1. Megalithism and Prehistoric Astronomy

The construction of megaliths are directly related to astronomical phenomenon, including the sun, moon, and stars. The general phenomenon of megalithism in Europe is a very old one in archaeology. These monuments, built with massive stones sometimes weighing more than 50 tons, appear all over Atlantic Europe (see Fig.1).

According to Vítor Oliveira Jorge (1987) the understanding of european megalithism only truly set off after dendrochronology was used to calibrate radiocarbon dates. Colin Renfrew (1999), based on the new dates, introduced two fundamental aspects to the study of prehistoric Europe: first, that the megaliths were independent creations of Neolithic peoples of the European Atlantic coast; and second, he tried to explain its origins in palaeo-sociological and demographic terms, thus suggesting that the similitudes between European megaliths were due to similar social conditions (Jorge 1987, p.219). Renfrew proposed that the farmers belonging to the Neolithic culture, advancing from the Middle East, as genetic analysis suggests (Cavalli-Sforza 2001), upon reaching the expansion barrier that is the Atlantic coast, would have developed a particular sense of territory. The megaliths would be erected to mark those territories.

More recently however, with more data from the Mesolithic, archaeologists are beginning to see the Neolithic, at least the Early Neolithic, as a continuation of the Mesolithic, as opposed to a cultural invasion by Middle-Eastern farmers (Tilley 1994). Tilley argues that Neolithic monuments were built in places that were already important and being used by Mesolithic populations as 'ceremonial meeting-points on paths of movement' (Tilley 1994, p.109). He also suggests that their purpose, or at least, one of them, was to draw attention to natural markers in the landscape. These markers would probably have a symbolic, social significance, much like the natural markers in Aboriginal Australia's walkabouts.

Another hypothesis is that these monuments marked astronomical events. In fact some view the celestial sphere as a natural extension of the landscape, and thus archaeoastronomy as a tool complementary to landscape phenomenology (Sims 2009). Neolithic cultures had no written language, which means that there are no written records of their beliefs, their cosmology. One can only infer them from what they have left behind, namely the megaliths themselves, occupation sites, and other archaeological artifacts.

Archaeoastronomy helps by discerning whether astronomical objects like the sun, moon or stars, or events, like full moon or summer solstice, held any significance for the megalith builders. As an example, Ruggles (1999) did a massive survey of over 300 megalithic monuments of various kinds in the British Isles. His results show a preference for alignments with lunar standstill. The movement and position of the moon was of obvious importance . Such an enormous amount of monuments oriented towards a single event, especially one that only occurs every 18.6 years, must mean that it had some special meaning of significance for the people who built them under intensive labor conditions. These monument also probably served to attract massive gatherings and ensuing rituals (Sims 2006; Sims 2009).

According to Sims (2006), that the lunar standstill was an important ritualistic event comes naturally out of a transition, a counter-revolution, from an egalitarian, femalecentred, moon-based Palaeolithic hunter-gatherer cosmology (Knight 1995; Marshack 1991) to a hierarchical, patriarchal solar cosmology characteristic of Neolithic pastoralists and farmers.

A massive survey of around 2500 megaliths in Southern Europe has been done by Michael Hoskin (2001) with help from various colleagues. They found that most megalithic monuments have an orientation due East, or between the Summer Solstice sunrise and the Winter Solstice sunrise azimuths, although there are outliers. They concluded that the sun was at the centre of megalithic cosmology in Mediterranean Europe, with a few punctual exceptions.

This paper's focus is on a particular group of megaliths, the dolmens, or antas (in the original portuguese) of central Portugal. These monuments are very simple in their architecture: they have a central polygonal chamber, made out of megalithic orthostats, typically 5 or 7, which is where the bodies and other votive offerings were laid. The chamber might then have an entrance with or without a corridor, also built with orthostats. Both the chamber and the corridor were covered with covering stones and the whole construction covered by a mound of earth and/ or stones, the tumulus or cairn. At its prime, a typical dolmen would look like an artificial mound, with a single opening (see Fig.2).

In order to infer the cosmology associated with these megaliths it is essential to understand what the archaeological records tell us about their builders, and how astronomy could fit in their lives, which is explored in section 2. In section 3 the data gathered in a new survey of dolmens in central Portugal is presented and discussed. It is argued that the naive association between megalithism, astronomy and Neolithic seasonal economy put forward by previous authors needs to be reviewed in light of the evidence. Future avenues of research are considered in section 4. For the more avid readers appendix A includes all the methodological details of the survey and appendix B all the raw data gathered.

2. The Megaliths of the Mondego platform and their Builders

The region of central Portugal is the second densest in the country in megalithic monuments dating back to the Neolithic. The Mondego platform, located in central Portugal (see Fig. 3) is bordered by the Central Massif to the southeast, the Marginal Massif to the west and northwest, and the Douro basin to the north. The area covers most of the drainage basins of the upper Mondego, the Vouga and part of the Douro rivers. The Central Massif, especially the Serra da Estrela, offers good pastures during the spring and summer seasons and has evidence of human usage since the fifth millennium BCE. The archaeological record has allowed archaeologists to come up with a model for the life of local communities during the Neolithic which is presented next.

2.1 The Early Neolithic

Discoveries of the past twenty years, especially of occupation sites in the Mondego basin, have allowed a clear picture of local life during the fifth millennium BCE, the Early Neolithic, to emerge (Senna-Martinez and Ventura 2008a, pp.318-9). Small communities sustained themselves by small game hunting, the gathering of acorn and other winter fruits, and transhumant pastoralism. The raising of ovicaprids - sheep and goat, introduced to the region by the first Neolithic settlers - suggests that spring and summer were spent on high ground pastures and winter on lower ground (Senna-Martinez et al 1997, p.664). According to Senna-Martinez and Ventura, later developments can be interpreted as the consolidation and intensification of this seasonal form of economy. Pottery fragments attributed to this period show characteristics belonging to the traditions of the Early Neolithic from Estremadura, to the southwest, and Andalusia, to the southeast, allowing for the possibility of a dual origin (Senna-Martinez and Ventura 2008b, p.80).

2.2 The Middle Neolithic

The first megaliths were built at the turn of the Middle Neolithic, around 4000BCE, roughly a thousand years after the first Neolithic communities showed up in the region (Senna-Martinez and Ventura 2008a, pp.317-50). This first phase is characterized by monuments with a small polygonal chamber and a short corridor or none at all. Some tumuli have complex structures in front of them, including atria (see Fig. 4). These structures have been interpreted as 'scenic spaces', with the monuments being used as temples, with closed, possibly reserved, spaces (the chamber and corridor) and open spaces for the general audience (the frontal atrium). Also, in some dolmens one finds the first instances of megalithic art in the region. Artifactual evidence associated with the burials are of the same kind as that found in early tombs of neighbouring groups, namely Évora/Reguengos/Montemor in Alentejo, Serra da Aboboreira, Galicia in northern Spain, and Estremadura (Senna-Martinez et al. 1997, p.662).

These monuments can be seen as 'true anchors in the landscape for populations that, on the other hand, have a high seasonal mobility' (Senna-Martinez and Ventura 2008b, p.82). In this way Senna-Martinez and colleagues fit the monuments into their economic model: the necropoleis, by marking the territory, would legitimize their winter occupation.

2.3 The Late Neolithic

In the middle of the fourth millennium BCE there was a cultural shift that affected megalith building in the region. This is now understood as the transition to the Late Neolithic (Senna- Martinez and Ventura 2008a, p.333). Megaliths belonging to this phase are generally bigger and possess developed corridors with heights that are different to those of the chambers, now usually composed of 7 or 9 orthostats, and include intratumular corridors and frontal atria (see Fig. 4 and Fig. 5). This phase is also characterized by complete abandonment of smaller monuments from the previous phase. Big monuments within a necropolis are rare in this phase, as they tend to show up in isolation. The funerary deposits recovered from these sites also show an increase in complexity of the funerary rites, with new elements having been introduced, like arrowheads. Occupation sites in the Mondego basin appear in close proximity to the necropoleis and suggest an autumn and winter occupation, as evidenced by the presence of central hearths and earth ovens to roast and keep acorn. This is interpreted by Senna-Martinez and colleagues as further evidence that these sites were being used as part of the seasonal model they proposed.

2.4 Megalithic Astronomy

During Hoskin's survey of Mediterranean tombs, twenty-one dolmens in the Mondego basin and a further twenty-two in adjacent river basins were surveyed between 1994 and 1997 (Hoskin 2001, pp.104-6). The orientation of the dolmen's axis, that is, the orientation of the entrance or corridor was measured. The tombs on the Mondego basin were classified as oriented to the sunrise, even though there are a few outliers. Tombs in the adjacent Vouga, Paiva, Torto and Coa basins were classified as oriented towards the sunrise/sun-climb. This has been interpreted as a evidence for a solar-based cosmology, focused on the darker months. Senna-Martinez et al. (1997, p.669), assuming that the orientation of the tombs was chosen at the time of their building, claimed that the results are consistent with the mentioned seasonal economic model. Most of the dolmens were found to be oriented towards negative declinations, which, given the assumption, would indicate that they were built during the autumn or winter seasons, when the pastoralists would be in lower ground. Also, the only known megalithic alignment in Beira Alta seems to be aligned with sunrise at winter solstice (Senna- Martinez et al. 1997, p.669).

However, there are many problems fitting this model to the entire dataset. First of all, only Mondego basin monuments seem to prefer negative declinations. The set of orientations of monuments in other river basins show no such preference and, roughly, half of these monuments have positive declinations. Also, two of the huts in a winter occupation site have their openings towards the north-east (Senna-Martinez et al. 1997, p.670), indicating a possible positive declination, and hence a summer sunrise, even though one can't be sure without the horizon altitude.

To explore these issues, thirty one dolmens in the area were surveyed in March and April of 2010, of which eleven had't been previously measured by Hoskin's team. All measurements were made using a compass, clinometer and a GPS unit. For those interested, a discussion on the employed methodology in included in appendix A, including the reasons behind the choice of using a compass, the inherent dangers of doing so, and how to work around them. The raw data for all the surveyed sites can be found in appendix B.

3. The Data and its Implications

In Fig. 6 the declination of all measurements have been plotted. The figure includes the axial orientations, represented as coloured dots, the windows of degeneracy (see section A.2), plotted as error bars, and other measured orientations represented by crosses (see table 2, appendix B). The open circles around the dots represent measurements of sites where horizon visibility was reduced, and thus have an uncertainty of a couple of degrees. The red and pink horizontal lines mark the declinations for solstices and lunar standstills respectively, calculated for the time period and region under study using Starry Night planetarium software. Twenty of the dolmens surveyed had already been surveyed by Hoskin (2001), and his measurements are represented by the diamonds in Fig. 6. There are differences in the declination values between Hoskin's survey and this one. Most of the new values are within 3º of Hoskin's, which might indicate a small systematic error or simply due to the problem of precision, as discussed in appendix A.1

3.1 Testing the Seasonal Economic Model

One can combine the two data sets and plot all the declinations for each river basin. This has been done in Fig. 7 below. Dolmens with reference number higher than 31 were surveyed by Hoskin only and they are indexed in the order they appear in the literature. Looking at the figure it is clear that dolmens in the Mondego river basin do have a preference for negative declinations. This, as mentioned before, fits nicely into the economic picture of the Neolithic communities as described in section 2.

There are, however, four dolmens of the Mondego river that are oriented towards positive declinations. Of these, ref.no.28 is discussed below, while the other declinations, those of ref.no. 25 and 32, 33 (of Hoskin's dataset: Lapa da Recainha and Dolmen de S. Pedro Dias), if they are meant to be aligned with a sunrise, can be observed respectively nine, thirty-seven and twenty-six days before or after an equinox. These can still fit the same seasonal framework, by noting that equinoxes are not very obvious events in horizon astronomy (Ruggles 1999, pp.150-1). It is possible, then, that they were not observed as markers of time. Instead, Mondego basin communities would have been more flexible, and this was probably tied to the weather: after a very long winter, a community might have stayed in lower grounds well after the equinox, moving up to Serra da Estrela only when it was warm enough.

For the Vouga and Paiva river dolmens, however, when both data sets are combined, it seems that they are almost evenly distributed, that is to say, there doesn't seem to be any preference for either positive or negative declinations. Whereas for dolmens of the Torto and Coa river basins there seems to be a preference for positive declinations, but the samples are too small for one to be able to say anything about them. The data is therefore calling for one of two possibilities: i) either the economic model proposed by Senna-Martinez and colleagues applies only to Mondego communities or ii) megalithism doesn't fit the economic model in such a naive way.

In favour of the first possibility, there is the fact that no occupation sites have yet been found in river basins other than the Mondego and so there is no precise picture of Neolithic life in those areas. The dolmens are themselves small and could have been built by small communities that could very well have independent existences, with different ways of life and/or ideologies. It is true that funerary artifactual remains are of the same kind as the ones found in Mondego tombs, but this shouldn't come as a surprise as they are also similar to the ones found as far south as Alentejo as as north as Galicia, even though life in the plains of Alentejo, for example, had to have been different. There are also small but significant architectural difference between dolmens in the different river basins, especially between the Vouga and Mondego rivers (Ventura 2010, personal communication).

The second possibility is, perhaps, much more tenable. This is to say that megalithic ideology is much more complex than previously assumed. For instance, the megaliths in the Vouga, Paiva, Torto and Coa basins are on higher elevations than the ones in the Mondego, this would go against a possible construction during winter, which is the lower ground season. Also, in this respect, as mentioned above, two of the habitation huts are oriented towards north-eastern azimuths. If the same logic that was applied to the megaliths was applied to them a paradox between the evidence for winter use (the hearths and earth ovens) and the orientation towards summer sunrises emerges. A new model would then be needed, and such a model would need to explain all the data, including why Mondego basin monuments prefer negative declinations. A few of the outliers identified by this survey suggest alternatives.

3.2 Outliers and Alternatives

The Orca de Pendilhe (ref.no.5) has a very low declination, lower than both the sunrise or moonrise lower extremes. This is most likely due to the fact that this tomb's corridor was reconstructed (Cruz 2000, p.35), whereas the orientation of a line parallel to the two surviving original corridor orthostats gave a very different declination (the cross of ref.no.5 in Fig. 6), well within the lower limit for the southern major lunar standstill. Thus, this dolmen suggests a moonrise alignment, possibly to a southern major standstill limit.

Another particularly interesting outlier is the Ameal necropolis in Carregal do Sal, composed of two Middle Neolithic dolmens, Orca 1 (ref.no.28) and Orca 2 (ref.no.29), separated by a mere seventy-five metres (Pinto and Senna-Martinez 2006, pp.51-2). The former's opening is oriented towards a very high positive declination, higher than the summer solstice and northern major lunar standstill declinations, and this is also one of the possibilities for the second dolmen, whose badly damaged state made it difficult to discern which one was the original entrance (all three hypothesis are marked by crosses in Fig. 6, two of which are almost superimposed) (Ventura 1998, pp.30-51). It is therefore a possibility that this very early necropolis was oriented towards the northern sky. This might indicate a circumpolar star based ideology, most likely associated with death (the dolmens were, after all, tombs), not dissimilar to what is found in the Egyptian Predynastic and Early Dynastic (Malville et al. 2007; Lehner 1997, p.33; Belmonte 2001). If this is true, the local Middle to Late Neolithic transition would have entailed a cosmological transformation not unlike Sims' counter-revolution in the British Neolithic (Sims 2006), but starting from a star-based cosmology. There is evidence of such a stellar to solar transformation in the early periods of Ancient Egypt as well (Hart 2005, p.153).

4. Concluding Remarks & Future Work

In this paper it was argued for a reinterpretation of the data concerning alignments of megalithic dolmens in central Portugal and beyond. In his conclusion, Hoskin extends Senna- Martinez and colleagues's assumption that the corridor and entrances to the tombs were 'laid out to face sunrise on the day that construction began' (2001, p.213) to the whole of the Iberian Peninsula. Such a view considers all the outliers as anomalies. There is however a different way to approach the dataset, and that is not to disregard the outliers which should also be explained by the model. Some of the stones in these tombs required more than 30 people just to move them, and the average amount of time to build a Late Neolithic dolmen exceeds the 880 hours of work (Senna-Martinez and Ventura 2008a, p.337), that is more than a month if nothing else was being done, but most likely two to three months of labour. It is very difficult to accept the builders wouldn't notice if they had done a mistake, that they wouldn't stop working or readjust a tomb that wasn't aligned to the right astronomical event.

In this sense, the entire dataset for central Portugal, which includes many monuments oriented to positive declinations, is evidence that a new model, a new way to tie-in megalithism with the Neolithic lifestyle and thus infer their ideology, is needed. Discovery of occupation sites in the Vouga and Paiva regions would help get a better archaeological picture of these regions. Statistical analysis might also shed some light on this fact, and it would be interesting to see whether the dolmens in these basins are statistically distant from the Mondego ones in the approach taken by González García and Belmonte (2010), indicating the possibility of differing groups, and possibly ideologies, in the area.

Some of the tombs suggest a lunar or even stellar alignment. The possibility for alignments not with sunrise, but with moonrise is also not discounted by the dataset. This would be in line with previous findings in the British Isles (Sims 2006; Ruggles 1999). The Ameal necropolis, dating to the Middle Neolithic and therefore one of the earliest, suggests a northern sky alignment. There is another dolmen dated to the same period and near Ameal, Orca 2 de Oliveira do Conde and although it was found in a badly damaged state, it might still be possible to measure the orientation of its entrance (Pinto 2005, pp.52-3). There is also at least one other dolmen close-by, Cabritos 2 in the north bank of the Douro basin, surveyed by Hoskin, that also has a high positive declination of 47º (Hoskin 2001, pp.106-7), practically the same as Orca 1 do Ameal (ref.no.28) and that might be worth visiting. These possibilities will be the focus of a second stage of the survey.

Acknowledgements

The author would like to thank the, too numerous to name, local councils and museums that were helpful not only in allowing the measurements to be made but also in providing documentation on the sites, as well as all those that accompanied him during the survey, and those that read and commented on this manuscript. This work was done while the author was being supported by FCT (Portugal) fellowship no. SFRH/BD/27249/2006.

The dolmens of central Portugal were covered by a tumulus or cairn, always round or slightly ellipsoidal, with a single opening where the corridor (and other scenic spaces, if present) lies. The very circular symmetry of these monuments, their simplicity when compared to British, Irish or French monuments of the same period, the lack of contemporary archaeological remains outside the chamber and scenic spaces, and the paintings within the chamber, and in some cases the corridor as well, suggests that the only meaningful axis was the corridor/entrance axis. The key measurement to take was then the orientation of this axis. All measurements were made using a compass, clinometer and a GPS unit. In this appendix, the employed methodology is explained.

A.1 The Problem of Precision

Even when a long corridor has survived intact, or been accurately restored, the determination of it's orientation is plagued by difficulties. The inherent non-uniformity of the orthostats used, as well as irregularities in their laying, might mean that their builders either were limited by technology, or that a perfect alignment of the corridor walls simply wasn't important to them. In either case approximations have to be done on the field to determine the central axis of the tomb. This means that slightly different techniques will give azimuth values that can differ by as much as two to three degrees (Hoskin 2001, pp.11-2). Under such conditions, measuring to an accuracy of arc-minutes, as can be achieved by using a theodolite, seems superfluous as the instrument's accuracy won't translate into orientation accuracy.

Whenever a corridor was present the axis was determined by halving the lengths of it's openings, placing a surveyor's rod on each and taking care for them to be as vertical as possible (see Fig. A1). They were then used as foresights to measure the azimuth, az0, with the compass. For Middle Neolithic tombs, however, or where the corridor didn't survive, a yet greater approximation was taken by halving the length of the chamber entrance, and using the centre of the backstone as a second foresight.

A.2 Degeneracy in Orientation

However, one shouldn't assume that our Western notions of axis, orthogonality, or even of corridor and line, were popular six thousand years ago. In fact, megalithic art seems to prefer curvilinear motifs to straight lines (Jorge 1998; Lewis-Williams and Pearce, 2009; Power 2004). Therefore even though, from one's modern understanding, the corridor is precisely oriented, the intention of its builders might have been a different one. It is possible that the corridor was only meant to indicate a general direction, and a feature of the landscape would indicate the precise point of the alignment. Another alternative is that light, from sun, moon or star, was meant to shine within the chamber, but not necessarily on the backstone. We then have a range of possible axes that, if looking only to the corridor or chamber entrance, one cannot say which ones are meaningful. This is akin to the concept of degeneracy in quantum mechanics, where one cannot distinguish between two quantum states with the same energy level. To distinguish them one has to measure other quantities, much like in this case where one needs to take into account the horizon landscape and the presence, or likelihood of presence, of drawings or engravings within the chamber's orthostats.

This inherent degeneracy in the exact orientation can be accounted for by measuring the window of visibility to the horizon, from the chamber, that each corridor permits. In this way one measures a maximum azimuth range that includes all the horizon points at which light from a rising astronomical object would shine within the chamber. This is, of course, a one dimensional measure of said degeneracy. A more complete measurement would have to include the height of the corridor and chamber entrances, that is, the maximum altitude that the sun, moon or star could be so that its light would shine within the chamber's orthostats. However, most of the surveyed tombs have no surviving corridor cover-stones so such a measurement is an approximation at most, but this should be considered in the future.

This was achieved by using a steel surveyor's tape to determine the diagonals of the corridor, that is, the maximum lines of visibility that the first and last sets of corridor orthostats permit from within the chamber. Surveyor's rods were then placed along these lines and their azimuths taken (see Fig. A1 and Fig. A2). In the appendix these measurements have been labeled azN and azS, meaning the azimuth, respectively, north and south of the axial azimuth az0. This means that the northernmost horizon position (lowest azimuth) that, for e.g., sunlight can shine within the chamber is azN, illuminating the right side of the chamber. The other extreme position, azS, is the southernmost rise position for light to shine, this time, on the left side of the chamber.

Measuring such a range while in the field, also allows one to focus attention on that particular region of the horizon when looking for features that might have been used as foresights, or simply just markers, for astronomical risings (see Fig.A3).

A.3 Magnetic Deviations

Using a compass on the field, even when it makes more sense to use one than a theodolite, is still problematic due to local magnetic deviations (Ruggles 1999, p.165). These include local magnetic anomalies, due to ferromagnetic material in the vicinity, and the magnetic declination, that is the difference between magnetic north and true north. Local magnetic anomalies are relatively easy to minimize. One needs only to measure azimuths in both directions so that later one can take the average of the two measurements. During the survey this was done for all measurements of horizon azimuths. The magnetic declination, however, varies from place to place, and from time to time, and even though it is relatively easy to find it in military or other detailed maps, or even estimated values on the web (via NOAA's Geophysical Data Center), there are nagging uncertainties involved in both, and it is always better to measure it on the field.

One way to do this is to use a combination of compass and GPS measurements. By taking measurements of azimuths of visible landmarks whose coordinates can then be measured with the GPS unit, one can calculate their true azimuth and compare it with the magnetic one. If this is done for multiple landmarks, lying radially from the dolmen, one can take the average of the magnetic deviations.

This technique proved to be very exact, always giving a magnetic declination that was, at most, within a degree of the estimated value obtained from NOAA's website. For six or seven measurements this comes down to 0.6º, with an error of only 0.4º for at least one site (ref.no.1). Note, however, that backsighting was not done for these measurements, but one would expect that the resulting values would be further improved by this.

B. Corpus Mensurarum - Spring 2010 Survey Data