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THE AURORA OF 1192
« on: February 09, 2017, 05:14:35 pm »
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the following work is the product and property of professor lynn
h. nelson, University of Kansas. If it is redistributed or quoted
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         THE AURORA OF 1192: ITS CAUSES AND EFFECTS
 
                    Lynn H. Nelson
                 University of Kansas
                    January 1992
 
Since the publication of Ladourie's Histoire de climat depuis
l'an mil in 1967, historians have generally accepted that the
European climate deteriorated after about the year 1000. They
have seen this deterioration as a cause of the Great Famine of
1315-1317, a factor the Black Death of 1347, and contributing to
the depression of the fifteenth century. There has been little
demand for a more precise chronology, and even less for a cause.
Since it was noted that there were few sun spots during the
period, and since someone coined the term, "The Era of the Quiet
Sun," historians have been more or less content to accept a lack
of solar storms somehow caused the deterioration of the medieval
European climate.
 
That causation is in fact quite unsatisfactory. For over a
century, all attempts to establish direct correlations between
solar flares and rainfall in the central plains of the United
States have been unsuccessful. The cycles of rain and drought
appear to be independent of the cycles of solar storms, despite
continued attempts to prove the contrary.  We should not be
content with causes for past events that cannot be shown to cause
the same events today.
 
Besides, there were many other significant geophysical events at
the time that cannot be accounted for by "a quiet sun." We need
an explanation of medieval climate change that is more precise
and comprehensive. I would like to offer such an explanation this
evening, and to do so by working my way back from the aurora of
1192.
 
At dusk on Wednesday, 15 January 1192, the inhabitants of Anchin,
in eastern Flanders, witnessed a terrifying sight. An anonymous
monk reported that
 
     ...At the setting of the sun and in the dusk of night, many
     people saw a kind of terrible fire that filled the entire
     northern part of the globe [of the sky].1
 
Baldric, a monk in the nearby monastery of Ninove in Aalst, noted
that
 
     ... throughout Gaul, a fire was seen in the night sky such
     that each person thought that the neighboring village was on
     fire.2
 
Observers at Cologne and Quedlinberg in Germany, at Prague in
Bohemia, and in the North of England reported similar sights. It
is clear from their descriptions that they were all reporting a
great aurora. All agreed that this fiery sky was something
extraordinary, and some saw it as a harbinger of the terrible
famine of 1197.
 
It is curious at first sight that the chroniclers should have
considered the sight a singular one, and that the populace should
have behaved as if they had never before seen such a display.
When the oldest men and women of one district were asked, they
claimed that they had never heard of or witnessed such a sight in
the entire lives. What makes this strange is that eleventh- and
twelfth-century chronicles not infrequently note bright spears
and swords in the sky, armies clashing, the combat of fiery
serpents, and other patterns of light that were clearly auroras.
At the same time, auroras are infrequent but not exceptional in
these regions today; some three or more can be seen in any given
year. Why should people have been so astonished at the aurora of
1192? Why should they have claimed that they had not seen such a
thing before?
 
The reason is that they did not recognize that the various plays
of light they had seen in the northern skies were actually
different manifestations of the same phenomenon. It was not that
they had never before seen an aurora, but that they had never
before seen this particular type. Its red color, size and
intensity, and lack of defined shape confirms that it must have
been what geophysicists call a type A arc.3 This is a rare and
very impressive display, and is often mistaken by modern
observers for the light of forest fires. Perhaps it would be well
to review the causes and dynamics of the aurora before proceeding
further.
 
Although the exact mechanisms are still matters for research, the
basic cause of auroras has been relatively well proven. During
solar disturbances, bursts of solar radiation fall upon the
magnetic fields surrounding the Earth and distort them
considerably. Charged particles are expelled from the sun at
speeds of up to six hundred miles per second. After a journey of
a little less than two days, they strike the terrestrial magnetic
fields and flow along these fields to the dark side of the
planet. Many of these solar particles are contained by these
fields but those with higher energy penetrate into the
atmosphere. The descent of these particles ceases at some point,
and they begin to diffuse, colliding with atmospheric atoms and
luminescing.
 
Different colors and shapes of luminescence are produced at
different altitudes. The type A red arc that was seen by the
monks and villagers in 1192 was produced by high energy particles
diffusing about 150 miles above the Earth. That wall of light may
have been as much as three thousand miles long and a hundred
miles high.4 It was a rare sight because these walls of fire are
only seen when the geomagnetic latitudes are driven far southward
and billions of tons of air are carried half-way around the
world.5 The only force capable of doing this is the torrent of
solar radiation that accompanies a great solar storm.
 
Solar storms are characterized by unusually numerous solar
flares, commonly called sun spots. Sun spots have been
systematically observed and recorded by Chinese scholars for
centuries. Their observations were made without telescopic aid,
and there is room to suspect that there were defects in some of
their records. The Chinese counts frequently fail to correspond
to what have been established as regular cycles of solar
activity, and comparisons of Chinese and European counts during
the early modern period have sometimes disclosed significant
discrepancies. With allowances made for such shortcomings,
however, the Chinese observations still provide the most
important and reliable single source of data for past solar
activity. It is possible to reconstruct from these records at
least the broad outlines of solar activity for the last thousand
years.6 It is also possible to relate specific European phenomena
with specific Chinese sun spot counts.
 
From about the year 850 to 1000 the Chinese scholars noted
relatively few sun spots, but their counts began to increase
after 1000 and reached a high point shortly before 1130. There
were some reflection of this in European chronicles.  Ralph,
archbishop of Canterbury, died in 1126 and an English annalist
who recorded the event also noted that sailors reported seeing a
broad wall of fire in the northeastern skies.7
 
The sunspot count dropped immediately after 1130, and almost none
were recorded until 1192. The number for 1192, however, was the
greatest the Chinese had ever recorded. Improved techniques of
observation and recording may have had something to do with this,
but it is clear that immense amounts of matter and energy were
spewed out of the sun in 1192. This torrent fueled the aurora
that startled the population of northwestern Europe and started
some monastic chroniclers wondering if this extraordinary event
might have any significance for the coming harvest.8
 
In fact, the aurora of 1192 marked the end of any major solar
disturbances for a long while. It was not until 1375 that the
Chinese observers registered a few years of relatively high sun
spot counts, and European chronicles recorded a flurry of auroras
during that same period. The years from 1192 to 1375 marked one
of those lulls in solar activity that have been called a "quiet
sun."  It was also an era marked by a cold, unsettled climate
throughout the northern hemisphere and a general decrease in
vegetation throughout the globe. The correlation was not exact
however, and, as we have noted before, the evidence up to now
suggests that solar and terrestrial weather patterns are
independent of each other.
 
The intensity of the Earth's magnetic fields is not independent
of solar activity, however. Solar radiation fed into those fields
increases the intensity of geomagnetism,9 so one would expect
that the strength of the Earth's field would diminish in an era
of a quiet sun. This was not the case in the twelfth and
thirteenth centuries, however. There are various methods of
determining past geomagnetic intensity, and the results of
several different methods obtained from several different parts
of the world are in general agreement.10 After reaching a peak of
intensity in about 900 A.D., geomagnetic strength began to
decline rapidly until about 1100. Its intensity then rose again
and stayed at a high level until about 1800. If this increase was
not caused by increased solar radiation, it must have been caused
by the  Earth itself. This brings us to the subject of
geodynamics.
 
The inner core of the Earth is a rigid, metallic, and perhaps
somewhat lopsided ball surrounded by a liquid metallic outer
core. The outer core, in turn, is enveloped by the Earth's crust,
which is covered with a film of water and surrounded by a mantle
of air. Most geophysicists presently accept the theory that the
solid inner core rotating within a liquid mantle acts as a great
dynamo that generates the Earth's magnetic fields.11 In the
absence of other factors, an increase in geomagnetic intensity
should reflect a increase in the momentum of the Earth's inner
core, increased convection within the liquid mantle, or both..
 
There is evidence that both were the case in the twelfth century.
The rotation of the earth is slowing under the tidal influence of
the moon. The decrease is quite small, about 2.4 milliseconds per
century. It takes two and a half million years for the length of
the day to increase by one minute at this rate. However, there
are several measurements that indicate that between about 1000
and 1800 AD the average rate of decrease in the rotation of the
Earth was not 2.4 milliseconds per century, but only 1.4. An
accelerating force equal to 1.0 millisecond per century was being
generated somewhere. The only apparent factor that could produce
an increase in geomagnetic intensity and a decrease in the decay
of the Earth's rotation is an increase in the momentum of the
inner core. The added momentum of the core would increase the
convection currents within the mantle, and this, in turn, would
increase the amplification of the magnetic field generated by the
inner core.
 
Similar, but shorter-lived, changes in core momentum have been
detected or inferred in modern times, so such massive events do
occur. Although there is no method at present of demonstarting
that such an event took place around 1000 A.D. it is clearly the
best available explanation of two significant geophysical
anomalies during the period: the intensification of geomagnetism
and a change in the rate of the Earth's rotational decay. Until a
better explanation is presented, we may assume that the momentum
of the Earth's inner core increased sometime around the year 1000
and ask ourselves what the effect of such an increase might have
been.
 
One effect would have been the displacement of the magnetic pole,
and this in fact occurred. The magnetic pole traces a complex
westwardly path around the celestial pole, but the velocity of
its movement correlates directly with geomagnetic intensity.12
From 300 to 900 A.D., the pole passed over fifty degrees of
longitude. During the next six hundred years, from 900 to 1500,
it travelled over two hundred and fifty degrees and moved from
the vicinity of Murmansk to northern Canada.14
 
Any increased momentum in the core would be distributed in time,
and we should expect some disturbances as part of that force was
transferred to the Earth's crust. As a matter of fact, the
chroniclers and annalists of northern Europe recorded an
increasing number of earthquakes from about 1100 to about 1130.
After 1130, they slowly became less frequent, and almost none
were noted after 1202. We can presume then that an increased
internal momentum began to reach the surface of the Earth in
about 1130 and that the process was substantially completed by
1200. What might the effects of this transfer be upon the waters
and winds?
 
The dynamics of the surface of a sphere are such that objects
with greater momentum move toward the equator and balance their
greater force by increasing the distance of their movement.
Confining our attention to the North Atlantic, we note that ocean
currents generally shifted southward. The southward displacement
of the Gulf Stream in turn forced the equatorial current that
runs from Africa to the Caribbean Sea further south. A larger
portion of these warm equatorial waters began to flow into the
South Atlantic rather than feeding into the Gulf Stream. The sea
levels in the North Atlantic, reduced by this loss, were
replenished by a current of cold surface water from the Arctic
seas. Overall, the North Atlantic grew cooler, and a weakened
Gulf Stream reached Europe in the Bay of Biscay or even further
south.
 
Not all of the momentum was taken up by the oceans; some was
absorbed by the atmosphere. The increased momentum of the air had
much the same effect as that of the water; there was a general
southward displacement of wind currents. The jet stream that
controlled the course of the Westerlies now passed over North
Africa.14  Although the Westerlies still provided western Europe
with warm moist air, the volume of this flow was no longer
sufficient to hold back the cold, dry masses of Arctic air, and
the hot, dry winds from the Sahara. Northern Europe began to
experience a cooler and wetter climate, and southern Europe a
warmer and drier one. Both were subjected to sudden and violent
changes of weather.
 
When did these changes occur? That is a difficult question to
answer, since there was a complex of factors involved. It is
possible to suggest a general chronology of events, however.
There was an increased momentum in the Earth's rotation beginning
about 950. By the year 1000, this force had affected the
convection of the outer core to such a degree that the intensity
of the magnetic field began to increase and the magnetic pole to
shift rapidly. Some of this force began to reach the Earth's
surface in about 1050, and earthquakes became increasingly
frequent.
 
The air and water of the North Atlantic began to absorb some of
this momentum at this time and to shift southward. By about 1120,
this displacement had become substantial, and floods and
windstorms became relatively common events. By 1150, the warm air
of the Westerlies was no longer sufficient to block sudden forays
of arctic air into western Europe, and there was an ever-present
danger of severe hailstorms. By 1200, the transfer of momentum
and its compensation by a southward shift of winds and waters was
complete. Earthquakes grew more rare, extensive portions of
Scandinavia could no longer support agriculture, the growing
season in northwestern Europe had been reduced by three weeks or
more, and the average temperature had declined by almost three
degrees centigrade.
 
How valid is this reconstruction?  It is certainly not
conclusive. Like any historical theory, it is an attempt to
explain an historical record. As is the case with any historical
record, our understanding of the geophysical past may change with
new discoveries and more sophisticated interpretations. For the
time being, however, this sequence of events provides a coherent
and relatively precise explanation of the deterioration of the
climate of medieval Europe.
 
It also places the beginning of that deterioration considerably
earlier than has been generally accepted. Many aspects of the
Twelfth Century Renaissance -- the extensification of
agriculture, the reclamation of lands, the growth of a non-
agricultural middle class, the development of long-distance
trading in bulk commodities, an increasing concern for the
homeless population -- have commonly been regarded as the results
of a population growth that led to the Malthusian climax of the
mid-fourteenth century. It would now appear that a deteriorating
climate may also have been a contributing factor.
 
That deterioration of climate was a gradual process and it
effects were mitigated in some degree by advances in agricultural
technology. It was reflected primarily in later springs and
earlier winters, and people seem to have been only vaguely aware
of the degree to which the festivals of the solar calendar were
becoming dissociated from the agricultural works that had
traditionally accompanied them. Plow Monday, the first Monday
after Epiphany and the traditional beginning of early plowing,
was still celebrated even though the fields were usually frozen
or too waterlogged to enter. The feast of Saint Barnabas on 24
June was still considered the proper time for the reaping of
winter wheat, although harvests were increasingly delayed to mid-
July or even later.
 
I have used the phrase "deterioration of climate" several times
without suggesting what it may have meant in human terms. It
meant basically that the already tight schedule of the
agricultural year was compressed by about a month, and that the
key operations of plowing and planting, and reaping and
threshing, were conducted during seasons of quite variable
weather.15 One example might be sufficient to indicate the
difficulties under which the peasants now had to operate.
 
In 1202, ten years after the appearance of the great aurora, an
unknown monk of Anchin wrote that
 
     During the moon of August, both in its rising and
     descending, it was excessively rainy and quite cold. This
     caused great difficulties in many places, for the wet
     sheaves were stored in the granaries, and the hay was not
     yet cut and could scarcely be gathered. And what caused
     almost everyone to grieve [even] more, the ripening and
     harvesting of the vines turned out to be even later.16
 
It is illuminating to piece together what was happening here. The
sheaves in the granaries must have been those of the winter wheat
harvest, since the spring wheat was not harvested until after the
hay was cut. It would appear, then, that rain had delayed the
winter wheat harvest until the end of July or beginning of
August. There had been no dry spell during which the peasants
could spread the sheaves, thresh them, and winnow the grain. Even
a covered threshing floor would not have helped with wet sheaves.
All they could do to save their harvest from rot and mildew was
to keep their sheaves in the granaries, and hope for a dry spell.
 
Up to a point, the hay in the meadow was safer standing than
being cut, but as time passed, the danger grew of its falling and
rotting in the fields. Nevertheless, there was no dry place to
put it, so it could not be cut. Without winter fodder, many of
the stock would die, and there might not be enough animals to
plow the fields in the coming year. At the same time that the
grass was beginning to rot, the time for plowing the fallow and
harvesting the spring wheat was passing, and the wet weather made
it impossible to do either. The cold delayed the maturation of
the grapes, probably into October.18 The late, wet vintage meant
less and poorer wine. If the peasants had been hoping to sell
their wine to compensate for their crop losses, they were
disappointed. The peasants of Anchin could see that they were
well on the road to famine.
 
They and most of the other inhabitants of the region had been in
the same position in 1192 and had considered themselves fortunate
that a dry spell had allowed them spread their sheaves and thresh
their grain. Most of the moulds could beaten off the kernels and
blown away during winnowing, and the kernels were only slightly
damaged from the infestation. They could not have known that some
of those moulds had left toxins behind in the grain.
 
Rye was particularly susceptible. The same habitat that nourished
penicillin over the centuries was also favored by the deadly
ergot. Ergot toxin is and was extremely powerful and slowly
destroys nerves and blood vessels. The pain of the dying nerves
is excruciating and is compounded by the gangrene that soon
afflicts the extremities, particularly the feet. The only
treatments were amputation and suicide. The complete ignorance of
what was happening made it all much worse. The sufferer felt as
if his feet and legs were on fire, and they soon grew as black as
if they were charred, and were finally consumed.
 
The popular term for the affliction was "the fires of Hell," and
the monk who was so astonished at the aurora of 1092 soon
wondered if the burning sky in January might not have had some
connection with the paupers that began arriving at his monastery
in March, crying out that their feet were on fire. And so we have
returned once more to the great aurora of 1192, and it is finally
time to put it in its proper place.
 
We can now see that the aurora had nothing to do with the great
changes that were underway. The causes did not lie on the surface
of the sun, but in the depths of the Earth. The fact that the
great aurora appeared about when the Earth's surface had finally
absorbed its share of the increased momentum of the globe was
quite coincidental. It is important to recognize, however, that
discerning individuals considered that some massive physical
event might lie behind the sudden outbreak of the fires of Hell
and the great famine that was even then beginning to build in
scattered districts. They were right, of course, but they were
looking in the wrong direction.
 
Much of my discussion this evening has been devoted to the
workings of vast and impersonal forces. It is only appropriate,
then, that I should close by focussing on a individual, even if
he was simply a nameless victim of what was happening. In the
middle of the thirteenth century, a monk of Sens by the name of
Richer, undertook to write histories of the various churches and
monasteries of the district and to recount some of the miracles
associated with their patrons. He narrated the following story
associated with the church of St. Hildulf.
 
     In another year, which in its course had come to the feast
     of St. Hildulf (11 July), there was a certain peasant of the
     village of St. Prix who had loaded an ox-cart full of hay.
     When he was returning to his home with his cartload of hay
     and was already close to the village, it began to grow
     threatening. The sky, which had seemed clear and pure enough
     before, was suddenly darkened. The thunder roared, the
     lightning flashed, and there was a downpour of hail of such
     great force that the peasant was hardly able to stand.
     He was weakened by the growing force of the storm, shaken
     and stoned by the balls of hail. He was not able to find a
     refuge unless he abandoned his cart and oxen. which he did
     not delay in doing. He hoped to hide beneath the wagon, a
     hope that was dashed when there unexpectedly arose such a
     strong wind that it not only overturned the cart itself, but
     scattered the hay all across the width of the fields, and
     left the peasant without a roof.
 
     What more? The peasant was robbed of his goods and flogged
     by the wind; he was stoned by the hail, strangled by the
     flood of rain, pounded down by the thunder, lashed at by the
     lightning, and so he was brought almost to the edge of
     death. While he was lying there all battered, the people of
     the village came up, looking to see if the storm had in any
     way damaged the crops in the ground. After they had found
     that it had not, they discovered the peasant lying half-dead
     next to his cart. As they were carrying him home, they
     concluded that he deserved everything that had befallen him
     because he was lacking in faith and had tried to avoid
     paying out for the honor of God and his saint Hildulf. For
     this reason, this sort of thing was bound to have happened
     to him.17
 
It is hard to believe that Richer did not have his tongue firmly
planted in his cheek when he wrote this, and yet he had a serious
message in mind. His nameless peasant had been almost killed by a
sudden storm that had lost him his cart, his hay, and possibly
his oxen. He had been brought to the edge of death. His neighbors
had been so frightened that they might have lost their crops that
they did not even see the body of their neighbor lying in the
road until they were satisfied that their grain was safe. As they
carried him home, they tried to fathom what forces governed such
phenomena of nature, and they decided that all of this fury had
struck their neighbor because he was stingy in paying his share
for support of the local festival.
 
Their desire to know the cause of weather was not in itself
comical, but their sense of the proportionality between cause and
effect was definitely eccentric. Richer had some idea of the
tremendous forces embodied in even a local thunder- and hail-
storm, and knew enough to realize that such massive events do not
arise from petty causes.
                           ENDNOTES
 
 
1. "Continuatio Aquicinctenses," MGHSS 6: 428, 13.  The "holy
fire" was probably an outbreak of ergotism. Jackson Tartakow, and
John H. Vorperian, Foodborne and Waterborne Diseases. Their
Epidemiological Characteristics  (Westport CT: Avi Publishing
Company, Inc., 1981), pp. 167-168 provides a brief discussion of
ergotism within a discussion of epidemics in general. Claude
Moreau, Moulds, Toxins and Food, translated by Maurice Moss (New
York: John Wiley & Sons, 1979), is an authoritative, although
technical, treatment of fungal diseases.                         
                 
2. "Balduinus Ninovensis chronicon,"  MGHSS 25: 537, 40.  The
chronology of the  "Annales maxima Coloniae," MGHSS 17: 803, 4-5,
is not as certain, but the chronicler of Cologne reports,
although less dramatically, a similar event: "In this year, a
great and wonderful fire was seen in the sky, mixed with the
stars."
 
3. See William Petrie, Keoeeit. The Story of the Aurora Borealis
(New York: Macmillan, 1963), chap. 4 for a discussion of the
various forms of auroras. Although the text is brief and
elementary, the accompanying illustrations are quite fine.
 
4. Petrie, Keoeeit, p. 50.
 
5. The concentration of atmosphere provides the atoms of air
necessary for the solar debris to produce such an intense light
at such altitudes.
 
6. A.D. Wittman and Z. T. Xu, "The Behavior of Solar Activity as
Inferred from Sunspot Observations 165 BC to 1986," Secular Solar
and Geomagnetic Variations in the Last 10,000 Years, edited by F.
R. Stephenson and A. W. Wolfendale  (NATO ASI Series: Series C,
Vol. 236, Boston: Kluwer Academic Publishers, c.1988), figures 1
and 3, pp. 135-136. It should be noted that Wittman sets the
value of the Hale sunspot cycle at 22.232 years. According to
this computation, the Aurora of 1192 occurred some four years
before the peak of the third cycle 1130. See also M. R. Attolini,
M. Galli, and T. Nanni, "Long and Short Cycles in Solar Activity
During the Last Millennia," Secular Solar and Geomagnetic
Variations, fig. 1, p. 36. Both figures combine Western and Far
Eastern observations. The majority of observations for the period
in question are Eastern.
 
7. Noted in Petrie, Keoeeit, p. 64.
 
8. This activity was soon over, and few, if any, sun spots were
observed until another flurry occurred around the year 1375.
Other periods of a "quiet sun" occurred 1490-1590 and 1660-1830.
Ronald T. Merrill and Michael W. McElhinny, The Earth's Magnetic
Field. Its History, Origin and Planetary Perspective (London:
Academic Press, 1983), p. 11.
 
9. See Merrill and McElhinny, fig. 4.5, p. 102; fig. 4.6, p. 103;
and fig. 4.10, p. 113. One should also note, however, that much
more needs to be done before these variations can be clearly
distinguished from the effects of the movement of the geomagnetic
pole.
 
10. Merrill and McElhinny, The Earth's Magnetic Field, chapters 7
and 8, provide a comprehensive discussion of the geodynamo
theory, its problems, and recent refinements.
 
11. Merrill and McElhinny, The Earth's Magnetic Field, pp. 294-
297.
 
12. See Morrison and McElhinny, The Earth's Magnetic Field, table
4.1 and figure 4.4, pp. 100-101.
 
13. One should also note that this tendency in the oceans and
atmosphere would have had a braking effect on the momentum of the
solid Earth by increasing its diameter. This would have shifted
momentum to the solid earth, allowing the wind and water to move
northward once again, where they would have absorbed momentum
from the solid Earth once again, shifted southward and repeated
the process. The transfer of momentum must have been an extremely
complex affair.
 
14. The economic historian might consider whether the reduction
of the working time of the agricultural laborer from ten months
to nine might not have offset much of the increased productivity
gained by the improvements of agricultural technology during the
course of the eleventh and twelfth centuries.
 
15.  "Sigeberti continuatio Aquicitense," MGHSS 6: 421, 17.
 
16. A number of depictions of the Labors of the Months, generally
from the later twelfth century, assign the vintage to October.
 
17. "Richeri gesta Senoniensis ecclesiae," MGHSS  25:268, 35-39.


 
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