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High Tides and The Canterbury Tales : In one of his famous poems, Chaucer may have described a rare astronomical configuration that actually occurred in the 14th century.

When the English poet Geoffrey Chaucer died in the year 1400, exactly six centuries ago, he left behind an unfinished collection of stories known as The Canterbury Tales. They begin with these famous lines:

Whan that Aprill with his shoures soote
   When April with its showers sweet
   The droghte of March hath perced to the roote . . .
   Hath pierced the drought of March to the root . . .
   Thanne longen folk to goon on pilgrimages . . .
   Then folk long to go on pilgrimages . . .
   And specially from every shires ende
   And especially, from every shire's end
   Of Engelond to Caunterbury they wende.
   Of England to Canterbury they wend.


Each of the tales is told by a member of a group of pilgrims, and many of them contain references to astronomy. These are unusually sophisticated, which is not surprising if we remember that Chaucer was expert enough in science to write a treatise on the astrolabe. Some of the most intriguing astronomical allusions are those found in the story told by the Franklin, a country landowner who admires chivalry and noble ideals.

"The Franklin's Tale"

The Franklin begins by describing the marriage of a knight named Arveragus and his beautiful wife, Dorigen, who live on the rocky coast of Brittany. While the knight is away at war in England, Dorigen is inconsolable. Whenever she walks along the cliffs near her castle, she sees the menacing black rocks offshore that have caused the deaths of so many mariners and will endanger her husband when he returns.

Meanwhile, a young squire named Aurelius has fallen secretly in love with Dorigen. At a garden party in the springtime he dares to reveal his love and ask for her favors. She replies playfully that she will agree to his embraces if he will remove all the rocks from the coast of Brittany. Aurelius at first despairs, but he then returns home and prays to the Sun to cooperate with the Moon in causing an exceptionally high tide that will cover up the rocks, so that he might then hold Dorigen to her promise. Aurelius specifically asks for a flood tide "so great that by at least five fathoms [30 feet] it oversprings the highest rock in Brittany." But the high tide does not come during that spring or summer, or even during the next two years, and Aurelius languishes as he waits in vain.

Finally, Aurelius and his brother travel to the town of Orleans to consult a scholar, a learned cleric (the Clerk) who possesses much special knowledge of the workings of the heavens. After asking an enormous fee the scholar agrees to help, and the three proceed to the Brittany coast where "through his magic" he seems to make the rocks disappear, apparently under the waters of a high tide. The Franklin ends the story by relating how each of the characters shows nobility: Dorigen tells her husband of her rash promise and agonizes over being unfaithful, Arveragus tells his wife she must keep her word, Aurelius releases her from her promise, and the Clerk of Orleans waives his fee.

One aspect of this tale has always seemed rather odd to Chaucer specialists. After all, the ordinary cycle of high and low tides is nothing that the poet's audience would find surprising, let alone amazing or magical as the plot requires. In the course of our analysis, however, we discovered an explanation -- Chaucer may be describing a rare astronomical configuration and an exceptionally high tide that actually occurred in the 14th century.

The wording of the tale is quite specific regarding the weather and the time of year, even naming the month when the three travelers arrive at the Brittany coast:
   And this was, as thise bookes me remembre,
   And this was, as these books make me remember,
   The colde, frosty seson of Decembre.
   The cold, frosty season of December.
   Phebus wax old, and hewed lyk laton,
   Phoebus [the Sun] grew old, with a hue like copper,
   That in his hoote declynacion
   That in his hot declination,
   Shoon as the burned gold with stremes brighte;
   Shone as the burnished gold with streams bright;
   But now in Capricorn adoun he lighte,
   But now in Capricorn adown he lights,
   Where as he shoon ful pale, I dar wel seyn.
   Whereas he shone full pale, I dare well say.
   The bittre frostes, with the sleet and reyn,
   The bitter frosts, with the sleet and rain,
   Destroyed hath the grene in every yerd.
   Hath destroyed the green in every yard.
   Janus sit by the fyr, with double berd,
   Janus sits by the fire, with double beard,
   And drynketh of his bugle horn the wyn;
   And drinketh from his bugle horn the wine;
   Biforn hym stant brawen of the tusked swyn,
   Before him stands brawn of the tusked swine,
   And "Nowel" crieth every lusty man.
   And "Noel" cries every lusty man.


The cry of "Noel" suggests a time in the latter part of December, shortly before or after Christmas. The same part of December is indicated by the mention of the two-faced Roman god Janus, an allusion to the approach of January. Chaucer's reference to the Sun in Capricorn also helps us pin down the time of year, for medieval astronomers defined Capricorn as the range of ecliptic longitude from 270[degrees] to 300[degrees]. The Sun reached its southernmost declination as it entered Capricorn on the day of the winter solstice, about December 13th during Chaucer's lifetime. The abundant seasonal clues show that this passage describes a "cold, frosty" day that must fall between December 13th and December 31st.

The Clerk's Calculations

At the Brittany coast the Clerk of Orleans works night and day until "at last he hath his time found" for the high tide. The Clerk calculates lunar and solar positions from a set of "Toledan tables," a reference either to those prepared in the 11th century by the astronomer al-Zarqali at Toledo, Spain, or to the Alfonsine Tables compiled at the same city in the 13th century under the direction of King Alfonso X (S&T: March 1985, page 206). Chaucer gives us one of the most complex astronomical passages in all of English literature as he describes the calculations and the resulting high tide that hides the rocks:
   His tables Tolletanes forth he brought,
   His Toledan tables forth he brought
   Ful wel corrected, ne ther lakked nought,
   Full well corrected, there he lacked nothing
   Neither his collect ne his expans yeeris,
   Neither his collect nor his expans years,
   Ne his rootes, ne his othere geeris,
   Nor his roots, nor his other gear,
   As been his centris and his argumentz
   As are his centers and his arguments,
   And his proporcioneles convenientz
   And his proportionals convenient
   For his equacions in every thyng.
   For his equations in everything.
   And by his eighte speere in his wirkyng
   And by the eighth sphere in its working
   He knew ful wel how fer Alnath was shove
   He knew full well how far Alnath was shoved
   Fro the heed of thilke fixe Aries above,
   From the head of that fixed Aries above,
   That in the ninthe speere considered is;
   That in the ninth sphere considered is;
   Ful subtilly he kalkuled al this.
   Full subtly he calculated all this.
   Whan he hadde founde his firste mansioun,
   When he had found his first mansion,
   He knew the remenaunt by proporcioun,
   He knew the remnant by proportion,
   And knew the arisyng of his moone weel,
   And knew the arising of his moon well,
   And in whos face, and terme, and everydeel;
   And in whose face, and term, and everything;
   And knew ful weel the moones mansioun
   And knew full well the moon's mansion
   Accordaunt to his operacioun,
   Accordant to his operation,
   And knew also his othere observaunces
   And knew also his other observances
   For swiche illusiouns and swiche meschaunces
   For such illusions and such mischances
   As hethen folk useden in thilke dayes.
   As heathen folk used in those days.
   For which no lenger maked he delayes,
   For which no longer made he delays,
   But thurgh his magik, for a wyke or tweye,
   But through his magic, for a week or two,
   It semed that alle the rokkes were aweye.
   It seemed that all the rocks were away.


To find the Moon's ecliptic longitude, a medieval astronomer would begin by noting the Moon's mean position at an initial epoch, called a radix or "root," and then would add up the tabulated mean motions during the time interval elapsed to reach the given date, expressed as a sum of "collect years" (centuries and 20-year periods), "expans years" (individual years counted from 1 to 19), months, days, hours, and minutes. Calculating the angle from the mean place to the true place of the Moon involved consulting the tables for such quantities as the "equation of center," "proportional minutes," and "equation of argument" -- exactly the terms employed by Chaucer in this passage.

Finding the Sun's position required a similar use of arguments and equations, with an additional complication alluded to by Chaucer's mention of "Alnath," a medieval name employed both for the single star Alpha Arietis and also for the stars in Aries that formed the first lunar "mansion." The 28 mansions were groups of stars near the ecliptic used as reference stations for the daily motion of the Moon during the sidereal month. Chaucer uses the changing distance between Alnath and the "head of that fixed Aries" (the vernal equinox point, where the ecliptic intersects the celestial equator) as a way of measuring precession. This was important for any solar calculation, because medieval theory placed the Sun in a geocentric orbit with the directions of apogee and perigee (called aux and opposite aux) at fixed positions among the stars in the "eighth sphere," which executed both a steady precession and an oscillating motion called trepidation relative to the vernal equinox point in the "ninth sphere." The precession calculation was needed to locate the major axis of the Sun's orbit, find the true place of the Sun, and thereby deduce the Moon's phase.

Chaucer scholars have long referred to this section of "The Franklin's Tale" as a problem passage, notorious in its difficulty, and some do not go much beyond noting that a new or full Moon will produce a high tide. Phyllis Hodgson went so far as to judge that "this passage with its involved and highly technical account of the Clerk's astrological calculations need not be taken too seriously. Although Chaucer himself was a master of the subject his purpose here is artistic -- to emphasize the Clerk's expertness and surround the central event of the tale with an aura of mystery" (The Franklin's Tale, 1960, page 99).

But the complexity of this passage suggests to us that the Clerk of Orleans is making a very difficult calculation, perhaps to find the time of an astronomical configuration that would produce the most extreme possible tide range. His trick is similar to that of Mark Twain's Connecticut Yankee in King Arthur's Court, who, because he is able to predict a solar eclipse, makes people believe he caused it.

Covering the Rocks

Several independent factors contribute to produce exceptionally high tides. (1) Spring tides of increased range occur twice monthly, when the Sun and Moon are in syzygy (that is, when the Moon is new or full) and their individual tide-raising forces combine for a greater net effect. (2) Twice a year, at the times known as the "eclipse seasons," new and full Moons occur with both the Sun and Moon near the nodes of the lunar orbit. A solar or lunar eclipse then occurs, as does an additional enhancement of the tide-raising forces. (3) Perigean tides of increased range occur once per month, when the Moon is nearest Earth. (4) The tide-raising force of the Sun is maximized once per year, at the time of the Earth's perihelion.

In certain years it is possible for all four of these conditions to be met almost simultaneously. Writing in 1913, the Swedish oceanographers Otto and Hans Pettersson described such remarkable events and observed that this situation "produces an absolute maximum of the tide-generating force." In his 1986 work, Tidal Dynamics, Fergus Wood concurs. He also makes a passing reference to an event he calls the "absolute high tide experienced in A.D. 1340," describing it by the phrase "maximum perigee springs, a very rare circumstance."

Intrigued by this reference to an extreme tidal event in the 14th century, we used the methods in Jean Meeus's Astronomical Algorithms (Willmann-Bell, 1991) to search for the dates of eclipses with the Moon near perigee and the Earth near perihelion. Our computer program looked for alignments by following the motions of five imaginary lines: the line joining Earth and Sun, the line joining Earth and Moon, the major axis of the Moon's orbit, the line of nodes of the Moon's orbit, and the major axis of the Earth's orbit. A perfect alignment of all five lines never actually occurs, so we searched for eclipses with no pair of them misaligned by more than 10[degrees]. Our results are given in the first table on the facing page.

A striking pattern is evident from this list. The dates fall in groups, and these are separated by intervals of more than 1,000 years when no such events occur at all. Our calculations make precise the rarity of these alignments and also confirm the 1340 date mentioned by Wood. Moreover, the resulting high tides fell in the second half of December, just after the winter solstice and with the Sun in Capricorn -- exactly matching the circumstances described by Chaucer in "The Franklin's Tale"![*]

To demonstrate that medieval astronomers could have recognized the unusual nature of this event, the second table on page 48 includes the times that we calculated by hand from a copy of the Alfonsine Tables. For while early scholars lacked our modern concept of tidal forces, they definitely associated tidal ranges with astronomical phenomena. A 13th-century treatise described spring tides by saying that "when the Sun and Moon are in conjunction, the power of the Moon becomes stronger and the tide increases and becomes strong." The same work referred to perigean tides by observing that when the Moon "approaches the point nearest the Earth, its power increases, and then the rise of the sea is strong." Several treatises associated a period of high tides with the winter solstice and therefore, indirectly, with the time of closest approach between the Earth and the Sun. Chaucer would have understood, at least in a qualitative way, that the celestial alignments in December 1340 would significantly influence the tides.

Even though the precise ports visited by Chaucer on his trips to France are not known, the Brittany coast has long been famed for its remarkable tides. At St. Malo the mean tide range is 26 feet, spring tide ranges average 35 feet, and perigean spring tides with ranges exceeding 44 feet are possible. Even greater tides occur at Mont-St. Michel, only a short distance east of St. Malo. For centuries tourists and pilgrims have walked out to the abbey of Mont-St. Michel at low water, then watched the rapidly rising flood tide make an island of the site at high water.

Chaucer and 1340

But if Chaucer visited France in the 1360s and 1370s and wrote The Canterbury Tales during the 1390s, why would he be aware of a high tide that occurred in 1340? We can suggest two possible reasons.

First, Chaucer must have become familiar with tides in the Thames River when he served as controller of the customs office and supervised construction of wharves in the port of London. He was also appointed to a royal commission to oversee repairs to walls and ditches on the lower Thames. Chaucer might have been obliged to ask the oldest and most experienced mariners about the highest tides they had ever seen.

The second possibility is more intriguing. In his 1977 biography John Gardner places Chaucer's birth "around 1340, possibly early in 1341." When Chaucer was learning about astronomy, astrolabes, and astronomical tables during the 1380s and 1390s, it is plausible to imagine that he might have investigated his own horoscope. Chaucer may have discovered the remarkable tide-raising configuration in 1340 while using the Alfonsine Tables to calculate solar and lunar positions near the time of his birth!

By the time he wrote The Canterbury Tales, Chaucer was well versed in the celestial science of his day. We suggest that he called on this special knowledge and used the skies and high tides of December 1340 as inspirations for the central plot device in "The Franklin's Tale."

Don Olson and Edgar Laird teach astronomy and English, respectively, at Southwest Texas State University, where Tom Lytle is a physics graduate student. The authors are grateful for research assistance from Ed Wallner and Fergus Wood.

[*] A less-perfect alignment occurred during the "cold, frosty season of December" in 1999, when the winter solstice, a full Moon, and lunar perigee all fell on December 22nd, only 12 days before the Earth reached perihelion. Unusually strong astronomical tides may have contributed to the ecological disaster that began on December 26th as intense gales broke up a two-week-old oil slick and polluted 250 miles of the French coastline from Brittany south. The winds then moved inland, uprooting 60,000 trees near Paris.
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Author:Olson, Donald W.; Laird, Edgar S.; Lytle, Thomas E.
Publication:Sky & Telescope
Date:Apr 1, 2000
Words:2895
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