In Geoffrey Chaucer's 14th century story "The Franklin's Tale," the faithfully married Dorigen tells an adulterous young squire that she will surrender her favors, but only if he can guarantee her husband's safe return from sea by submerging the coastal rocks of Brittany. Dorigen is sure her virtue is safe--until a local cleric raises a great flood tide that covers the treacherous shoals. Dismissed by most scholars as one of the convenient miracles commonly used in medieval stories, there may be truth to the story, according to a study in April's Sky and Telescope: The sun and moon actually conspired in Chaucer's time to cause an exceptional tide that may have covered the 10-meter-high French rocks.
As an accomplished astronomer, Chaucer undoubtedly knew how to predict the tides by the relative positions of the sun and the moon. Both have gravitational tugs that create a tidal bulge on Earth, but the two forces rarely pull in the same direction. Occasionally, however, the sun, Earth, and moon all line up, causing lunar and solar eclipses as well as exceptionally large tides.
Aware of Chaucer's technical prowess, astronomer Donald Olson, English professor Edgar Laird, and graduate student Thomas Lytle from Southwest Texas State University in San Marcos wondered if Chaucer could have been inspired by a real flood tide. To answer the question, Olson plugged a standard set of equations for calculating the positions of the sun, moon, and Earth into his computer and searched for dates when all three bodies were aligned within 10 degrees of each other. To his great surprise, one such alignment occurred on 19 December 1340, very near the year of Chaucer's birth.
To check that Chaucer could also have made this calculation, Olson repeated it by hand using only the medieval Alphonsine tables of the positions of celestial objects, a book Chaucer owned. The results were almost exactly the same. "We believe that the story is based on a real event," says Olson, "This shows again how expert Chaucer was [in astronomy]."
"This is a really good claim, and I am horribly jealous," says astronomer and historian Brad Schaefer of Yale University. But Schaefer cautions that many of the miracles in medieval stories are clearly fanciful. And the trouble is, there is no independent way to check whether Chaucer really made the calculations.