Today an eclipse was seen throughout much of the US, reaching totality in a coast-to-coast strip (we were in the 80% range). So it seems like a good time to blog about eclipses.
These days astronomers can predict eclipses by calculating the paths of earth and moon. (Eclipses of the sun are caused by the moon passing between us and the sun, eclipses of the moon by the earth passing between sun and moon. But you knew that.) There was no way for medieval people to make such calculations. (The US is going to have another total eclipse of the sun in 7 years. Plan ahead.)
Medieval people were therefore always surprised when an eclipse showed up. Although there are plenty of modern notions that medieval people thought they were caused by demons, or thought a dragon was eating the sun (or moon), this is not true. Medieval people knew perfectly well what an eclipse was, even if they couldn't predict it. After all, even if you don't know that the earth goes around the sun, rather than vice versa, you could certainly understand how the moon could block the light of the sun or the earth the sun's light reflected from the moon.
Remember, medieval people assumed the earth was a globe, just like the moon. The "earth was thought to be flat" fable was invented in the nineteenth century.
Some eclipses may well have passed unnoticed. If it's a cloudy day, even an 80% eclipse of the sun won't look like much of anything. And a cloudy night would mean you'd totally miss a lunar eclipse. But medieval people saw and reacted to eclipses.
The usual explanation was that God had sent the eclipse to mark some major upcoming event. After all, the normal understanding was that God was actively involved in His creation, even though they couldn't always figure out the hints He tried to give them. If anyone important died shortly after an eclipse, then it was understood that the eclipse had been sent to warn about this event. After the death of Henry I of England, many recalled a recent solar eclipse.
It is possible to calculate eclipses after the fact as well as ahead of time. Astronomers have noted an eclipse that took place in the spring of the year we call 29 AD, which some have identified with the "darkness at noon" that the Bible says accompanied the Crucifixion.