15 October 878 AD: A Lunar Eclipse was observed in Ireland on this day. A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a full moon.

Retrospective astronomical observations confirm what the monks witnessed: A lunar eclipse at 3:53 am on the 15 October 878. The moon was just fully immersed in the umbra and 15 October that year did indeed fall on a Wednesday – the fourth day (feria) of the week.

There was a lunar eclipse on the Ides [15th] of October,
the fourteenth of the moon, 
on the fourth feria about the third watch.

Annals of Ulster 878 AD