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Established 16/3/2000

Star circleLunar Stone: Calculating the synodic and siderial months
and the exact length of the yearStar circle

Knowth, one of the large mounds of the Boyne Valley complex, has become famous in recent years for its huge array of 'Neolithic art'. The site has been said to contain over a quarter of all known art from this period in Europe, and now that excavations are winding up at the site after nearly 40 years, a full inventory of the art has been made.

But a question must be asked of this amazing written record, which comes to us from as as far back as 3,300BC, at which time the construction of Knowth began. 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 arcane and mysterious, or even astronomical?

I believe that much of Knowth's art can be explained properly when viewed from an astronomical viewpoint. In the context of the Boyne Valley complex of sites, Knowth begs to be interpreted fully and properly, and in my opinion that process must begin with astronomy.

Crescent moon symbols on the stone
Crescent moon symbols on the Knowth Lunar Stone.
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, semi-circles, crescents, lines, lozenges and other features. The casual visitor will also notice that many of these forms appear present an astronomical theme, revealing an interest in the heavens.


American Author Martin Brennan has suggested that this kerbstone at Knowth was used for making a 27-day lunar calculations. Although he's on the right track, I believe it goes much further, and 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.

Lunar kerbstone

Our lesson on how to solve 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. With this system of classification, we can now explain the symbols, how they were counted and, what the 'secret' knowledge contained on the stone really is. Then the symbols will be seen as parts of a unified whole.

In my opinion, the symbols can be interpreted 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.

27-day count - inwards
27-day count - outwards
Fig.1: The 27-day count working inwards
Fig.2: The 27-day count outwards


This is the count identified by Brennan. It begins on the extreme right of the stone, working (as shown in Fig. 1 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 (Fig. 2), 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!

The 29-day count working inwards
The 29-day count working outwards
Fig.3: The 29-day count working inwards
Fig.4: The 29-day lunar count working outwards


The second count was not identified by Brennan in his studies of the stone. It is important to the overall understanding of the stone, and shows that the level of understanding of the lunar movements was quite high in such supposedly 'primitive' times. 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.


Just as the exact length of the year in days can be calculated at the Calendar Stone, so too can it be done with this Lunar Stone. 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.


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. We disagree with that viewpoint. With enough circumstantial evidence from the interpretation of the carvings, the sites and the myths, we can put forward a hypothesis – that the Boyne Valley builders were astronomers, who studied the complex movements of the Moon and planets. So far, no serious effort has been made to disprove the astronomical function. Is it the conclusive solution to what was happening in the Boyne Valley over 5,000 years ago? Maybe not, but it goes a long way to solving an age-old puzzle - what were these enigmatic sites about?

Other pages of interest:
The Calendar Stone - complex lunar cycle calculations.
Equinox sunlight at Knowth - Sunlight in the western passage.
Inside Knowth west - 5,000-year-old megalithic engravings.
Stellar engravings - the Stone of the Seven Suns at Dowth.

I am, as always, greatly indebted to the masterful astronomer Charlie Scribner, who has been teaching me the astronomy of the ancient people over the last year.

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All information and photos, except where otherwise stated, copyright, © Anthony Murphy, 1999-2015
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