White, Raymond E.

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Apéndice 5
DETERMINING THE ORIENTATION OF LE BASSIN
MONUMENTAL DE BIBRACTE
Raymond E. WhiIe*
1. INTRODUCTION
On 21 July 1990, my colleague, Dr. Donna E.
Swaim (D.E.S.)’,andl (R.E.W.) visited the archacological
site on Mont Beuvray (the mountain was known to the
Romans as “Bibracte”, and was, in fact, visited once
by Julius Caesar), near Autun, France. It is currently
under investigation by the “Projet Internationale du
Mont Beuvray”. The visit was made as part of a longer
journey we had made as co-leaders of a University of
Arizona travel-group,visitingprchistoric sites ofcultural
and astronomical importance.
Our visit was made at the express invitation of a
joint French/Spanish archacological team lcd by Drs.
Martin Almagro-Gorbea (A-G)2 andJeanGran-Aymerich(G-A)3 because they have discovered a hassin they
presumed was astronomicaly oriented. The ground
orientationof/e bassinmonumentalhasbeenprcviously
determined from the orientation of the archacological
grid systcm established by carlier investigations; the
objcctive of the present work was to establish the
relationship of the ground orientation of le bassin to
the Horizon, or Alt-Azimuth, System of astronomical
coordinates.
2. THE SITE, AND OBSERVATIONS MADE
THEREFROM
A series of timed observations for magnetic bearing
were made with use of a Brunton Pocket Transit, a
veryruggedandportablemagneticcompass (±20reading
error) and this author’s own personal digital wristwatch
(±Ls reading error). An independent log of the watch
error was maintained throughout the whole, six-week
long, joumey, only a three-day portion of which was
spent at Bourges and at Bibracte; the rate of the watch
error was found to be rigorously linear over the six-
week interval. As a result, the watch error on 21 July
1990 of obscrvations was found to be 12.0 s FAST;
alí timesof theobservations were corrected accordingly.
Three separate obsevations for the magnetic bcaring
of tbe major axis of le basgin were made, looking
generally from the SSW to the NNE; the average of
the threeobservations was 390 withnil standarddeviation
as alí three values were, fortuitously, thc same.
Next, the magnetic bearing of the sun was observed
at three different time intervals; the observational data
is shown in the short table, below. The magneticbearing
of the shadow of the sighting post of the Brunton
Compass cast by the sun was observed on the face of
the compass itself while thc compass rested levelly on
the grotsnd near le bassin. Such a technique results in
a back azimuth actually being observed; the correct
magnctic bearing is obtaincd by adding 1800 to the
back azimuth.
UT. 21 JULY 1990 Mag. Bearing of sun
Le bassin monumental is a low, lozenge-shaped or
quasi-elliptical, masonry structure which appears to be
a water collection basin; it is roughly 10.5 m. long
in the major axis and 3.6 m. long in the minor axis.
In the carlier publication (1989) by A-G and G-A, the
orientation of le bassin was made relative to the grid
system put in place by archacological surveyors. From
this work, they state that the long (major) axis of the
elliptical basin is directed 34,50 East of Magnetic
North.
* Steward Observatory, The University of Arizona, Tucson, AZ
85721, USA.
Humanities Program, Thc University of Arizona, Tucson, AZ
85721, USA.
2 Departamento de Prehistoria, Universidad Complutense, MA-
14h 23.Sm
14 32.8
14 41.8
2440
2500
2560
Table. The Timing and Magnetie Bearing Data for
the sun at Bibracte
The geographic position of the site (LE = 6022~32~4
= 46~55’37’) is welI known (A-G and G-A 1989);
together with the individual times of the observations,
thc True Azimuth of thc sun may be computed from
thc daily tabulations of apparent solar position in The
‘This value oftbe Longitude is with respect to the Prime Meridian
al Greenwich; it has been derived from the vatue used by A-G (1989),
which isrelative to the Paris Meridian, anil using L’(Paris)=2020’14E
(Astronomtcal Almanac 1981, p. 129).
DRID.
CNRS, Musde du Louvre, PARIS.
276 RAYMOND E. WHITE
Asíronomical Almanac 1990. Thc difference between
the True Azimuth of the sun and its Magnetic Bearing
at the samc time gives a calibration of the amount of
the so-called “magnetic deflection”; the mean value,
and its standard error, for the three separate evalua-
tions of the magnetic deflection value is as large as
the evaluation itself, it is 206 ± 2~5 (True North is
East of Magnetic North). While the standard error of
the mean deflection value is consistent in its value
with the estimated reading error of the magnetic
compass.
The azimuthal orientation, then, of the major axis
of le bassin is: 390 Mag. —2~.6 = 360.4 True; conse-
quently, the True Azimuth of the minor axis is 360.4
+ 900 = 1260.4. This value may be compared directly
to the predicted value for the December Solstice sunrise
point for a site with a fiat astronomical ho-rizon at
the same geographic position, i.e.: 1250.6 (figure 5.1).
The difference between the two values, 00.8, may be
accounted for easily by the effcct on the observed path
of the sun by a low-lying hill in the line-of-sight to
the sunrise point; having visited the site, there are
abundant low hilís inOppidum‘5 immediate neighborhood.
If the hill is no more than three-quarters of a degree
high, as seen from le bassin, the actual sunrise will
be slightly dclayed for a minute or so ... but the observed
azimut of the point of actual sunrise will be shifted
by 00.8 further towards the South. Theseapparent shifts
are the result of the diumal path of the sun being tilted
to the Horizon by, roughly, 430 towards the South. The
situation of a 10~ mask occluding the true horizon has
been illustrated in figure 5.2: the apparent rising point
of the Sun (at the time of December Solstice) has been
Diurnal Path or Sun at Bibracta. Dec. Solatíce Sunrise
20
AY.. SIl lo
90 ‘00 lIC 120 130
E rsA,ímuo> 1-1 ->
o
u’
1.10 >50
Fig. 5.2.—Diurnalpath ofSun atRibra cte. DecemberSolstice
Sunrise. E/se effect of an opaque mask (“Id!!”) upon the perceived
risingpotnt of<he Sun al <he latitude offlibra cíe: <he daity (“diurna!”)
path of Use sun is shown os an iníerrupíed set of filted squares when
1/se son is behind 1/se rnask, and a connecíed set offilled squares odien
it is wetl up aboye o/se rnask. The abscissa is catibrated it, tens of
degrees of True Azimut/o, frono due East (E) at 90’ <o 150’
shiftcd southwards (towards greater values of the true
Azimuth) by nearly 160 from the sun’s rising point on
the true hotizon. It is, therefore, imperative, that an
accurate profile of the local horizon should be determi-
ned for the site. The problem facing thc Bibracte
archacological work is that the area around thc site
has become hcavily forested which blocks the view
of the local horizon available to the Celts. Topographic
maps of the area can be used, alternatively, to deter-
mine horizon profiles of sufficient accuracy for this
phase of the work.
3. DISCUSSION
It is clear that the minor axis of le bassin monu-
mental de Bibracte is oriented towards thc local point
of sunrise on the December Solstice, the most important
day of the year for prehistoric agrarian populations and
for use in their calendrics: the last day of the sun’s
journey to the South ... if alí goes wcll!. Por the carly
agrarian peoples, the daily raising and sctting of the
sun was not the phenomenon guaranteed by the Laws
of Physics and Orbital Mechanics we take for granted
today: each day was an affirmation of their trust in
their gods.
Phcnomena which we know, now, to be cyclic and
governed by PhysicalLaw werc,by and large, perceived
to be “accidental” occurences by thc Ancients. Grcat
uneasiness must have been generatedamongstthc general
populace by te sun’s annual excursion into the South:
as it went further and further southwards, and the
exterior temperature dropped lower and lower, thc
people must have become more and more anxious as
the sun approached the December Solstice. Thc people
must have wondered: would the sun turn around and
head back northwards ... as it had always done before
or, this time, continue its southerly journey into
Eternal Night and freezing cold?. Imagine the relief
when, each year, the sun stopped and thcn turned back
northwards. “Magically”, it would soon become warm
again. It certainly would have been a fine excuse for
a celebration!. The RomanSaturnaliawere the celebratory
feasts of the sun’s ultimate return to the North, which
were derived from the similar festivals of the Ancient
M
Fig. 5.l.—Ground orientation of le basgin monumental de
Bibracte, showing the deviation ofTrue Northfrom Magnetie
North and the direction of the December Solstice rising sun
(VIS).
ORSENTATION
Greeks and dedicated to their god Chronos (te Romans
Saturn). the god of time.
It is equally clear that more precise observational
tcchniques need to be brought into play at Bibracte:
hand-held, even ground-supported, portable instru-
ments don’t have the necessary accuracy with which
tomake observationalcalibrations ofmagneticdeflections
and attachmcnts of local to astronomical coordinate
systems. The differencc of exactly 36,4-36,16 = 0,240
bctwccn thc measured magnetic bearings of the long
axis of the basin by A-G (Apéndice 6) and by this author
is not significative. However, if orientation information
about structures on site is to be gleaned from their
relativealignments within the grid system, then abundant
care must be taken to ensure that alignment errors of
the entire grid are individually no worse than ±1’.
4. AC KNOWLEDGEMENTS
Wc. D.E.S. andR.E.W., thank Drs. Martin Almagro-
Gorbea and Jean Gran-Aymerich for inviting us to
participate in the activities at their rcsearch center at
Bourges. and for permission - and encouragement -
to visit the site of le bastin monumental de Bibracte;
their logistical support to us while we were at Bourges
is recalled here with great pleasure and appreciation.
REFER ENCE
Almagro-Gorbea. M. and J. Gran-Aymerich, 1989,
“Le bassin monumental du Mont Beuvray (Bibracte)”,
Monuments et Mémoires de la Fondation Piot 71,
pp. 21-41.
277



Determining the Orientation of the Bassin monumental de Bibracte

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Determining the Orientation of the Bassin monumental de Bibracte

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