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Here we discuss the orientation of the tumulus of Bryn Celli Ddu along the direction of the sunrise on the summer solstice. Probably, this was also the original orientation of the henge, on the site of which the tumulus was built. Keywords: Archaeoastronomy, Archaeology, SunCalc software
The Summer Solstice at Bryn Celli Ddu
Amelia Carolina Sparavigna
Politecnico di Torino
Here we discuss the orientation of the tumulus of Bryn Celli Ddu along the direction of the sunrise on the summer solstice. Probably, this was also the original orientation of the “henge”, on the site of which the tumulus was built.
Keywords: Archaeoastronomy, Archaeology, SunCalc software
Bryn Celli Ddu is a prehistoric site on the island of Anglesey (map in the Figure 1). As told in , the name means a very evocative 'mound in the dark grove'. Today the site is occupied by a burial mound and people can visit its interior through a stone passage leading to the burial chamber (Figure 2). A mysterious pillar within the burial chamber, “carved with sinuous serpentine designs, and the fact that the site was once a ‘henge’ with a stone circle”, have attracted the interest of researchers. In particular, archaeoastronomers are attracted by the passage of the mound which is linked to the summer solstice.
Figure 1 - Map of the region (Courtesy Google Earth).
Figure 2 - Left. The mound and the northeast entrance (courtesy Rhion, Wikipedia). Right. The mound in a Google Earth image.
Norman Lockyer was the first researcher that proposed the link between Bryn Celli Ddu and the summer solstice . Lockyer, scientist and astronomer, credited for the discovery of helium, is famous among archaeoastronomers for his book “The dawn of Astronomy”, 1894 . In this book, he studied the role of the sky in the temple-worship and mythology of the Ancient Egyptians. Then, in 1906, he published his work on “Stonehenge and Other British Stone Monuments Astronomically Considered” . This was the first systematic study of megalithic astronomy . In this study, he argued that the passage in the tumulus of Bryn Celli Ddu marked the summer solstice. But, as told in , at the time Lockyer’s belief was met with a great deal of skepticism. At last, in 1907-98, the research by Christopher Knight and Robert Lomas showed this solstitial orientation to be true [1,5]. Then, Steve Burrow, curator of Neolithic archaeology at the National Museum of Wales, also supported the case for summer solstice alignment with a direct experience. As reported in , “he had to visit the site twice before his discovery. On the last day of his second visit he said he was "absolutely elated" when the sun filtered through nearby trees and entered the chamber along the five metre-long entrance passage”.
About the alignment of the passage, let us also remember that in 1991 , Miranda Green, in her book on the sun-gods of ancient Europe, proposed Bryn Celli Ddu aligned along the direction of the sunrise on the First of May. However, this is not possible, as we can evidence using a software giving the sunrise direction on satellite maps. Actually, many software applications exist which can give us information about time and azimuth of sunrise and sunset. Particularly interesting are those applications that are showing solar azimuths on satellite images: among them, we have Sollumis at sollumis.com and SunCalc at suncalc.net that we have largely used in previous papers (see for instance [8-14]). SunCalc.org is providing also a free Android app for smartphone and tablet. For the motion of the moon, we can use MoonCalc.org or the Photographer’s Ephemeris, at http://app.photoephemeris.com/. Alignments along the moonrise and moonset lunar standstills can be investigated by means of these tools as we did in Refs. 15 and 16 for instance. In the case of Bryn Celli Ddu, let us use SunCalc.net. We can see the results in the Figure 3.
Figure 3 - Thanks to SunCalc.net, we can see the direction of sunrise on June 21 and the First of May. Only the alignment along the sunrise on the summer solstice is possible. The yellow line represents the direction of the sunrise, the red one that of the sunset. The orange arc shows the apparent motion of the sun in the sky. In the left panel, the sun is reaching the highest altitude at the latitude of Bryn Celli Ddu.
From the Figure 3, we understand that the rays of the sun reach the passage only a few days around the summer solstice. It is therefore an alignment different from those of the sites of Maeshowe, Orkney, and Newgrange, Ireland, for instance. These two sites are linked to the winter solstice. In the case of Maeshowe, the “entrance corridor is so placed that it lets the direct light of the setting sun into the chamber for a few days each side of the winter solstice, illuminating the entrance to the back cell” [17,18]. We can see the alignment in the SunCalc simulation (Figure 4, left panel).
In the case of Newgrange, the tumulus is famous for the illumination of the passage and chamber by the sunrise on winter solstice (see Figure 4, right panel) . “Above the entrance to the passage of the mound there is an opening called a roof-box. On mornings around the winter solstice a beam of light penetrates the roof-box and travels up the 19 metre passage and into the chamber. As the sun rises higher, the beam widens so that the whole chamber is dramatically illuminated” .
Figure 4 – Simulations by SunCalc.net on December 21 for the sites of Maeshowe (left) and Newgrange (right). Maeshowe is aligned along the sunset, whereas Newgrange is along the sunrise direction.
It is evident that these three tumuli were linked to the sun. In fact, these sites were displaying a hierophany - a manifestation of the sacred - being the buildings able to bring to light the periodic behavior of the sun. As told in  for Newgrange, the tumuli were displaying a “one-off” phenomenon “in the solstitial hierophany” [22,23].
The orientation of Bryn Celli Ddu is very interesting. It is towards the summer solstice, that is, it is oriented like the site of Stonehenge . In fact, as explained in , before being a tumulus, Bryn Celli Ddu was a stone circle. “In around 3000 BC a henge monument was constructed. … Some 1000 years after the henge was built, the site was radically altered. … In its place a passage grave was built” . This could explain the orientation towards the summer solstice of Bryn Celli Ddu, different from that of the Maeshowe and Newgrange tumuli. The orientation was probably that of the previous henge, a place of worship and celebration of the summer solstice.
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 A very small effect of the Earth’s axis precession has modified the hierophany at Newgrange as discussed in [20,23]. Today it is slightly different from that observed by the builders of the structure [20,23].
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Sparavigna, A. (2017). The Summer Solstice at Bryn Celli Ddu. PHILICA.COM Article number 1057.