Medieval monks and scribes made detailed descriptions of lunar eclipses that are now helping present-day researchers study a cluster of mysterious volcanic eruptions on Earth. The chroniclers’ writings noted a reddish orb surrounding the eclipsed moon and unusual instances where the moon disappeared entirely from the sky. At the time, these events were thought to foretell calamities.

Today, scientists have found that an exceptionally dark eclipse is associated with a large amount of volcanic dust in the atmosphere. S├ębastien Guillet, a senior research associate at the Institute for Environmental Sciences at the University of Geneva, believes that medieval manuscripts contain important information about a string of large volcanic eruptions that occurred during the Middle Ages.

Over a five-year period, Guillet and his colleagues scoured 12th and 13th-century European, Middle Eastern, and East Asian sources for lunar descriptions. These descriptions, combined with ice core and tree ring data, allow scientists to date some of the biggest volcanic eruptions the world has seen with more accuracy.

Of the 64 total lunar eclipses that occurred in Europe between 1100 and 1300, the study found documentation of 51. In six of these cases, the documents reported that the moon was exceptionally dark, corresponding with five major volcanic eruptions identified from traces of volcanic ash found in polar ice cores.

Medieval_MapOne of the powerful eruptions, the 1257 Samalas eruption, stands as one of the largest volcanic eruptions of the past millennium. The resulting volcanic aerosols blocked sunlight and caused widespread climate disruption. Historical records show that the following summer in Europe was one of the coldest recorded over the past millennium.

The researchers believe volcanic eruptions took place three to 20 months before the dark eclipses, based on observations of more recent eruptions and their effect on lunar eclipses. By putting together the information from ice cores and medieval texts, scientists can now make better estimates of when and where some of the biggest eruptions of this period occurred.

The study provides insights into how past volcanism affected not only climate but also society during the Middle Ages, and helps to shed light on the onset of the Little Ice Age, a period of cold weather between 1280 and 1340 that disrupted harvests and saw the advance of European glaciers.


The research underscores the value of historical records in understanding Earth’s past and the impact of natural disasters on society. By studying the writings of medieval monks and scribes, we can learn more about our planet’s history and the challenges we may face in the future.

Reference: Hunt, Katie. “Medieval Moon Observations Reveal ‘Mysterious’ Volcanic Eruptions.” CNN, 6 April 2023,

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