accounts of lunar eclipses, including Teika’s, to understand how volcanoes affected Earth’s climate during that era.

“What I find particularly fascinating about these results is the intersection of ancient knowledge and modern science,” lead author Sébastien Guillet, a paleoclimatologist at the University of Geneva, tells’s Charles Q. Choi. “It’s remarkable to think that observations made by monks hundreds of years ago are still valuable today.”

Guillet came up with the idea for this unprecedented study while listening to Pink Floyd’s album The Dark Side of the Moon, per a University of Geneva statement.

All but one of the eruptions examined by the researchers had not been recorded at the time—for the most part, no written accounts of them existed. Still, previous research had estimated their rough time frame based on ice cores from Antarctica and Greenland, which revealed layers of volcanic dust. With ice data as a blueprint, the team of researchers spent five years scouring medieval lunar eclipse records to pinpoint the details of the eruptions.

“By putting together the information from ice cores and the descriptions from medieval texts, we can now make better estimates of when and where some of the biggest eruptions of this period occurred,” co-author Clive Oppenheimer, a geologist at the University of Cambridge, says in the statement.

During the study, scientists examined almost 400 accounts of 119 lunar eclipses from the High Medieval Period, between 1100 and 1300 C.E. Of these, they connected volcanic eruptions with five eclipses that were described as dark, as well as two that were described as reddish, per By combining this data with evidence from tree rings, satellite observations and models, scientists determined that volcanic events probably occurred within 3 to 20 months before each of these eclipses.

The ash blasted from volcanoes can block sunlight, dramatically cooling the climate. In 1815, for example, the Indonesian volcano Mount Tambora erupted and initiated what became known as the Year Without Summer—a frigid year with crop failings that killed at least 100,000 people over a decade. Volcanic eruptions during the High Medieval Period are thought to have triggered an era of global cooling called the Little Ice Age, which lasted from 1350 to 1850.

Now, amid human-caused climate change, studying the cooling effect of volcanoes provides a strong model for scientists to understand how solar geoengineering could impact the Earth. This controversial strategy for cooling the planet would inject reflective particles into the stratosphere to send some of the sun’s light back into space, imitating the impact of volcanic ash. Scientists hope this new study can provide more information on how such an idea might work.

“Volcanic eruptions are our only—albeit imperfect—analogue for how the Earth’s climate might respond to future solar geoengineering,” Kevin Anchukaitis, a palaeoclimatologist at the University of Arizona who was not involved in the study, tells Nature News’ Jeff Tollefson. “Better models should lead to a better understanding of the benefits and risks of geoengineering.”

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