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Eclipse records date volcanic eruptions from the 12th and 13th centuries.

Una página de un texto antiguo con un dibujo de círculos y otras formas representando un eclipse.

This is a legally approved Spanish translation of an Eos article.

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On the night of December 2, 1229, the courtier and poet Fujiwara no Teika, observed the Moon rising from the mountains of Kyoto. The Moon was partially eclipsed, and as it continued to be eclipsed completely, the bright disc gradually darkened until it completely disappeared.

“Los ancianos nunca la habían visto así”, escribió Teika en su diario unos días después. “Verdaderamente fue algo a que temerle. Efectivamente, en mis setenta años nunca había escuchado o visto [de algo así]”.

The findings of Teika and other ancient observations of rare dark moon eclipses have gained new significance after scientists used these records to date notable volcanic eruptions that occurred almost a thousand years ago. The discoveries, published in Nature, could help solve major questions about vulcanism and climate change.

“We had no idea that we could come across this beautiful text – it was pure luck.”

During the COVID-19 lockdown in March 2020, Sébastien Guillet, a paleoclimatologist from the University of Geneva, came across the transcription of Teika’s diary, called Meigetsuki, for the first time and was truly moved. “We had no idea we could find such a beautiful text – it was pure luck.” For years, he has combed through hundreds of medieval manuscripts searching for records of lunar eclipses that described the color of the moon in its entirety.

If there were any details, the majority of the descriptions, made by clerics, monks, crusader soldiers, or common urban people, were of reddish “blood” moons, typically seen during a total lunar eclipse on a clear night. However, a few describe a dark moon, such as the one Teika saw.

Between the 12th and 13th centuries, there were several highly active volcanic periods within the past 2,500 years. Scientists have determined, through analysis of sulfate index layers in Greenland and ice cores from Antarctica, that there were seven significant volcanic eruptions between the years 1108 and 1286. However, only one of these eruptions, the 1257 eruption of Samalas on the island of Lombok in Indonesia, has been linked to a specific volcano or location.

“These eruptions are very intriguing as they occurred during a critical moment,” Guillet explains. This was the transition from the anomalous Medieval Warm Period to the much colder Little Ice Age. “Did they contribute to this transition, and if so, to what extent? I do not believe there is a clear consensus among scientists about the answers to these questions.”

Scientists need to date these events in order to determine causality. However, while ice cores are valuable records, the dramatic events documented in them cannot be reliably dated. Ice shifts over centuries, and entire sections of the cores can be lost during drilling and extraction.

“If not dated correctly, with a margin of error of 1 or 2 years, it becomes very easy to attribute something to a volcano that may have simply been a natural variability,” says Guillet. It could also be wrongly attributed to the wrong volcano, added Clive Oppenheimer, co-author and vulcanologist at the University of Cambridge.

The beauty of eclipses is that we know exactly when they occurred. Furthermore, crucially, the color of the Moon during total eclipses has recently proven to be a good indicator of whether there has been a massive volcanic eruption.

Many volcanic eruptions release gaseous sulfur and particles into the troposphere, but only the largest and most explosive events inject them into the stratosphere, where they are dispersed around the Earth forming a veil of aerosol that blocks sunlight. These events temporarily cool the climate – the eruption of Tambora in 1815 is famous for causing the “year without a summer” in 1816 – also affecting eclipses.

During a regular total lunar eclipse, when the Earth moves directly between the Sun and the Moon, the atmosphere only allows red light waves to pass through and illuminate the lunar surface. However, if the stratosphere is filled with sulfated aerosols, even those red rays are blocked and do not reach the lunar surface, leaving it dark.

Based on recent major eruptions, such as Krakatoa in 1883 and Pinatubo in 1991, scientists have determined that this effect on eclipses lasts for approximately one year.

Registros revelados

In ancient manuscripts from Europe, the Middle East, and East Asia, dating from 1100 to 1300, Guillet and his colleagues have found hundreds of mentions of total lunar eclipses. Except for Teika’s diary, none of the nearly 200 sources from Japan, China, or Korea mention the color of the Moon.

“Combining all of these lines of evidence presents a new and elegant perspective.”

However, medieval Christians, mostly in Europe, saw the Blood Moon as a sign of the impending apocalypse, so they paid close attention to these details and provided information about the color and brightness of the Moon in 36 eclipses – more than half of the number of eclipses that could have been visible during that period of time.

The records show eclipses with dark moons in the years 1110, 1172, 1229, 1258, and 1276, contemporaneous with five of the seven major volcanic sulfate records found in ice cores. (Several records after the other two eruptions describe a typical Blood Moon, leading Guillet and colleagues to speculate whether the plumes from these volcanoes only reached the troposphere and may have been located near Greenland, resulting in an exaggerated record.)

After examining manuscripts, the group compared eclipse data with global aerosol simulations, climate models, and temperature reconstructions from tree ring growth information to date these five eruptions within a few months.

Un texto antiguo dentro de un libro.

A passage from the Anglo-Saxon Chronicle mentions the occurrence of a dark lunar eclipse on May 5, 1110. Translated from Old English, it reads “On the fifth night of the month of May, it seemed that the Moon shone brightly in the evening, but then gradually its light diminished, so that by the time night arrived, it had completely vanished, and no light, orb, or anything of it was visible.” Credit: Bodleian Libraries, University of Oxford, CC BY-NC 4.0.

“El juntar todas estas líneas de evidencia es una nueva y elegante perspectiva” dijo Charlotte Pearson, una geoarqueóloga y dendrocronóloga de la Universidad de Arizona, quien no ha estado involucrada en este estudio.

Volcanic eruptions and eclipses are key points that aid researchers in synchronizing chronologies collected from various sources, such as ice cores and tree growth rings, according to the statement. “The more you can put everything on the same timeline, the better you can explore what these events meant for people.”

Medieval chronicles not only recorded dark moons, but also frosts, bad harvests, and famines, he said. Records of eclipses can help link these events to major volcanic eruptions. If there is enough volcanic ash in the stratosphere to darken the eclipsed moon, it is likely enough to cool the climate – something scientists can confirm by examining tree ring growth patterns in years with little growth.

“Esta información de los eclipses puede proveer una forma completamente independiente de probar el esparcimiento del polvo de estas grandes erupciones medievales a través de la atmósfera”, dijo Oppenheimer. Este conocimiento también podrá ayudar a los científicos a predecir tales efectos en futuros eventos volcánicos.

—Kate Evans (@kate_g_evans), Escritora de ciencia

Norma A. Portillo’s translation was made possible through a partnership between Planeteando and GeoLatinas.

This article is part of our ENGAGE resource, which provides science news for teachers to use in their lessons. Take a look at all of our ENGAGE articles and let us know in the comments how you incorporated this particular article into an activity with your colleagues.

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