BALTICA10

The Orientations
of Central Alentejo
Megalithic Enclosures
FERNANDO
PIMENTA,
LUÍS
TIRAPICOS
If we consider the Sun or Moon to be the most probable
astronomical targets, there is apparently an interest
in declinations around that of the equinoxes. It is
generally accepted that there are technical difficulties
and no clear reasons for precise equinoctial orientations
(Ruggles 1997). Nonetheless several natural
signs from flora and fauna can be used together with
astronomical events to mark seasonal changes. If for
northern latitudes, the extreme limits of the solar and
lunar azimuths can represent a strong motivation for
special rituals, in lower latitudes where there is a more
temperate climate, a similar motivation can occur at
the beginning of spring and autumn.
The surveyed sites can be thought as a scenic/theatrical
space facing the “stage” of the rising heavens.
238
Fig. 6. Circular bar graph for the symmetry-axis declinations
of 8 enclosures.
Curve fitting was done using LAB Fit 4 software that
handles variables’ uncertainties and produces the full
resulting covariance matrix. This software also provides
an error propagation calculation based on the
standard expression for the absolute error. This function
was used for the determination of the final uncertainties
for arctan(q) or arctan(-1/q), and also for the
declinations. For the declination calculation the astronomical
refraction effects were calculated using G. G.
Bennett’s formula (Meeus 1991, p.102) for a temperature
of 15º and a pressure of 1010 mbar. We used an
uncertainty of 0.5º for the horizon elevation, including
the uncertainty in the refraction effects, and 1” for the
latitude (measured by GPS).
Discussion
As is clear from Figure 6, the orientations are all in a
narrow range, close to the direction of due east. Since
there is a very low probability of this happening by
chance (~7x10 -7 , using the expression 5 n*(θ range
/360)
n-1
(6), with n=8) and there are no common horizon features
that could justify such a pattern, we believe that
only two explanations are possible: either an astronomical
target (Sun, Moon or planets) or a construction following
the slope, and thus as a consequence facing the
far horizon, since we verified that the azimuth of the
symmetry axis and the azimuth of the steepest slope
have a correlation coefficient of 0.7.
4
da Silva, Wilton P. and Silva, Cleide M. D. P. S., LAB
Fit Curve Fitting Software (Nonlinear Regression and Treatment
of Data Program) V 7.2.36 (1999-2007), available
from www.labfit.net.
5
See Ruggles 1999, p. 95.
Conclusions
We can conclude from the data that the enclosures do
not seem to have been built just following the slope,
but instead probably pointed to an astronomical target.
There seems to be an interest in declinations that correspond
to the Sun at the beginning of spring or end
of summer or to the Full Moon at the beginning of autumn
or end of winter.
It is interesting to compare our results with the declination
distribution for the dolmens in the Alentejo according
to Michael Hoskin 6 (Hoskin 1998, 2001, 2002).
While possible solar declinations are also possible lunar
declinations, Occam’s razor argues here in favour
of a solar orientation, since there are no exceptions outside
the declination range from –24º to +24º. Hoskin
interpreted this distribution as an orientation towards
the rising Sun at the end of winter or the beginning of
autumn, and probably the latter, since agriculture demanded
less attention at that time of year, leaving time
available for the construction of communal tombs. It is
possible that, through cultural continuity, ritualistic use
of the enclosures around the beginning of autumn or
the end of winter, and particularly at Full Moon at the
beginning of autumn, could have led, later in the Neolithic,
to the construction of dolmens oriented towards
the rising eastern horizon and particularly to the Sun,
mostly in the same period of the year.
Acknowledgments
We wish to thank Nuno Crato, Carlos Daniel Paulino,
Clive Ruggles and Manuel Calado for their helpful
comments on early drafts of this paper. We wish also to
express our gratitude to Andrew Smith from Adelaide
University for the horizon elevation and distance-to-
6
See Figure 9.