'Mysterious' volcanic outbursts are revealed by mediaeval moon measurements.

Medieval observations of the moon are helping present-day researchers study a mysterious cluster of volcanic eruptions on Earth. Monks and other scribes from the era made detailed descriptions of lunar eclipses, when the moon is fully in Earth’s shadow. An exceptionally dark eclipse is associated with the presence of a large amount of volcanic dust in the atmosphere, according to Sébastien Guillet, a senior research associate at the Institute for Environmental Sciences at the University of Geneva. Guillet believes medieval manuscripts contain an important source of information about a string of large but little-understood volcanic eruptions on Earth. Over a five-year period, Guillet and his colleagues scoured 12th and 13th-century European, Middle Eastern and East Asian sources for lunar descriptions, which, when combined with ice core and tree ring data, are allowing more accurate dating of what scientists think must have been some of the biggest volcanic eruptions the world has seen.

The study published April 5 in the journal Nature found documentation of 51 lunar eclipses that occurred in Europe between 1100 and 1300. In six of these cases, the moon was exceptionally dark. These dates correspond with five major volcanic eruptions identified from traces of volcanic ash found in polar ice cores. One of these 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, leading to one of the coldest summers recorded over the past millennium. 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.