Ancient Astronomers of the Stone Age

Ancient Astronomers of the Stone Age

Some time after 6,000 years ago, a remarkable community of people arose on this island. They were the megalithic builders – farmers, astronomers and engineers. They left monumental, indelible structures crafted from stone and earth which were to stand the test of aeons of time. These people were responsible for creating Ireland's most remarkable monuments. They were the Neolithic sky watchers.

Their constructions are Ireland's best known, most explored, and possibly least understood, monuments. The most famous of these, Newgrange (known in ancient Irish manuscripts as Síd in Broga), is a magnet for tourists, who flock to the Boyne Valley every year in huge numbers. About a quarter of a million people visit the monument evey year. The nearby megalithic passage mound at Knowth is open to tourists except in winter, and the third major Brugh na Boinne site, Dowth, is also open to the public. So what is it that attracts people to these sites? What do they come to see? What are they told about these remarkable monuments?

Even to the casual visitor, it is clear that there is something remarkable about Newgrange, Knowth and Dowth. The stark symbolism etched deeply into the huge megaliths is a written record which comes to us across over five millennia. At Newgrange, the huge stone outside the passage entrance is highly decorated with huge, swirling spirals. At Knowth, nearly every stone is decorated, and the site has been hailed as having the largest collection of megalithic art in all of Europe – in fact, over a quarter of all known megalithic art in western Europe is at Knowth and its satellite mounds. A few kilometres to the east at Dowth, there are more decorated stones. At Loughcrew, 40 kilometres west of Brugh na Boinne in County Meath, there are the ancient cairns of Sliabh na Calliagh, the mountain of the witch, again featuring vast amounts of ancient carvings.

Aurora at Newgrange
Aurora borealis (northern lights) at Newgrange (Síd in Broga)

Among the familiar patterns, such as zig-zags, waved lines, spirals and lozenes, there are some decorations which are distinctly astronomical in nature. At Dowth, there are stars and sunwheels; at Newgrange, there are carvings that look like a representation of Orion's Belt; at Knowth, there is a wealth of astronomical imagery – crescents and moon shapes, stars, circles, spirals, sundials and astral imagery, and possibly even a map of the moon. At Loughcrew, there are suns and sunwheels, stars and much more. 

Pick up almost any amateur astronomy book at your local bookstore and in the history of astronomy section you will invariably read that the first astronomers were the ancient Babylonians, or the Chinese. You will also read about how the Greeks helped to enlighten the world with their many scientific and astronomical discoveries. Some of these books are progressive enough to mention that there were ancient astronomers at Stonehenge, but most overlook Newgrange and Ireland, which is evidently a place where competent astronomical study was taking place over 5,000 years ago.

Orion and Taurus over the Boyne and Newgrange
Orion, Taurus and the Pleiades over the Boyne and Newgrange.


The Winter Solstice sunrise event at Newgrange, where the sun shines into the long passage on the shortest days of the year and illuminates the central chamber, is the most heralded event in the Irish cultural calendar, and attracts major media attention every year. Most people know about it, and many gather at the famous mound every December to witness the event, even though most of them have to make do with seeing the event from outside. It is a famous example of ancient astronomy in action in modern times, and is a fitting beginning to our exploration of the ancient Irish sky watchers.

At dawn on Winter Solstice every year, just after 9am, the sun begins to rise across the Boyne Valley from Newgrange over a hill known locally as Red Mountain. Given the right weather conditions, the event is spectacular. At four and a half minutes past nine, the light from the rising sun strikes the front of Newgrange, and enters into the passage through the roofbox which was specially designed to capture the rays of the sun.

For the following 14 minutes, the beam of light stretches into the passage of Newgrange and on into the central chamber, where, in Neolithic times, it illuminated the rear stone of the central recess of the chamber. With simple stone technology, these wonderful people captured a very significant astronomical and calendrical moment in the most spectacular way.

The sunlight appears to be split into two beams – a higher beam and a lower beam. This is in fact true, the lower beam being formed by the doorway to the passage. It is the light which enters through the roofbox, however, which reaches the central chamber.

Solstice sunlight in the passage of Newgrange.
Winter solstice sunlight in Newgrange, viewed from the entrance. There is an upper beam and a lower beam. The upper beam reaches the chamber.

For a very short time, the beam of sunlight enters the chamber, illuminating the floor. It is a narrow beam, only 34cm wide at the entrance and narrower in the chamber. Originally, the beam would have struck the rear chamber orthostat (C8) and its light would have been reflected on to another chamber stone, C10, which contains the famous triple spiral. After just 14 short minutes, the beam disappears from the floor of the chamber, retreats down the passage and once again the heart of Newgrange returns to darkness.

Newgrange does not stand in isolation as an astronomical device. The Winter Solstice sunrise phenomenon is not the only function of Newgrange. In the book Uriel's Machine, Robert Lomas and Christopher Knight speculated that the roofbox and passage of Newgrange may have been used to track down Venus during specific moments in its eight-year cycle. Certainly the evidence for study of Venus is abundant in the Boyne Valley.

Above the roofbox of Newgrange, there are a series of eight markings, which the authors have suggested could represent the eight years of the Venus cycle. This eight-year cycle of Venus ties in very closely with the metonic cycle of the moon, and may have been recorded elsewhere at Brugh na Boinne as we will see later. Before the excavation of Newgrange began in the 1960s, there was a folk belief in the Boyne Valley, recorded by Joseph Campbell in his book Primitive Mythology: The Masks of God, that the light of the morning star shone into the chamber of the monument on one day in eight years.

Lunar astronomy

Many astronomers know that the moon's path through the sky, although inclined slightly to the sun's path, will take it into positions which are shared by the sun at certain times of the year. The points where the imaginary line of the moon's path crosses the line of the sun's path are called nodes. It is when the moon is at a node that it sits on the ecliptic, and when the nodes are located above Orion, in the gap between Gemini and Taurus, and in the gap between Sagittarius and Scorpio, then the Moon shares the sun's Summer Solstice and Winter Solstice positions, respectively.

This only occurs twice during a single rotation of the nodes, which takes 18.6 years. So every nine years, on just a few occasions, a full moon or waning gibbous Moon which rises in the Sun's Winter Solstice position can, technically speaking, shine into Newgrange.

On July 5th 2001, a group of amateur researchers (most of whom I know, including Richard Moore), were given special permission by Duchas (as the OPW was then known) to access the Newgrange chamber to see if they could witness the full moon from the interior of the chamber. Regrettably, some cloud cover meant the moon was obscured for the crucial minutes after it rose, but the cloud did clear in time to allow the group to see the moon from within the passage. Although this observation did not prove conclusively that the moon can be seen from the chamber, it goes without saying that if the moon's position can coincide with that of the winter solstice sun, then it would be possible to see moonlight in the chamber of Newgrange under the right conditions.

There may be more evidence to support a lunar function at Newgrange. The front of the mound is decorated with a brilliant façade of milky-white quartz, and some researchers and archaeologists believe the whole mound may originally have been covered with this brilliant stone. Perhaps Newgrange was supposed to be the earthly reflection of the moon?

More research needs to be done on a possible lunar alignment at Newgrange. The Irish name for Newgrange is Brugh na Bóinne (earlier Brug na Bóinne). The word Bóinne, from which the River Boyne is derived, means "white cow", and the ancient goddess Bóinn may have been associated with the moon. Indeed, some researchers have pointed out that the period of gestation of a cow is equivalent to nine and a half synodic lunar months. Professor Ronald Hicks has pointed out that the moon was known as an lair bhán, the "white mare". The word Brugh is interesting too. Traditionally it has been interpreted as meaning "mansion" or "house", but there is a word Brú which I have found to mean "womb" (MacCionnaith Foclóir, 1938). Could the real meaning of Newgrange be "The womb of the Moon???" The symbolism and interplay between the various elements involved leads to further speculation about the whole purpose of the site. We can imagine a full moon rising over the hill of Red Mountain, shining across the valley, over the Boyne River, which has the same meaning as the Milky Way in the sky, and may in fact have been seen as its earthly reflection. The Irish for Milky Way is Bealach/bóthar na Bó Finne – the way/road of the white cow. Perhaps the quartz façade on the front of Newgrange is supposed to be another representation of the Milky Way?

Maybe some day we will see spread across the front pages of the world's newspapers and news websites the wonderful image of moonlight flooding into the chamber of Newgrange.

Frank Prendergast's study

Serious academic research, as opposed to that of us amateurs, has also revealed interesting astronomical functions at Newgrange. Archaeoastronomer Frank Prendergast has produced data which shows that even the large standing stones in front of the entrance of Newgrange, known as the "Great Circle", were astronomical and calendrical in function. Although carbondating has placed the construction of the Great Circle to about 2000BC, over a thousand years after the construction of Newgrange itself, Prendergast shows that the stones functioned properly at that time. His research demonstrated how the shadow of GC1, the megalith adjacent to the entrance, would have crossed the lower part of the three spirals on the west side of K1 at the winter solstice; that the shadow of GC-1 would have crossed through the centre of the three spirals at the period when the south declination of the sun was half its annual maximum; that the shadow of stone GC-2 similarly crosses the same three spirals at the equinox; that the alignment of GC5 to GC3 pointed towards sunrise when the north declination of the sun was at half its annual maximum; it would also have been possible to observe the summer solstice sunrise by sighting across the top of GC1 and GC-2.

Our own work has shown how the astronomers of Newgrange may also have used the star Deneb, in the swan constellation Cygnus, to help track down the position of the sun during the night before Winter Solstice. The mystery is heightened by the attachment of certain swan myths to Newgrange, such as the famous romance story of Aonghus and Caer. Aonghus' mother was Boann, the Moon, and his father, the Dagda, owned Newgrange.

The story tells how Aonghus fell madly in love with a maiden who visited him while he slept. She visited him in his dreams for a year, and all this time he could not touch her because she would disappear. His mother Bóann searched Ireland for the maiden, but was unable to find her after a year of searching. Aonghus enlisted the help of his father, the Dagda, who in turn sought out Bodb, who was the Tuatha Dé Danann king of Munster. Bodb revealed that the maiden was, and brought Aonghus to meet her at Loch Béal Dragan (Dragon's Mouth) in Tipperary. Bodb explained how Caer was from Sídh Uamhain, an "otherworld residence" in Connacht.

Caer's father revealed to the Dagda that his daughter went in the forms of a bird and a girl on alternate years. At the following Samhain (November) she would be a bird at Loch Béal Dragan, and the Dagda instructed Aonghus to go there and call her to him. He did so, and found her in the shape of a beautiful white swan, in the company of thrice fifty others. She went to him, and he too became a swan, and they embraced each other and flew three times around the lake. They then flew together to Brugh na Bóinne and put the dwellers of that place to sleep with their beautiful singing. Caer remained with Aonghus in the Brugh after that.

The story says they took the form of swans and lived IN the Brugh. Is it purely coincidence that the swan constellation is cruciform in shape, like the Newgrange passage? Maybe, and we have to remember there are other cruciform passages in ancient megalithic mounds and cairns in Ireland. But the mystery deepens with the addition of the Fourknocks alignment.

Not a lot of people seem to know this, but if you plot the line of the direction of Winter Solstice sunrise from Newgrange on a map, this line intersects the small Megalithic mound of Fourknocks, 15 kilometres to the southeast.

(INFO: Sunrise 3150 +/-100 BC = 133°54 +/-4' Range of azimuths calculated by Tom Ray: 133°49' - 137°29)

In other words, Newgrange "points" to Fourknocks, although Fourknocks is not visible from Newgrange. Fourknocks, in turn, points to a very unusual azimuth – around 17 degrees east of north, way beyond the northernmost range of the rising sun or moon. What we need to resolve is whether the passage of Fourknocks is aligned on a significant star, or constellation. And, in the epoch when this small mound was constructed, between 3000 and 2500BC (Gabriel Cooney, 2000), the star Cygnus would be rising at this point.

The second significance of Deneb relates to its precessional importance. Throughout the entire 26,000 cycle of precession, Deneb remains mostly a circumpolar object, never setting below the horizon and being visible to observers at this latitude every night of the year.

Interestingly, in the epoch 3000 to 2500BC, Deneb is at its lowest point in the entire precessional cycle. At this time it grazes the horizon, and just about sets below the horizon at due north briefly during this time, before rising again to remain visible to those who would wish to watch the star.

Another aspect of the Aonghus-Caer romance is interesting. It is the reference to the "Lake of the Dragon's Mouth", where Aonghus found Caer. This could be a possible reference to the constellation of Draco, which is a prominent constellation in the northern hemisphere of the sky.

The constellation is particularly relevant in the Neolithic period, because the main star of Draco, called Thuban, was the pole star for a few hundred years around the date 2,800BC. It is also fascinating to our story in light of the fact that Cygnus the constellation is located very close to the head of the dragon in the sky.

Other constellations may also be featured at Newgrange. Kerb stone 52 is one of the finest decorated stones of Neolithic origin in Ireland. Located at the northwestern side of the great kerb, it is diametrically opposite the entrance stone, kerb stone 1, and features a vertical line down the centre. A line plotted between kerb stone 1 and kerb stone 52 points in the direction of summer solstice sunset.

Interpretation of megalithic art is a fraught and objective area. I believe it's possible that kerb 52 contains representations of the belt stars of Orion, and that the large "star" bored into the vertical line represents Sirius, the brightest star in the night sky. Sirius shared the same declination as the winter solstice sun around 3,150BC when Newgrange was built (ie Sirius would have been visible in the chamber of Newgrange when it rose at night), and it is possible the people who constructed Newgrange were well aware of this coincidental alignment, and therefore they may have used the passage of Newgrange to watch the precessional drift of Sirius over long periods of time, even if it was not constructed for that purpose.

The neighbouring stone, kerb 51, may contain representations of the constellation Cassiopeia with its familiar W shape. This is another constellation which would skim the horizon in the late Neolithic, and its distinctive shape would have been readily identifiable, as it is today. We know from ancient myths that this constellation was identified with a very important deity by the ancients. Charles Squire, in his 1912 volume Celtic Myth and Legend, says: "For the children of Dôn were certainly gods of the sky. Their names are writ large in heaven. The glittering W which we call "Cassiopeia's Chair" was to our British ancestors Llys Dôn, or "Dôn's Court". This same children of Don have been identified with their Irish counterparts, the children or people of the goddess Danu, and Danu herself was the Irish Cassiopeia.

This is another constellation which may have been featured as a target of the Fourknocks construct, and the familiar zig-zag patterns carved on some of the lintel stones in Fourknocks may be associated with this constellation. Author Martin Brennan, whose pioneering work on Neolithic astronomy and art in Ireland has been widely published, suggested that the quadrangle shapes on these lintel stones may be connected with the head of the constellation Draco, which as we have already seen was the polar constellation in the Neolithic.

Red sky over Dowth
A red sky over Dowth.


From Newgrange, we move further to the east, and possibly further back in time, to the megalithic cairn at Dowth, located less than a mile away. Dowth in Irish is Dubad, which means "darkness" or "place of darkness". It is the only one of the three great mounds of Brugh na Bóinne, the others being Newgrange and Knowth, which has not been excavated and reconstructed in modern times.

Some crude archaeological work was carried out here in the middle of the 19th century, around the time of the Great Famine. By crude, I mean the typical Victorian grave-robbing archaeology where the recovery of valuable finds and treasure is the main objective, and where little concern is given to returning archaeological sites to their original state. It was during this work that much of the top of the mound was removed, resulting in the huge crater in the mound which can be seen today. There are two known passages at Dowth, both on the western side. Only part of the kerb of stones is exposed, along the east, south and southwest of the mound. Many of the kerbstones remain buried. But some of those which are exposed are very exciting, especially kerb stone 51 on the east side, which has been named the "Stone of the Seven Suns".

It contains what appear to be suns, or stars, with rays coming out from the centre, and with the whole surrounded by a circle. There are seven of these suns in total, six of which are contained within circles. Attempts have been made to explain the meaning of these symbols – some say they are representations of the sun at different times of the year, others say they represent celestial bodies such as comets. One thing seems certain – they represent heavenly bodies of some form. It really is an impressive stone.

It is mythology, and particularly the ancient story about how Dowth was built, which reveals an ancient astronomical symbolism which may help to explain something about the meaning of the "Place of Darkness" and the Seven Suns stone. The story comes from the Dindshenchas, a collection of ancient stories about Irish placenames, and concerns Bresal, who was the ruler of the time.

"In his time there fell a murrain on kine in every place in Ireland, except for seven cows and a bull that increased strength for every farmer in his time. By him is built the solid hill in the likeness of Nimrod's tower, so that from it he might pass to heaven, - that is the cause why it was undertaken". The story continues to tell how Bresal's sister stopped the Sun from moving so that there would be 'no night but bright day' until work reached completion. Unfortunately, they committed incest and the Sun went down . . .

The men of Erin left the task incomplete, saying: "... since darkness has fallen upon our work, and night has come on and the day is gone, let each depart to his place. Dubad (darkness) shall be the name of this place for ever."

Given that there are seven "suns" on kerb 51, and that the mythology about Dowth speaks of a bull and seven cows, it seems possible that the site has some connection with the constellation of Taurus, the bull, which contains the open cluster the Pleiades, known as "The Seven Sisters". This constellation was very important around the time the Boyne Valley mounds were being constructed, as it housed the sun on the spring equinox, that very important moment of the year when the sun's path along the ecliptic crossed the celestial equator heading northwards. In astrology, it is the sun's position among the zodiac stars at this time which determines the current "age" – i.e. the "Age of Taurus".

Another interesting phenomenon which occurs at this time is what is known to astronomers as a heliacal rising of the Pleiades. This happens when the stars in question rise at the eastern horizon but are quickly lost in the glare of the rising sun. It is interesting to note that the Egyptians, and the Dogon tribe in Africa, among others, used the same Dowth-like sun-wheel symbols to signify a heliacal rising.

If these sun-wheel symbols do represent the heliacal rising of the Pleiades, it could tell us something very significant about the Neolithic people – they were aware of the great cycle of precession, the slow wobble of the Earth's axis which causes the celestial pole to shift over time, resulting in the vernal equinox point, that place where the sun crosses the celestial equator, moving backwards, or westwards, through the zodiac over a 25,920-year period. This vernal point moves just one degree (about two widths of the full moon) every 72 years, and spends on average 2,150 years in each of the twelve constellations of the zodiac.

A total lunar eclipse over Dowth. Its builders would have watched eclipses such as this one.

The astronomical function of Dowth is not in question. In 1980, Martin Brennan and Jack Roberts, who suspected the southern passage and chamber was aligned on the winter solstice sunset, gained access to the chamber, and with the help of fellow researcher Hank Harrison, filmed the beam of light from the winter solstice sunset as it reached forth across the floor of the chamber and struck the bottom of one of the decorated chamberstones.

This few minutes of 8mm cine film, which I had the rare chance to view in November 2000, helped along with other discoveries in that year to prove something which Brennan had believed from the time he had begun his research into the Neolithic sites – that Newgrange did not exist in isolation as an astronomically-aligned structure.

The winter solstice alignment of Dowth South has in recent years become the focus of study for artist and author Anne Marie Moroney, who has spent the last four winters recording, photographing and studying the phenomenon. This study is documented in her beautiful book Dowth: Winter Sunsets (Flaxmill Publications).

She says that between November and February the rays of the evening sun reach into the passage and then the chamber of Dowth South. During the winter solstice the light of the low sun moves along the left side of the passage, then into the circular chamber, where three stones are lit up by the sun. The convex central stone reflects the sunlight in to a dark recess, lighting up the decorated stones there. The rays then recede slowly along the right side of the passage and after about two hours the sun withdraws from Dowth South. Interestingly, she also says that the builders of the passage mounds seemed to realise that the human eye would be harmed by watching the sun directly. By directing the sunbeams through a small opening, the slightest changes in the position of the sun could be observed safely. The northern passage too may be astronomical in function, although an entrance shaft erected by the OPW in the early 20th century prevents any sunset from being watched from within the passage. Some researchers have suggested it is aligned on the cross-quarter day sunsets on November 8th (Samhain) and February 4th (Imbolc), and Anne-Marie Moroney has carried out some preliminary measurements and studies which would support this hypothesis.

The northern passage is inaccessible to the public, and was disturbed in the early Christian period by the construction of underground storage chambers called souterrains. However, those lucky enough to know someone with connections in the world of archaeology who can access a key to the gate of the northern passage, are in for a treat. That's exactly what I got when I was able to enter the northern passage and chamber in November of 2000. The chamber of Dowth North is an eerie place to be. Hidden from daylight, and sunken into the ground, it is cold, dark, damp and claustrophobic. The modern electrical lights do not work, the chamber is currently only accessible through a 70-foot souterrain, and the passage orthostats lean together such that when you walk up the passage, you have to squeeze through the stones. It's a strange experience, to say the least. But Dowth North could be the oldest cruciform passage in the Brugh na Bóinne complex.

The interior of Dowth North seems to carry on the astronomical theme present on some of the great kerbstones outside the mound. The chamber stone C7 is particularly well decorated, featuring a number of stellar symbols, concentric circles, a small spiral, linear markings and other features such as small inverted V shapes.

The astronomical theme was also picked up by George Coffey, keeper of Irish antiquities at the National Museum, a century ago. Coffey noticed that many of the star/sun symbols at Dowth were repeated at Newgrange and Loughcrew. Perhaps at some time in the future, the concrete shaft will be removed and once more the light will be allowed into Dowth North. For the time being, it remains off bounds even to the sun.

Interestingly, a number of white quartz stones have been found near what would have been the original entrance to this passage, and this could be another link with the Moon-Milky Way theme which we picked up on earlier at Newgrange. Dowth is also mentioned in myth as the place where Bóinn was buried – perhaps this was a reference to the full moon shining into the Dowth chambers before setting, once more plunging the place of darkness into, well, darkness.

There have been some suggestions in recent times that there may be a third passage at Dowth, on the eastern side. Just to the left of the Stone of the Seven Suns is a very interesting kerb stone featuring a vertical line down the centre. Other entrance kerb stones, such as kerb 1 at Newgrange and the entrance kerb stones at Knowth's two passages, feature a vertical line. With no major excavations planned at Dowth in the near future, it seems unlikely that we will know if there is a third passage for the time being.

One other archaeological site near Dowth is extremely worthy of a mention. Labelled Site Q on archaeological maps, it is a structure known as a henge, or an embanked enclosure. It is said by some to be the second-largest such ringed structure in Ireland.

Entering the ring through its southwestern opening, the huge scale of the site and the height of the surrounding banks give one the impression of a giant ampitheatre. If this was some kind of ceremonial site, it certainly smacks of grandeur and hugeness. There is a second opening, to the northeast of the ring, which may or may not have been contemporary with the construction of the site. But if one stands outside the structure, to the southwest, and lines up the two entrances, this is the exact line of summer solstice sunrise, an event which I have been witness to. From the air, the Dowth henge is egg-shaped, just like Newgrange, and the fact that it may share a winter solstice sunset alignment with nearby Dowth is interesting indeed.

Knowth in the summer.


From Dowth, the Place of Darkness where the winter sun sets, we must now turn our attention to the third great Brugh na Bóinne mound, Knowth, which has in recent years been causing great excitement in the fields of both archaeology and astronomy. In the 1960s, Knowth was just a large mound in a field, with no sign of any stones visible. Since then, under the direction of Professor George Eogan, the whole site has been thoroughly excavated and has thrown up more than its fair share of treasures, decorated stones, surprises and mysteries.

Probably the biggest reason Knowth has caused such a sensation is its plethora of megalithic art. Many of its 127 kerb stones are decorated, some highly so. Knowth also has two passages, facing east and west. In our astronomical exploration of Knowth, we first turn to one of the most exciting stones in the Boyne Valley – the kerb stone which was named the "Calendar Stone" by Martin Brennan.

Brennan proposes (and I agree with him) that this kerb stone at Knowth proves the people of the Neolithic were competent astronomers, who had made observations over great periods of time, and were able to pass on their astronomical knowledge from generation to generation. The stone presents a format that can be used to track the synodic month, and from it we can obtain very important calculations of large subunits of the 19-year cycle of the moon known as the Metonic Cycle.

This stone demonstrates that the Neolithic people who constructed the mound were aware that the solar year, which is 365 days long, does not contain an equal number of synodic periods of the Moon. But it also shows they were aware of the great 19-year Metonic Cycle and studied the movements of the moon over long periods of time.

A synodic period of the moon is marked by the return of the moon to the same phase, and is exactly 29.531 days long. So therefore, 12 lunar months, or synodic months, is exactly 354.372 days long. But this is a whole 11 days shorter than a tropical year. The Neolithic mound-builders knew this, and used the Calendar Stone to record their calculations of the numbers of synodic lunar months in tropical years.

25 synodic months is 738.275 days, which is 8 days longer than 2 tropical yrs.
37 synodic months is 1092.647 days, 3 days short of 3 tropical years.
49 synodic months is 14 days shorter than 4 tropical years.
62 synodic months is 5 days longer than 5 tropical years.

The Calendar Stone at Knowth
The Calendar Stone at Knowth.

It is this value in the sequence which is represented on the Calendar Stone at Knowth. There are a total of 31 "waves" across the stone, surrounded by representations of the moon – 29 of them – representing the 29 days of the synodic lunar month. If we double the number of waves, we get 62, representing 62 synodic periods of the moon, which, as we have already seen, is just five days longer than five tropical years. The sequence of counting synodic lunar months continues until we get a very close correlation between synodic months and tropical years: 99 synodic lunar months is only two days longer than eight tropical years. But even closer still is 136 synodic months, which ends about a day before 11 tropical years. And if we add 99 synodic months to 136 synodic months we reach the "Metonic Cycle": 19 tropical years is equal to 235 synodic lunar months, or 254 tropical lunar months. A tropical lunar month is defined by the amount of time it takes the moon to reach the same background stars again – it is equal to 27.322 days.

The Metonic Cycle gets its name from a Greek called Meton who lived in Athens in the 5th century BC, and who claimed he discovered this cycle of the moon himself based on simple observations. It now seems likely, in light of this new interpretation of the Calendar Stone at Knowth, that the Metonic Cycle was known about millennia before Meton ever lived. In Irish phraseology in the 18th century, the Metonic Cycle was known as Naoidheachta, literally meaning "nineteen" or "the nineteenth". This was recorded by General Charles Vallancey, whose interpretive linguistic work was largely derided and ridiculed, but whose work contains many interesting revelations.

The "waved line" feature of the Calendar Stone can also be seen to have supplemental counts on the left side, continuing from 31 to 32, 33 and 34. 32 synodic months is also a significant subunit of the Metonic Cycle, because when doubled it becomes 64 synodic periods which ties in with five calendar synodic periods of Saturn. 33 synodic months is one-third of the very important metonic subunit: 99 synodic periods ends just two days after eight tropical years. And 34 is one-quarter of another large Metonic subunit: 136 synodic months ending about one day before 11 tropical years.

Knowth Lunar Stone
A drawing of the Lunar Stone at Knowth.

Another Knowth kerbstone can also be used to calculate the lengths of the siderial and synodic lunar month, and the solar year. I call it the "Lunar Stone". In our look at this 5,300-year-old astronomical puzzle, we will interpret the symbols as follows: the crescent shapes are early and late phases of the moon; the circles are lunar phases close to full moon, the small spiral with a single crescent to the right represents the way the count is carried out, as identified by Martin Brennan; the wavy line represents numbers of lunations, or months, while the line underneath is a calibration bar marking a specific number of lunations or months.

The 27-day count begins on the extreme right of the stone, working towards the right, a total count of 11 crescents. The eighth phase is marked with a line to indicate a quarter moon. Then the three concentric circles are added to the count, making a running total of 14. Working backwards, we don't count the centre circle because it marks the turning point of the count, and work outwards, adding another two circles, total 16, and then the 11 crescents again, totalling 27. This first method of counting reveals the siderial month, the length of time it takes the Moon to make one complete circuit through the sky, in other words the time it takes the Moon to return to the same background stars.

The 29-day count works in a similar way, but this time, in addition to counting the outer two concentric circles of the triple circle, the additional double concentric circle on the far top left of the stone is also counted. So we have (working inwards) 11 crescents, plus three circles, total 14, and (working outwards) add two circles, plus another two circles, total 18, plus the 11 crescents again, totalling 29.

This, I believe, is the synodic lunar month count. While the siderial month marks the moon's return to the same background stars, the synodic month marks its return to the same phase. Both are important in calculating the 19-year Metonic Cycle. There are 235 synodic months and 254 siderial months in the Metonic Cycle. Interestingly, another Irish researcher, Gillies MacBain, has pointed out that the original total number of kerb stones around Knowth, 127, is half of 254 – the number of siderial lunar months in one Metonic Cycle. Could the whole site have been one large astronomical calculator, with complex movements of the moon seemingly figured out and recorded in stone?

As with the Calendar Stone, the Lunar Stone can be used to work out the length of year. It involves a simple calculation, using the stone as a guide, and the result is accurate. The length of the synodic lunar month is 29.5 days, and if we do as the stone suggests (in the waved line calibrated count of 12) and multiply the synodic period by 12 (in other words 12 lunations), we get 354.37 days, which is 11 days short of a complete year. One final addition of the 11 crescents will result in the accurate answer of 365 days.

Another very well adorned stone, kerb 15, seems to contain a combination of a large sundial device and a series of markings designed to help calculate the length of the year. Neil L. Thomas, in his study of the Irish Stone Age symbols of 3,500BC, believes he has identified the manner in which this stone device works.

Thomas says the stone is a unique statement, an exact 365-day, sixteen-month, four-week month, five-day week solar calendar. Brennan identified the large fan-like rayed device on this stone as being a sundial, and stressed the importance of the study of the sun's movements to the people who he called the "master diallers of the New Stone Age". Their dials were constructed, he says, so that they could tell time precisely, even to fractions of a second, but this was for the purpose of making exact observations simultaneously in different places.

There is another sundial at Knowth. It is carved onto the top of a kerbstone on the northeastern side of the mound. On the sundial at equinox, the sun rising in the east casts its shadow west, at midday it casts its shadow north and, as it sets, it casts its shadow east, completing a cross in its circle and defining time and space simultaneously. Brennan says the dial measures what are known as the unequal or "planetary" hours, which are shorter in winter and longer in summer. At the equinoxes it divides the day into eight equal parts, which are further subdivided into 16 parts. This corresponds to the solar division of the year into eight parts.

There is another type of dialling device at Knowth, outside the entrance to the western passage. It is a strangely shaped standing stone, which casts a shadow at an interesting time of the year.

Frank Prendergast said that measuring the shadow cast by a vertical pole, or gnomon, is known to have been used as a simple astronomical instrument from which the approximate time of day or even the altitude of the sun at local noon can be deduced, leading to a reasonably accurate definition of north-south.

Perhaps the builders of Knowth knew the value of gnomon shadows, because on the evening of maximum penetration of the setting sun into Knowth West, March 3rd, the standing stone casts a shadow on the entrance kerbstone. It does this for a number of evenings at certain times of the year, but it is on the day of maximum penetration that the shadow appears to line up against the central vertical line of the entrance kerbstone just as the sun sets. I managed to catch the progression of the shadow on film, and the result is interesting. Just as the sunlight is penetrating deep into the western passage, the gnomon shadow lines up perfectly with the vertical centre line on the entrance kerbstone. When it hits the stone, the setting sunlight becomes diffuse and the shadow disappears.

As the shadow event is happening outside, something very familiar is happening inside Knowth West in early March and early October. The long rays of the sun penetrate into the passage and illuminate the darkness, revealing a wealth of decorated stones inside.

Although similar to the winter solstice illumination of Newgrange, the Knowth West illumination has not been sufficiently recorded or publicly documented to show exactly how far beyond the bend in the passage that the sunlight actually penetrates.

Until I took a photograph from inside the western passage around sunset on the autumn equinox in the year 2000, I had never seen a photo of this phenomenon. And even this photo does not do the event justice – this was taken just as the sun was being covered by a large bank of cloud in the west, and moments after I took this picture the sunlight disappeared.

Because the western passage is oriented somewhat south of west, to about 260 degrees azimuth, it is not aligned on the equinoxes as some have claimed, but rather 18 days before the spring equinox and 18 days after autumn equinox. An American astronomer and researcher, Charles Scribner, has put forward a new theory as to how the alignments functioned.

If all of that wasn't enough to whet your astronomical appetite, there's always the work of planetary cartographer, Philip Stooke of the University of Western Ontario, Canada, who claims to have discovered a map of the moon carved onto one of the stones inside the chamber of the eastern passage at Knowth.

Stooke spends most of his time preparing maps of asteroids based on spacecraft observations, but he has also prepared detailed maps of the moon, and it was this field of study which made his eyes light up when he first saw a drawing of the carving in an archaeological book about Knowth which he read in Canada. He told BBC Science Correspondent David Whitehouse : "I was amazed when I saw it. Place the markings over a picture of the full moon and you will see that they line up. It is without doubt a map of the moon, the most ancient one ever found." And all from thousands of miles away. 

If his claim is true, it may lend some weight to the argument that Neolithic stone passages could have been used to track down moonrises and moonsets. In this case, the light of a full moon rising in the east may once have shone into the eastern passage of Knowth and illuminated the map in the end recess. Who knows?? Later phases of activity at Knowth took their toll – in the early Christian period the construction of souterrains seriously altered the entrance to the eastern passage, and modern archaeological reconstruction has seen the addition of a huge concrete slab at the entrance, so we may never know the full extent of any astronomical alignment the passage might have had.

Knowth would appear to be a monument inspired by astronomy and the heavens, and there is a wealth of discoveries yet to be made there. 

Sunset at Cairn T Loughcrew
Sunset at Cairn T, Loughcrew.


Before we finish our wonderful exploration of Stone Age astronomy, we must leave the Bend of the Boyne for a moment and turn our attention further west, towards the hills of Meath at a place called Sliabh na Caillaighe, the Mountain of the Hag, at Loughcrew. Here, scattered on the peaks of the Carnbane hills which overlook some breathtaking scenery, are a group of Stone Age cairns which may be even older than their Boyne Valley counterparts.

It was here, at one of the cairns called Cairn T, that Martin Brennan and Jack Roberts made a discovery in the spring of 1980 that was to put them on the front page of newspapers across the world. Cairn T, at over 900ft above sea level, stands in the central position and takes the most prominent place on the highest summit of a megalithic area that may once have contained as many as 50 to 100 cairns. It was March 17th, 1980, and Brennan and his friend Jack Roberts had just arrived at Cairn T after a drive from Dublin. This was the latest in a string of attempts to see the sunrise, but adverse weather conditions that spring meant they had not seen one single sunrise since March 1st.

Here is Brennan's account of what happened:

"We were winding up the mountain road when the disc of the sun broke on the horizon. We felt as if we were ten minutes late for an appointment made over 5,000 years ago. The lock on the modern door leading to the passage had frozen during the night, and as we struggled with it the rising sun was already above the horizon. When we drew back the door a narrow chink of light streamed down the passage and flashed into the end recess of the chamber."

"On the upper left of the backstone a rectangular patch of light was rapidly beginning to take form, brilliantly illuminating the entire chamber in a growing splendour of shimmering golden orange light. It was dazzling, and when we entered the chamber we stood back and gazed in awe."

"The light assumed a clearly defined geometric shape that was projected on to the upright backstone and moved diagonally across it, tracing the path of the sun against a mural of prehistoric art. What impressed us most was the careful and delicate modelling of the light beam by the huge stones forming the passage and chamber, and how the shape of the beam conformed to the patterns engraved on the stone. For the first time we were seeing the signs and symbols in the context in which the artist had meant them to be seen."

Brennan said it was clear that they were dealing with a solar construct capable of defining an individual day with far greater precision than Newgrange. The wider differences in the sun's apparent movement at equinox made it considerably easier to define the actual day of equinox at Cairn T than the day of Winter Solstice at Newgrange. Coupled with the help of the rock engravings, this created a remarkably precise asonomical instrument.

Cairn T ceiling stone
The ceiling stone in the rear recess of the chamber at Cairn T.

The stone which forms the ceiling of the end recess of Cairn T is also beautifully adorned, and contains a number of star-like and sun-like patterns. This stone, too, would receive reflected light on the morning of the equinox. Crouched in the small recess, with one's face to the entrance and one's back to the beautiful equinox stone, looking up at the ceiling gives one the impression of looking at some kind of star map or astral guide. Sunwheels, most notably those with eight radials, echo the pattern of sunwheels on the equinox stone. This highly-decorated stone, along with one on the ceiling of the east recess at Newgrange, was engraved before being positioned above the recess.

Some of the passage stones at Cairn T are adorned with huge numbers of round holes, or cup marks, and give the impression of some form of primitive star map.

Another interesting aspect of the Cairn T alignment is the fact that when you look out from the end recess through the entrance, the distant Hill of Slane can be seen in the centre of the view. In other words, the spring equinox sunrise viewed from Loughcrew Cairn T rises at Slane. Was this another intentional alignment? Is it merely coincidence that Saint Patrick was later to light his Easter fire here, and that Easter is calculated using the full moon closest to the spring equinox? Also coincidental is the fact that St. Patrick's Day is celebrated three days before the spring equinox in modern times.

There is a large mound on the peak of the Hill of Slane, although many visitors do not see it because it is shrouded in a clump of huge trees and access is difficult. This mound was said to have been the burial place of the Fir Bolg king Sláine, who reigned in the ancient province of Ulster, which at that time was the largest of the five provinces in the country. His territory was bounded to the south by the River Boyne, which in ancient times marked the border between the provinces of Ulster and Leinster.

It was in Drogheda, at a mound known today as the Millmount, that the mythical Milesian rulers Eremon and Eber divided the island into two kingdoms – one north of the Boyne and one south. The mound later became the burial place of Eremon's brother, the bard and astronomer Amergin, who famously chanted these lines when the Milesians first landed at the Boyne Estuary:

"What land is better than this island of the setting sun?
Who but I knows the place where the sun sets?
Who but I can tell the ages of the Moon?"

It seemed that Amergin and his brothers were well aware of the Boyne River's long-established link with astronomical study.

There is a fascinating alignment at Cairn L on Carnbane West at Loughcrew. It is inside the chamber of Cairn L that on the November and February cross-quarter days, sunrise penetrates into the chamber and illuminates a six-foot tall white standing stone.

This event was first rediscovered by Brennan on November 3rd, 1980. He describes it thus: "A flash of light pierced the darkness of Cairn L and illuminated the top of the standing stone. Instead of the usual slow, progressive entry of the beam of light, it had penetrated the chamber instantaneously." He said the beam of light was modelled so that it struck only the limestone pillar, and no other stones. He had witnessed precise astronomy at work, more than five millennia after this Stone Age timepiece was put together.

There are other alignments at Loughcrew too. The direction of summer solstice sunrise is marked by the lining up of four sites – Cairns P1, R2, T and U. Cairn I, which points to Cairn T, can be used to track the sun as it makes it way towards the equinox, while cairns S and U are aligned on the cross-quarter day sunrises, Cairn S on May/August cross-quarter, and Cairn U on November/February cross-quarter. Moonrises have also been observed at Loughcrew, and the most interesting of these is the Harvest Moon in August which shines into Cairn L and strikes stone 13 at the back of the end recess.

Winter solstice sunrise at Baltray
Winter solstice sunrise in line with the Baltray standing stone.


Before I conclude, there is one more site worthy of a mention. It is at Baltray, a village at the mouth of the River Boyne, where there are two megalithic standing stones, which have stood as silent sentries watching over the estuary for thousands of years. The larger of these two standing stones has a very flat, straight edge, which myself and two friends (Richard Moore and Michael Byrne) discovered had a unique alignment.

The stone points to two small islands out in the Irish Sea, called Rockabill, which in modern times have a lighthouse on the larger of the two islands. In December of 1999, we confirmed, after much speculation, that this was the location of the rising sun at Winter Solstice as viewed from the Baltray standing stone.

The alignment is a bit off in this epoch though, and the sun's rising position has drifted to the left in the years since the stones were erected. We are not sure exactly when the stones date to, but we suspect they are early indeed, perhaps as early as the Late Neolithic, which would place their construction some time between 3000 and 2500 BC. At least one prominent archaeologist has agreed that this might be the case. We know at Newgrange, thanks to the work of Frank Prendergast and Tom Ray, that the sun's rising position now is a whole degree, or two sun widths, to the left of where it was when Newgrange was constructed, and it is clear that at Baltray the sun's rising position is at least one and a half sunwidths left of Rockabill. We would prefer to leave it to the professional archaeoastronomers to tell us exactly when the stones date to, but it is sufficient to conclude that the alignment is of major importance.

This is the first major archaeological site one would encounter on a journey up the Boyne River, and because of the major astronomical theme of the many Stone sites along the river, it must be considered important that the very first site has a Winter Solstice sunrise alignment just as Newgrange does. These stones would have been important as a boundary marker in ancient times, marking the border between the ancient provinces of Ulster, to the north of the Boyne, and Leinster to the south. Legend tells us this is the strand where Cuchulainn met his son Connla for the first time and the two fought on the shore of the sea under the standing stones before Cuchulainn killed his son in the shallow waters of the incoming tide.

The stones may also have been used to track the position of the Moon, as is suggested in another myth about Rockabill which tells how the Sun god, Balor, was once trying to steal the magic cow, the Glasgabhlin, from Ulster. This cow, the story says, had an everlasting supply of milk. I have tentatively linked this cow with the Moon, and the calf with Venus, suggesting that the astronomical purpose of the stones is more complex than meets the eye.


There is something awe-inspiring about the huge megalithic sites of the Boyne region. Very few people leave Brugh na Boinne without having felt some deep connection with the ancient past, and fewer still leave without a clatter of questions on their mind about the original intent and purpose of these sites.

For the astronomer, these are surely exciting places to be. In one moment, an amateur astronomer visiting Dowth or Loughcrew or Fourknocks or Newgrange can bridge up to five and a half thousand years of history. For just as we today gaze at the heavens and watch the sun, moon, planets, and stars, so too did our ancient ancestors. 

Something of a consensus is beginning to form, even in the more conservative areas of archaeological and anthropological academic research, that much of what the people of the Irish Neolithic were about involved some form of heavenly study or knowledge. The field of ancient astronomical research, once left almost entirely to amateurs, is now very much a part of professional study of these sites.

And this is a good thing. If we can begin to look at our ancient sites as being connected, whether astronomically or otherwise, we can begin to see the cosmic landscape, in much the same form as it was originally crafted, by people who felt a connection with the cosmos in a much starker way than we do today. We may have discovered more planets, put a man on the moon and seen into the furthest reaches of space with our telescopes, but we have lost something too – a simple, but beautiful, relationship with the heavens which understands the harmony of the sun and the seasons, the moon and the tides, and their movement among the stars which reveals the longer ages in which things slowly change.

The astronomical landscape functions on more than simply one or two levels. It is what I call three-dimensional, and each site demonstrates this perfectly. At Newgrange, to take the most famous example, some would believe that it is a device for pointing out winter solstice. That is not the full picture. Newgrange is a complex astronomical and calendrical device. The people who constructed it knew this better than we do, and could probably tell exactly what day of the year it was simply by looking at the way the sun casts shadows on the stones. We have taken that simple observation away, and replaced it with wristwatches, and calendars, and computers and a host of devices and aids with which we can tell the day, or season, or year.

The people who built Newgrange, and Knowth and Dowth as well, were very familiar with the lunar cycles. We speculate that they studied the moon's movements because we see symbols outlining those movements on their stones.

And our understanding of their exact level of knowledge is still poor. There remains much work to be done. In this article, we have seen an astronomical interpretation of three or four kerbstones at Knowth. There are 127 in total, and many more decorated stones in the passages.

Maybe at some stage we will know and understand, and see the universe in much the same way as the megalithic astronomers. Until then, the journey of discovery and enlightenment is an exciting and fulfulling one.

© Anthony Murphy. This text is an edited version of a lecture I delivered to members of Astronomy Ireland in Dublin City University (DCU) in January 2002.

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Sirius, the Dog Star, shone into Newgrange when it was built.

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1 comment

What a detailed, informative article which filled many gaps in my knowledge of Newgrange which we love visiting when home in Ireland. Thank you!!

Margaret Owens

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