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 Location: Ynys Mon, Gwynedd, Wales.  Grid Reference: 53� 12' 30" N. 4� 14' 20" W.


      Bryn Celli Ddu: (Passage-Mound, Henge).

One of Wales most important megalithic sites.

This passage mound encompasses a single white pillar inside and, along with a few other select sites in Europe, is  believed to have been constructed with the ability to measure the year constructed into its design.

The passage is orientated towards the rising summer-solstice sun.

(Plan of Brynn Celli Ddu)



   Bryn Celli Ddu: 'The Mound in a Dark Grove'

Passage mound with structural similarities to contemporary Irish and Scottish monuments.

The use of the site appears to have changed over time. It started as a stone circle surrounded by a bank and internal ditch. In the centre was a standing stone covered in carvings, including spirals and zigzag patterns. A Passage grave was built inside the ditch, with a North-east entrance. Finally, the whole passage was covered with a cairn. Some time before the pillar and its surrounding megaliths were erected, the stone was taken down and buried in the middle of the original ring, outside the chamber. This stone has been taken from its ancient setting to be stored in the National Museum of Wales in Cardiff, and a replica casting has been erected in the socket of the original stone. (16).

An exploration of the site in 1928 discovered a pit at the centre of the henge in which a fire had been set, over which a stone slab had been laid with a human ear-bone beneath.

On the ridge to the north of the site is another tall standing stone.

The mound contains kerbstones, which would have originally been covered up to similar to those at the Boyne-Valley, Ireland.

The postholes discovered outside and previously thought to be contemporary with the tomb (c 3,000 BC), have been dated to over 6,000 years ago. (3).

A replica of the carved standing-stone now stands behind the mound and the original is in the Museum of Wales. The patterns on this stone have been compared to similar engravings found in the Brittany region of France.


Article: BBC News (18 June 2006)

Bryn Celli Ddu Aligned to Sun.

An archaeologist has discovered that the passage into a burial mound on Anglesey (Wales) was built to catch the rising sun on the summer solstice. Steve Burrow said he was 'elated' when the sun filtered in through trees as he sat in the Bryn Celli Ddu chamber. He made the discovery as he researched a book about burial tombs in Wales from 4,000-3,000 BCE. Carbon dating on the site has also revealed it may contain the oldest building in Wales.
     Mr Burrow, the curator of Neolithic archaeology at the National Museum of Wales, said 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. "The emotion of seeing something that was put there deliberately 5,000 years ago was amazing," he said. "I was the first person to be recording the event so I was trying to record it with stills and digital cameras as well as on a video camera, but I was jumping up and down."
     The site is owned by heritage body Cadw, which has part-funded a radio carbon dating programme at the site. Testing has discovered that post holes outside the entrance to the chamber are 3,000 years older than the tomb itself. This could point to the site having the remains of the oldest building in Wales, added Mr Burrow.
     A video of the sun rising and entering the Bryn Celli Ddu chamber can be viewed as part of an exhibition called Death in Wales 3,000-4,000 BCE, at the National Museum of Wales in Cardiff until 24 September.


Astronomical Alignments.

The 'shadow gauge' on the pillar inside indicates where in the solar year you are (from September to April). (16)

At the winter solstice, the dagger of sunlight would have reached the top of the pillar but for a concrete lintel recently placed to secure the roof. (16)

The pillar and stone edge are also positioned to accurately measure the Venus cycle and the winter solstice. (16)

The spring and autumn equinoxes are marked by the sunlight moving onto the carved rock behind the pillar and splitting to light up the sides of the chamber with its patterns and cup markings. (16)

On the summer solstice the runs rays illuminate the chamber through the reflectivity of the quartz. For a few minutes only it is directed to illuminate a small carved double spiral on stone slab left of the entrance. It has been shown that this shape is achieved by focusing and plotting the suns path over 366 days. (16)

The passage was blocked except for a small opening that didn't allow access. (light-box). Platforms of white quartz pebbles, hearths and a structure surrounding the skeleton of an ox, found outside the mound, suggest rituals. (1)

Outside the passage a solitary standing stone stands in line with the passage and the summer solstice alignment.



   The Bryn Celli Ddu 'Lightbox':

The alignment with the solstice sunrise links Bryn Celli Ddu to a handful of other sites, including Maes Howe and Newgrange, both of which also point to the midwinter solstice. It has also been suggested that a feature similar to the 'lightbox' at Newgrange may be matched at Bryn Celli Ddu (1).

Light-boxes are a megalithic construction feature that have so far only been recorded at three (possibly four) sites in the UK, with the two in Ireland (Newgrange and Carrowkeel - below) both having the same design, and the other two on the Orkneys (Maes Howe and Crantit) in Scotland.

Newgrange (left), and Carrowkeel (right)


All of these sites have been shown to have been deliberately constructed so as to allow the rays of the sun (and/or moon) into the interior of the passages for very specific time periods only. One of the stones from the light-box at Newgrange (right) has a particular design on it which can be found at two other passage mounds: Gavr'inis in France, and Four knocks in Ireland.


At Maes Howe, the light of the setting solstice sun was restricted by the closing of a 'portal stone', placed into the side of the passage. In this way, at the right moment, the stone could be closed across the passage, and the light would only pass over the top (as at Newgrange). The same design feature is also present in the entrances of the three sub-chambers, each of which also had a blocking-stone which closes most of the hole, but not all of it. (These stones now lay on the floor in front of the holes). This particular astronomical feature is similar to 'light-boxes' found in other passage mounds in Ireland and Wales (Newgrange, Carrowkeel). A similar feature is believed to have been found on the Orkneys at the recently destroyed/restored Crantit Tomb.


The Bryn Celli Ddu 'Light Box'.

The Bryn Celli Ddu passage mound does not have a 'light-box' per se, but it does have a sophisticated means of measuring the year incorporated into its design, using the entry of light into the passage at certain times, in a similar way as seen with light-boxes elsewhere in UK.

At Bryn Celli Ddu, the passage-mound was designed in such a way so as that the light of the sun at relevant times of the year would penetrate the chamber and cause a beam of light to be cast on a 'Declination Gauge' made by the tall, cylindrical pillar placed at the back of the hexagonal chamber.

The changing altitude of the sun over the year causes the light of the sun that reaches the inside of the chamber to move up and down the pillar over the year. Notches have been found which support this theory.


The positioning of the stones at the entrance and along the passage restrict the light into a narrow beam which can be seen to move up and down the pillar over the year.


(More about light-boxes)




Geodesy on the Prehistoric British Landscape:

It has been observed that Bryn Celli Ddu is related to other important geodetic sites through 'Pythagorean' geometry, as the following diagram illustrates.

(English Geodesy)


(Other Passage mounds)

(Other Welsh Sites)





1). J. Manley. Atlas of Prehistoric Britain. 1989. Oxford University Press.
2). Pitts, M. 2006. Sensational new discoveries at Bryn Celli Ddu. British Archaeology No. 89 (July/August): 6.
3). http://www.britarch.ac.uk/ba/ba89/news.shtml
16). Lomas and Knight. Uriel's Machine. 1999. Century Books, London.


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