Knowth - The Calendar Stone

Knowth - The Calendar Stone

Knowth, one of the large mounds of the Boyne Valley complex, has become famous for its huge array of megalithic art. The site has been said to contain over a quarter of all known art from this period in western Europe, and following the completion of 40 years of archaeological excavation and restoration of the site, a full inventory of the art has been made.

There is a question about the megalithic art of Knowth, inscribed into its great stones around 3300BC. The question is this – what does the art mean? Is it really art, whether abstract forms, or symbolism – or is it a mixture of abstract representations and drawings of real objects? Could the carvings be representative of something else, something astronomical?

I believe that some of Knowth's art can be explained when viewed from an astronomical viewpoint. I have argued, in Island of the Setting Sun: In Search of Ireland's Ancient Astronomers (with Richard Moore), that a more holistic interpretation of Knowth and the megalithic passage mounds can be made only when astronomy and cosmology are incorporated into the study of these great monuments.

The casual visitor will see that Knowth's stones are covered with a vast array of symbols. Many of these symbols are repeated, with common forms including circles, spirals, curves, zig-zags (chevrons), semi-circles, crescents, lines, lozenges (diamonds) and other features. The visitor will also notice that many of these forms appear present an astronomical theme, revealing an apparent interest in the heavens.

Calendar Stone
Kerb stone 52 at Knowth, which is known as the Calendar Stone.

The Calendar Stone

Kerb stone 52 at Knowth (pictured above) appears to demonstrate that 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 Calendar 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 Metonic Cycle of the moon. 

This stone may show us that the mound builders were aware that the solar year, which is 365 days long, does not contain an equal number of synodic periods of the moon (full moon to full moon or new moon to new moon). But it also hints that they were aware of the great 19-year Metonic Cycle (a later Irish word for this cycle is Naoidheachda, "the nineteenth" – see Newgrange: Monument to Immortality, 2012, p. 89) 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. Therefore, 12 synodic lunar months is exactly 354.372 days long. But this is 11 days shorter than a tropical solar 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 solar years.
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.

See a video showing how the Calendar Stone works here:

It is this value in the sequence (62 synodic months is 5 days longer than 5 tropical years) which is represented on the Calendar Stone at Knowth. There are a total of 31 "waves" across the centre of the stone, surrounded by representations of the moon – 29 of them – representing the 29 days of the synodic lunar month. These moon symbols consist of a series of 22 crescent shapes and 7 circles or double circles. I take the crescent shapes to represent the early and late phases of the moon, and the circles to represent that week in the middle of the month when the moon is full or almost full.

If we double the number of waves, we get 62 (31 x 2 = 62), representing 62 synodic periods of the moon, which, as we have already seen, is just five days longer than five tropical solar years.

The sequence continues until we get a very close correlation between synodic months and tropical years: 99 synodic lunar months is only 2 days longer than 8 tropical years. But even closer still is 136 synodic months, which ends about a day before 11 tropical solar 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. But our interpretation of the Calendar Stone at Knowth appears to demonstrate that the Metonic Cycle was known about long before Meton existed.

The Calendar Stone at Knowth (kerb 52)
The Calendar Stone at Knowth (kerb stone 52).

Calculating the year

Furthermore, it is possible that this kerbstone was used to calculate the exact number of days in the tropical solar year. There are 29 moons in total, 22 crescents and seven circular moons which are really double circles, and potentially signifying an extra count. There is a folk tale still in existence from Brittany to Scotland which says that you should never count stones more than once because you will never get the same number. The numbers and arrangements at Stone Age sites were chosen so that there were several ways of counting them.

If we count the moons, to get 29, and add the second set of circular moons, we get 36. If we double 36 (and the stone already suggests doubling with the waved line – 2 x 31 = 62) we get 72, and add the solar spiral, we get 73. Five times 73 is 365, the exact number of days in the year. Every fourth year, add the solar spiral to get 366. The "waved line" feature can also be seen to have supplemental counts on the left side, continuing from 31 to 32, 33 and 34.

Thirty-two 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. Thirty-three 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.

When considering these engravings we must remember that the method of observation does not require complex equipment or machinery, and does not involve complicated mathematics. What it involves is simple observation over long periods of time. These engravings were obviously an attempt by the Neolithic astronomers to pass on their knowledge to the next generation – a quest in which they succeeded. Five thousand years on we too can get an insight into the astronomical observations of our "primitive" ancestors, and we too can watch the heavens and observe the Metonic cycle for ourselves.

Knowth kerb stone 93 - Lunar Stone
Kerb stone 93 at Knowth - a possible lunar counting stone.

Counting moons

American Author Martin Brennan has suggested that kerb stone 93 at Knowth (pictured above) was used for making a 27-day lunar calculations. It is possible that this stone reveals a complex system of moon counting which shows that the ancient stone builders were competent astronomers, and used it to tell the length of the year. The secret of the stone lies in the way the symbols are counted. When you know the count, you know the meaning of the stone.   

Our lesson on how to interpret this 5,300-year-old puzzle begins with trying to classify the elements so that they can be identified as specific symbols and representations. We can classify the markings into easy-to-remember groupings: circles, crescents, waves, a spiral and a line.

The symbols can be interpreted as follows (and remember that this is just an interpretation, and the interpretation of megalithic art is a subjective area!): 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 following diagrams are based upon Brennan's drawings of kerb 93.

Knowth kerb 93 27-day count 1
Kerb 93: The 27-day count working outwards. 11 crescents + 3 circles = 14
Knowth 27-day count 2
The 27-day count inwards. +2 circles = 16 and add 11 crescents to make 27.

The 27-day count

This is the count identified by Brennan. It begins on the extreme right of the stone, working (as shown in the first diagram above) towards the right, a total count of 11 crescents. Brennan notes that 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 (second diagram), 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. It has to be pointed out that the lunar tropical month is almost exactly the same length as the siderial month. The difference is so small that it would take a keen observer over a century to notice!

Knowth kerb 93 29-day count 1
K93: 29-day count outwards. 11 crescents + 3 circles = 14.
Knowth kerb 93 29-day count 2
K93: 29-day count returning. + 2 circles = 16 +2 more = 18 + 11 cresc = 29.

The 29-day count

The second count was not identified by Brennan but is here presented as a possibility towards counting the 29-day synodic lunar month. The count works inwards exactly the same way as the Tropical Month (27-day) count shown above. 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, and has been identified on another great masterpiece, the Calendar Stone, also at Knowth. 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 cycle of the Moon, called the 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 kerbstones around Knowth, 127, is half of 254, or half the number of siderial lunar months in one Metonic Cycle.

Calculating the year

Just as the exact length of the year in days can be calculated at the Calendar Stone (kerb 52), so too can it be done with this Lunar Stone (kerb 93). It involves a simple calculation, using the stone as a guide. 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.


Some researchers say the art of the Stone Age was non-representational. It appears to be very abstract. Some say it cannot be decoded as having any real meaning. This quote from Jacob Streit's 1984 book Sun and Cross, quoted in chapter 2.3 of my book Mythical Ireland: New Light on the Ancient Past, is apt:

In the extensive literature on old Irish art and culture a certain attitude that seeks to simplify symbolism is frequently encountered. The cause may be that there is no authoritative interpretation of these phenomena or that all features of mythic religion are generally thrust aside from fear of being unable to handle them. In their place are irrelevant aesthetic consideration of themes and ‘ornaments,’ which are judged solely as decoration and considered as entirely external forms.

It is very difficult to reach definitive conclusions about the meaning of symbols carved into stone over five millennia ago. What we can say though, with more than an ounce of confidence, is that the builders of the great monuments of the Boyne Valley were competent astronomers. Newgrange alone demonstrates this. The breadth and complexity of their understanding of the movements of the heavens is unknown, but enough evidence has been advanced to suggest that the mound builders were more than just solar observers. In any event, it is simply not practical or useful to hinge your calendar upon two fixed points of the year – the solstices. Many primitive societies used a lunar-based calendar. There is ample evidence to suggest that knowledge of the moon's movements was not something beyond the reach of ancient peoples. In Island of the Setting Sun: In Search of Ireland's Ancient Astronomers and Newgrange: Monument to Immortality, I have outlined a substantial body of evidence which demonstrates that the megalithic builders were indeed possessing of a competent lunar astronomy. Even in the 18th century, Charles Vallancey found that peasants who could neither read nor write had a reasonable working knowledge of the Metonic Cycle and the Epacts of the moon.

Sunlight in Knowth's Western passage equinox 2000
Sunlight in Knowth's western passage, autumn equinox 2000.

Equinox sunlight in Knowth's western passage

Brief observations made by Mythical Ireland at Knowth's western passage on the autumn equinox, September 22nd, 2000, confirm that the sunlight does penetrate the passage at equinox. The event has rarely been seen, and had never been photographed until I took the above photo inside the western passage in the year 2000. This photograph clearly shows that sunlight enters the passage and illuminates the orthostats.

The event lasted only a few minutes before the sun sank below a large bank of clouds in the west. But it was a glorious moment as the sun's rays illuminated the southern wall of orthostats about one-third the length of the passage.

From outside, the sunlight on the wall of orthostats was clearly visible. At this time, 6:46pm BST on September 22nd, the shadow of the standing stone, or gnomon, outside the western passage entrance, was almost exactly in line with the carved vertical line on the entrance kerbstone. 

Inside the western passage, I could see that the sunlight extended some distance into the passage, but the sunbeam became very low to the ground the further in I went. After the sun went behind cloud, the light became diffused, and as my eyes adjusted to the dark I could see that the light extended right down into the passage, as far as the bend.


Kind thanks to Professor George Eogan, who not only took me on a personal guided tour of Knowth on July 19th, but then invited me to see the equinox event for myself. Professor Eogan was director of excavations at Knowth for around 40 years. I am also grateful to Charles Scribner for his help with trying to understand the lunar cycles. Thanks also to Chris Bruno for his wonderful animation showing how the stone works.

Sandstone Pillar at Knowth
A sandstone pillar outside the western entrance at Knowth. Around equinox, this stone casts a shadow on a vertical line on the passage entrance kerb stone.
Back to blog