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Ireland's ancient astronomers and the alignment of our prehistoric monuments

Anthony Murphy outlines how the ancient dwellers built clever structures to work in harmony with the cosmos.

Anthony Murphy Journalist, author, astronomer

MANY PEOPLE ARE familiar with the UNESCO World Heritage Site of Brú na Bóinne in the Boyne Valley in Co Meath, and the most famous of its monuments, Newgrange.

On the shortest days of the year, the rising sun casts a beam of light through a specially-constructed aperture above the passage entrance and illuminates the central chamber of the monument about 20m (70ft) inside the giant cairn.

None of this would seem remarkable except for the fact that the monument was constructed by a community of apparently primitive farmers who lived by the Boyne about 5,200 years ago. That’s fifty-two centuries ago, a long time for a structure to remain standing, and functioning, by any measure.

The great “mega-mounds” of Newgrange, Knowth and Dowth were put together using giant stones, many of them weighing over one tonne and some weighing up to 10 tonnes, in an era before metallurgy and the wheel.

An extraordinary expenditure of human labour was required for their construction, and when you include the many smaller mounds or passage-tombs dotted around them, it is clear that the monument complex of Brú na Bóinne is truly extraordinary.

Many of the large stones forming the kerb, passage and chamber of Newgrange were hauled to the site from the coastline, at Clogherhead, a distance by sea and river of 31km (19 miles) in each direction. The monuments constitute an awesome display of human ingenuity and dedication.

However, they are not merely giant tombs; they have a fascinating astronomical function as well.

No coincidences

While many people would be aware of the winter solstice sunrise illumination of the interior of Newgrange, much less well known and equally fascinating is that one of its sister mounds, Dowth, has a chamber that gets illuminated by the rays of the setting sun on the same day.

It is now broadly accepted that this is not a coincidence.

Just over 42km west of Newgrange, at the megalithic complex of monuments on the hills of Loughcrew, near Oldcastle in Co Meath, a smaller but perhaps even older passage-tomb (known as Cairn T) with a cross-shaped chamber similar to that of Newgrange is aligned so that the light of the sunrise at spring and autumn equinoxes shines onto a beautifully-decorated stone in its chamber.

The Stone Age cairn on the summit of Slieve Gullion in Co Armagh features a short passage leading into a small chamber where, at sunset on the shortest days of the year, the sunlight illuminates the interior. This passage-tomb actually points to Cairn T at Loughcrew over 60km to the southwest.

In 1999, Richard Moore, Michael Byrne and I discovered that the larger of two megalithic standing stones at Baltray in County Louth was arranged in a dramatic alignment with the Rockabill islands some 23.5km away in the Irish Sea.

On the shortest days of the year, an observer positioned at that large megalith would have seen the sun rising out of the larger island in the Neolithic. The stone is of the same type as those used in the construction of Newgrange.

Above are just a few examples of what appears to be the intentional astronomical alignment of prehistoric monuments in Ireland.

While the focus of all these would appear to be the sun, there is an increasing body of research that suggests there may have been alignments on other heavenly objects, such as the Moon, the planet Venus and even some stars, including the one known today as Sirius, the Dog Star, the brightest of all stars.

While the mysterious carvings on many of the stones at Brú na Bóinne and places like Loughcrew are said by archaeologists to constitute abstract and non-representational “megalithic art”, several of the stones at the largest of the trio of great hounds, Knowth, have been interpreted as calculations of lunar cycles.

Not just the sun

It is entirely appropriate to suggest that a farming community would have an obvious interest in the sun, which helped their crops to grow, but it is unlikely that their curiosity with astronomy stopped there.

In the Neolithic, the river Boyne was tidal at least as far as the eastern extremity of the Brú na Bóinne complex, and as the river was undoubtedly the main transport artery for the materials and workforce needed for building the monuments, it stands to reason that the mound builders would have studied the movements of the moon also.

It would be difficult to move large slabs of shale from Clogherhead to Newgrange by boat without recourse to the tides, and of course, the tides are regulated by the movements of the Moon.

A prolonged study of the wanderings of the moon would have revealed that its movements follow predictable patterns. Some researchers, myself included, have suggested that a competent observational knowledge of the movements of the moon in prehistory would not have been difficult for a society that had much clearer views of the night sky than we do today.

Without the hindrance of light pollution and atmospheric haze, the people of the New Stone Age would have enjoyed a pristine night sky, allowing them to track the monthly movements of the moon against the background stars of an ancient zodiac.

In time, they would have been able to perceive that even eclipses of the moon occurred in patterns and sequences that were entirely predictable.

One stone at Knowth, popularly known as the ‘Calendar Stone’, contains markings which can be interpreted as displaying a working knowledge of a 19-year cycle of the moon known to modern astronomers as the Metonic Cycle (so named because its discovery is attributed to a 5th century BC Greek astronomer called Meton).

The 19-year cycle, known to 18th-century antiquarian Charles Vallancey from his conversations with uneducated Irish peasants as Naoidheachda (literally ‘the nineteenth’), is a fortuitous occurrence whereby the lunar months and the solar year are finally seen to harmonise.

Tied very closely with the Metonic Cycle is the 18.6-year rotation of the lunar nodes, an occurrence that reveals the sequential and predictable nature of eclipses.

If all this sounds a bit complex, it really isn’t. The beauty of it all is that once one studies the moon’s movements even just for a few years, one is soon able to calculate the Metonic Cycle without having to wait 19 years to actually see it!

How might they have remembered all this information?

While it is true that most Irish myths and sagas were written down by Christian monks in the Middle Ages, some of the myths about Brú na Bóinne appear to refer to the astronomical function of the great mounds.

The story about Dowth, for instance, suggests that a total eclipse of the sun occurred while it was being built, and there are carvings on one of its kerbstones that look like depictions of a solar eclipse.

At Newgrange (anciently known as Síd in Broga or Síd Mac ind Oc), one story portrays the arrival of the sun deity, Dagda, into the mound at a time when he has made the sun stand still in the sky – a reference, I think, to the winter solstice when the daily movement of the sun along the horizon is seen to halt for a number of days.

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There is a reference too to the deity Eithne (another name for Bóinn) coming into Newgrange in the guise of a dog, and the astronomer in me wonders if this is maybe a reference to the fact that Sirius would have been visible to an observer prostrate in the chamber of Newgrange over 5,000 years ago.

Without time travel, it may never be possible to fully test these claims, but the remnants of stones and stories that have come down to us from a hoary age of the past certainly give us cause to ponder on the achievements of those Neolithic farmer-astronomers who lived in the Boyne Valley more than 5,000 years ago.

Anthony Murphy is a journalist, author, astronomer and tour guide from Drogheda in Co Louth. He has published nine books about the myths and monuments of prehistoric Ireland.

This work is co-funded by Journal Media and a grant programme from the European Parliament. Any opinions or conclusions expressed in this work is the author’s own. The European Parliament has no involvement in nor responsibility for the editorial content published by the project. For more information, see here.

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Anthony Murphy  / Journalist, author, astronomer

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