Astronomical Time within Clava Cairns

In North East Scotland, near Inverness, lies Balnuaran of Clava, a group of three cairns with a unique and distinctive style, called Clava cairns; of which evidence of 80 examples have been found in that region. They are round, having an inner and outer kerb of upright stones between which are an infill of stones. They may or may not have a passageway from the outer to the inner kerb, into the round chamber within. At Balnuaran, two have passages on a shared alignment to the midwinter solstice. In contrast, the central ring cairn has no passage and it is staggered west of that shared axis.

This off-axis ring cairn could have been located to be illuminated by the midsummer sunrise from the NE Cairn, complementing the midwinter sunset to the south of the two passageways of the other cairns. Yet the primary and obvious focus for the Balnuaran complex is the midwinter sunset down the aligned passages. In fact, the ring cairn is more credibly aligned to the lunar minimum standstill of the moon to the south – an alignment which dominates the complex since, in that direction the horizon is nearly flat whilst the topography of the site otherwise suffers from raised horizons.

Cairns at Balnuaran of Clava. plan by A. Thom and pictures by Ian B. Wright

Passageways to the midwinter sun are commonly found in megalithic cairns, such as Maeshowe further north in Orkney, and the rising midwinter sun at Gavrinis, 4Km east of Carnac, in Brittany.

Such cairns with passageways and chambers are habitually categorized as “tombs”, and their environments “cemeteries”, as at Balnuaran of Clava. This subconscious consensus hypothesizes cults of death (paralleling the Sun’s “death” in the west at sunset) even when there is little or no evidence of bodies. Later Beaker burials, often in mounds, were “a late development of that period characterized by Passage Graves” (Glyn Daniel. 1960. 209.) built in France between 1800 and 1700 BC. Antiquarian romanticism established the name “tomb” for passage cairns, even when the archaeology does not justify such an interpretation for passages and chambers in the cairns at Clava, which are from the late 3rd Millennium BC. Calling all chambers tombs provides a convenient illusion that their builders were not primarily interested in astronomy.

There are profound astronomical reasons why cairns might face the extreme solstice sun in midwinter or midsummer. Such alignments, also found in stone circles, between their stones and long-sights to horizon features where, from that site, the location of the extreme sun on the horizon, to north or south may be recorded to at least a minute of a degree. Their purpose was not just to record the extremes of the sun, but also to define the beginning and end of a single year, and count the number of days in between successive solstices.

The natural calendar for the Stone Age was tied to the Moon since there are about 30 days between full moons which can illuminate the night. Marks made on bone appear to show cyclicity so that hunters could see the lunar month as an invariant. (see Alexander Marshack’s The Roots of Civilization) Counting for many months would reveal an interaction, in northern climes, between those moon phases and the seasonality caused by the movements north and south of the sun on the horizon. A unique example survives, from the late-Palaeolithic, of a midden bone on which a tally of days over nearly three years was scored, showing lunar years of 12 moons, within detours where the count passes the extremes of the sun, exceeding the lunar year by 11 days.

Thai Plaque circa 10,000 BC, front face. The tally is of days, notated as to phases of the Moon and with excursions showing the solstices. from Alexander Marshack 1991.

Thus it is not unexpected that, after the ice age, the counting of time might have been synchronized within megalithic alignments to the extreme Sun. A count could then start at one solstice alignment and end at the next alignment, after exactly one solar year. This solar year would initially be 365 days long – the Practical year. At Le Manio, my brother and I found, in its Quadrilateral kerb structure (see below), a long count conducted along the alignment to the midsummer sunrise. It was exactly three solar years long when counted in inches. This monument presents the synthesis of counting with the solstice, once a constant unit of measure was adopted to represent each day (along an alignment to the solsticial sun) within one or more year count. One should therefore see megalithic interest in solsticial alignments as probably integrated with an associated counting of solar years, between stones aligned to the solstitial sun.

The solar year count found at Le Manio between points P and R of its Quadrilateral. Note symbolic use of 36 stones in a kerb. Site plan by Alexander Thom, measurements from Richard Heath & Robin Heath’s Survey of 2010.

Having seen the inch employed at Le Manio’s Quadralateral to count each day, at the NW Cairn of Balnuaran one finds a distance equal to 365 day-inches between the ridge of the left-hand entrance sandstone jamb and the white end-stone, at the far end of the chamber (Richard Bradley, 2000, 117-119.). A day-inch count started at the entrance, with the winter solstice sunset shining to the end-stone. When next the sun shone into the chamber, the count would end at that end-stone . That is, the inch used at Le Manio was also used at Balnuaran, to count the 365 days in a single year between solstice alignments. This was the only practical way to accurately define the solar year in the late stone age.

Location of a 365 day-inch count between entrance and chamber kerb at NW Cairn

After four years, the operators of such a count would notice an extra day arising between the four solar extremes: 1461 day-inches were counted and so the solar year must actually be 365.25 days long. This knowledge was then probably presumed by their counting procedures, as we do today with our calendar through having a leap day every four years.

The Corridor Count of the Nodal Cycle

The outer kerb to inner kerb of the chamber is about 18.6 feet, a number corresponding to the years between lunar maxima, either maximum or minimum standstills. The monument signifies both these alignments through the two stones of the stone circle, straddling the entrance to the cairn, and might correspond to a way of counting this 6800-day cycle of the two lunar nodes which causes the Moon’s range of rising and setting phenomena on the horizon, to expand and contract. An explicit 6800 day-inch count, though possible within larger monuments such as the circular monuments of Le Menec and Stonehenge Avebury Circle, could not be compressed to feet per year without developing a substitute for our arithmetic notation.

Le Menec’s cromlech has a forming circle of radius 17 megalithic rods, each rod containing 100 megalithic inches, 3400 such inches in all whilst the Aubrey Circle has a diameter of 3400 inches, so that both monuments would allow as 3400 day-inch count from say lunar maximum to minimum standstill and this could then be counter in reverse, to count from the minimum standstill to the maximum standstill.

The NE Cairn had a count for a single year, over 9.3 times smaller than at Stonehenge and Le Menec (nearly 11.4 times smaller). Le Menec is hard to date but probably succeeded Locmariaquer to the East and Le Menec is probably younger, perhaps 4000BCE, whilst Stonehenge I (including the Aubrey holes and two opposed barrows) was probably built around 3100BC. Richard Bradley’s Radiocarbon Chronology [1], suggests the NE cairn may  date to 2000BC and, if the Clava culture that built the cairns was a development of what had come before at Stonehenge and Le Menec, then more advanced techniques may well have evolved a counting technique which could compress the size of monument necessary to count the Moon’s nodal period, as just 18.6 year-feet long: the length of the NE passage which then represented a journey through time.

Significance of Lunar Maximum and Minimum Standstill

Whilst the Sun can only enter the NW and SE cairns at midwinter sunset, the southerly moonset varies over 18.618 years; from setting north of their entrances (a minimum standstill) to setting south of their entrances (at maximum standstill). For half of this long cycle, the moon will also shine into the cairns as it is setting – every lunar orbit. Clava cairns could symbolically mark the maximum and minimum alignments using the stone circle which encircles all three cairns. A major study by Henshall and Ritchie (2001) compared the axes of Clava cairns as below.

“It has long been known that Clava-type cairns are aligned towards the south-west quadrant, and this is certainly true at ten passage-graves … These alignments appear to be targeted on positions in the lunar [nodal] cycle.”

This is made clearer (below) in that the lunar minimum and maximum have two standing stones in the stone circle, “standing for” those standstills, relative to the passageway of the solstice sun.

Sightlines to lunar maximum and minimum

There is also a stone in the north-west, at the end of the proposed solar year count which has a sightline passing to the right of the right-hand solar end-stone, upon which the midwinter sunlight . The pencil of alignments (in blue) for the two lunar extremes, after crossing in the chamber[2], touch the ends of a set of white outer-kerb stones. The dark “moon” stone is defined by these two expanding sightlines, as shown inset.

One suspects that, before the cairn came to be formed, a set of stones was used to establish these sightlines which converge on the “solstice stone” in the above diagram. The original day-inch counting then came to be incorporated, informing the design of the finalized monument. Whatever is the case, the lunar cycle was clearly of interest and probably relevant to counting the Moon’s nodal cycle using a solar year count in day-inches.

All of this leads to the corridor length, taking this route for the year count, to be 18.618 feet and the unlikeliness of this can be explained away by realising that 18.6 year-feet are 223 mean solar months long. Solar months are by definition 1/12th of a solar year long and the inch is 1/12th of the English foot exactly because, in a Lunation Triangle counted in megalithic yards per month, the yearly excess of the solar year over the lunar year is exactly one foot and each inch of that foot must be due to the mean solar month being one inch larger than the lunar month[3].

The inch is 1/12th of an English foot, and the 365-inch length can be seen as 30.44 feet, which is the length in days of a mean solar month, by definition 1/12th of the solar year. It was therefore possible to divide the 365-inch length of the solar year count into twelve portions, as one can divide any line or rope into twelve parts. Markers could be placed in the rope and whenever the year count proceeded past a marker, another marker was moved an inch, from the beginning of the count; so recording the number of mean solar months that had passed.

After each year the count would have gone back to the beginning, whilst the mean solar month count had advanced by exactly one foot per year. If so, the 18.6-year cycle between lunar maxima or more probably the lunar minima at Balnuaran, must have started the solar year counting, and not the solstice sun shining into the chamber: this is fundamental though initially counter intuitive. To start the count accurately from a lunar minimum would require a reasonably long sight to a lunar minimum, in rising or setting, to north or south. Thom gives an azimuth contour for the horizon altitude as below, from which the astronomers may have chosen the sight for its low horizon altitude towards the setting midsummer sun and, additionally, to the moon’s rising at maximum standstill to the North.

Availability of low elevation Sightlines to extreme solar and lunar events on the Horizon

Analysis of the site’s sightlines, horizon altitudes and latitude indicate good sightlines were limited to the southwest (setting) and north east (rising), but the clear emphasis within the built structure preferred the south-west. There are further stone circles and tall standing stones in this general direction but for the NE cairn and its count an alignment to the minimum setting of the moon was probably extended from the solstice stone of its stone circle (at the 2 o’clock position), through the minimum stone opposite (8 o’clock position) to the  8 o’clock stone of the central ring cairn.

The Second Solar Count, to the Ring Cairn

The distance between the solstice stone and ring cairn stone is interesting at 334 feet long since this is the solar year when seen as days each worth an Iberian foot of 32/35 feet. This would seem an attempt to find a foot so as to find a solar year count but this arrangement of measures was previously found in a counted length over 47 lunar months (a useful eclipse period) at Crucuno near Carnac, Brittany, between monuments circa 4000 BC. Namely from Crucuno Dolmen and Rectangle, the latter a kerb monument similar to Le Manio’s Quadrilateral. At the Quadrilateral, one can observe a 945 day-inch count from the “sun gate” to stone 32 of its southern kerb. Dividing 945 days by 32 gives a very accurate approximation to the lunar year because 945/32 = 29.53125. Due to the numbers that factor 945 (27 x 5 x 7 or 27 x 35), 27 English feet equals 29.53125 Iberian feet so that one can simultaneously count lunar months as 27 feet and count days in Iberian feet.

At 334 English feet, the count between the solstice stone of the NW cairn and the 8 o’clock stone of the central cairn, counts 365.25 days of the solar year, in Iberian feet, whilst simultaneously counting 12.368 lunar months of the solar year, in lengths of 27 English feet. The count is then aligned to the lunar minimum and one can also note that the length of it equals the circumference measured for the stone circle surrounding the ring, to which the endstone of the count is part. Bradley also notes that the stone circles tend to be organised into a twelve-fold structure which, as a solar year, represents the ecliptic surrounding the Earth. The ring cairn may represent the Moon being 2 x 29.53125 English feet wide (outer kerb-to-kerb).

This idea of using the Iberian foot as the primary count, integrating with lunar months as 27 feet long, takes the story on from Crucuno, Le Manio and Table des Marchands, where the components of this possible counting method were established as per previous postings. It was a cunning numerical shortcut here used to provide a much larger count along a more accurate sightline, between stones 334 feet apart.

This changes perceptions of the ring cairn from solar to lunar significance. The passage cairns are on the Earth, letting in the midwinter sun, whilst the ring cairn has no passage and is angled to the sun in a way that resemble the lunar orbital tilt to the ecliptic. The ring cairn is therefore symbolic of the Moon itself which might seem rather flimsey assignation without the reasoning behind the counts and practical alignment to the lunar minimum.

The diameter of the stone circle is 59 English feet, making the radius 29.5 feet, the days in a lunar month and this causes the form of the NE cairn to be repeated between the stone circle, as central chamber of the NE cairn, and the count to the stone circle (marked by a standing stone) from the NE cairn, as the corridor of the NE cairn.

[1] The Good Stones: a new investigation of the Clava Cairns, Richard Bradley,monograph 17 of Society of Antiquaries of Scotlan, 2000, 117-119.


[3] When the megalithic yard, as excess of the three year day-inch count, was used instead to count months, the right triangle became partially normalised to the number of months before there would be one lunar month more in the solar count relative to the lunar count, i.e. 32.625 to 33.625 months. Employing megalithic yards to count months fully normalised the excess to being the English foot of which there were then 32.585 feet in the lunar year and 33.585 feet in the solar year. In this way, there came to be 12 inches in that foot, ping the twelve months in a lunar year and becoming the 12 mean solar months in the solar year.