During a total lunar eclipse, the shadow of the Earth slowly covers the Moon. When it is completely covered, the Moon acquires a distinct red coloring due to sunlight that filters through our planet’s atmosphere. Over 1,000 years ago, however, European observers reported a completely dark lunar eclipse.

A new study by an international team of researchers suggests in Scientific Reports that a series of volcanic eruptions released enough sulfur compounds into the stratosphere to darken the sky between 1108 and 1110 CE. 

There are several pieces of evidence to support this interpretation. The first is the presence of sulfates in ice cores from Greenland. And not just any old deposit, but the largest deposit of the last millennium. This distinct mark was previously attributed to Icelandic volcano Hekla that erupted in 1104, but over the last decade researchers have worked out that the ice core dates were slightly off, moving the deposit to 1108.

Another hint that Hekla was not responsible for the deposit was the discovery of a similar sizable deposition of sulfates in Antarctica. For this reason, the team suspects that a volcano closer to the tropics is the answer.

To explore this idea further, the team combined data from the ice cores with historical accounts from around the world. Between 1109 and 1111 CE, there are reports of famines across Western Europe connected to dramatic changes in weather. The Peterborough Chronicle reports of a dark total lunar eclipse in May 1110. These facts match well with the story revealed in the ice cores.

Not only that but the team found a report of an eruption from Mount Asama, the most active volcano in Honshū on the main island of Japan. Its eruption in 1108 was the largest known for this particular volcano. 

A statesman describes the 1108 event as such: "There was a fire at the top of the volcano, a thick layer of ash in the governor's garden, everywhere the fields and the rice fields are rendered unfit for cultivation. We never saw that in the country. It is a very strange and rare thing."

The team are not certain that Mount Asama alone is responsible for the dark lunar eclipse, but they are confident that it played a role and that the new dating of the ice cores is correct.