Alexander Thom

In a certain sense, then, one could say that the sun on O'Kelly's face, filtering through that window at Newgrange for the first time after who knows how many years, marked the birth of a new science. If Lockyer was its precursor and Hawkins the pioneer, credit for the first systematic theoretical treatment of this new discipline goes to an affable and very resolute Cambridge professor, Alexander Thom.

Thom began taking an interest in the astronomical orientations of the megalithic sites in Britain and in Brittany during the 1950s. His field studies, in which his son Archibald later joined him, extended over several decades and touched upon thousands of sites (Thom 1967, 1971; Thom and Thom 1978). According to Thom, the purpose of megalithic monuments was mainly, if not entirely, astronomical, and they were intended for the observation not only of the solar cycle but also of the lunar cycle (the lunar stations are the extreme points of the rising and setting of the moon through a cycle of 18.6 years; for more detailed information, see Appendix 1). Because observation of the lunar stations is fraught with practical difficulties, including the rather fundamental problem that the rising of the moon is not always observable, we must allow that megalithic astronomers were able to determine the points where the moon rises and sets on the horizon on the basis of observable intermediate positions. Clearly, familiarity with geometry and mathematics would be indispensable for such operations (even though as recently as the 1970s, archaeologists thought of megalithic man as a grunting, barely human beast, and that there are a few who still think so today). And here we see the second key point to have emerged from Thom's research, which is that the degree of knowledge of these disciplines had to have been at least up to the task of applying them practically. For example, Thom discovered that many large stone henges are not circular but ovoidal, or half circular and half ovoidal. These forms were obtained by using ropes and poles to lay out quite complex geometrical configurations, many of which are based on those right triangles that are called "Pythagorean"—that is, having all integer legs. The Thoms found evidence of their extensive use, particularly the 3-4-5 triangle, but also, for example, one whose sides measure 12-35-37 (122 + 352 = 372; try it to believe it).

One of the best conserved sites in which the geometrical constructions discovered by Thom are visible is Castle Rigg. Castle Rigg is a stone circle built in a splendid setting, on a plateau in the mountains around Lonsdale, England. It is made up of 35 large stones arranged according to a rather complex plan that was laid out in the following way. First, an alignment with the southern major lunar standstill was marked off with two monoliths set 32 meters apart. This line was then used as the base diameter for a semicircle

Professor Alexander Thom Stone Circles
Figure 2.12: Alexander Thom at the base of the Grand Menhir

inscribed onto the southwest side. On the northwest side, the figure was "ovalized" by the intersection of three circular arcs, the outer two with a radius equal to one third of the base diameter, the internal one with a radius equal to the distance from the point of intersection to the center of the semicircle. (Don't ask me why ...).

Another point that emerges from the enormous mass of data collected by the Thoms is the probable recurrence in the design of a great many megalithic monuments of a unit of measure equivalent to about 41.5 centimeters. It would be worth it to be able to officially name this unit of measure a "Thorn" as scientists do in physics, where units of measure take the names of the people who discovered the laws of the phenomena they measure (the newton, the ohm, etc.). Unfortunately, however, Thom let himself be swayed by the fact that the double of the unit he had discovered, 82.96 centimeters, is very close to the value of the modern English yard. This is probably coincidence, since all units of distance are vaguely similar because they derive from measures that are naturally available to us—the length of an arm, a forearm, the spread of both arms, a stride. Thom was obviously aware of this, but he nevertheless decided to call this unit of

Figure 2.13: Castle Rigg in Thorn's survey

Figure 2.13: Castle Rigg in Thorn's survey

82.96 centimeters a megalithic yard, probably in the interest of making it more easily legible (for British and American readers, anyway). He also introduced the megalithic inch, equal to 1/40 of a megalithic yard.

As a consequence of choosing this value for the yard, double of the unit probably utilized by the megalithic cultures, all the measurements consisting of odd-numbered multiples of the real unit come out with a fraction dangling clumsily off the end, not only compromising clarity but also encouraging the pettiness of those would present it as proof that Thom was a mad fool for thinking that megalithic man used the English yard. So, despite several pieces of evidence found by Thom, many scholars continue to doubt the existence of the megalithic yard. Even worse, I myself have listened firsthand to an archaeologist (whose shall go unnamed) maintain that Stonehenge was built "using the length of a stride.'' If this is so, we can imagine the following exchange (Stonehenge, 4500 years ago):

"Would you mind moving that 50-ton trilith just a bit to the right? I'm concerned that the solstice sun may not get through.''

It is abundantly obvious, or should be anyway, that any architect of any period would need a precise unit of measure in order to plan a monument as complex as a henge, and the notion that something like, say, Avebury could have been built "by eye'' is patently absurd. Thom's idea, however, goes well beyond this, for in his view megalithic societies used the same unit of measure in a large variety of monuments, and this has far greater ramifications than the relatively simple problem of designing a specific monument. There are many sites, some quite distant from the others, where the use of the megalithic yard looks convincing, though it is difficult to ascertain whether a "standard" was used or whether it was the similarity of some human-related measures (e.g., the arm) that influences Thom's measures. At Woodhenge, for example, though all that remain are the post holes, it is still possible to make measurements as to how the oval rings of wooden posts were laid out. There are six such rings with circumferences of 40, 60, 80, 100, 140, and 160 megalithic yards and whose axes of symmetry are oriented toward the summer solstice (perhaps one day someone will figure out why there is no 120-yard ring). Other examples can be found on Lewis Island, in Scotland, home of Callanish, a complex megalithic structure composed of a stone circle 12 meters in diameter with a large monolith and the remains of a burial chamber in the middle. Two parallel lines of stones run east of north for 80 meters, while other lines marked by stones point east, south, and west, giving the site the aspect of a bent cross. The interest in astronomy of the builders of Callanish is apparent in both the north-south alignment and the northeast arm, which indicates the direction of the i >

20 metres

Figure 2.14: Callanish moonset azimuth at the southern major standstill (Hawkins 1965, Ruggles 1999). Twelve kilometers from Callanish is the beach at Dalmore where a neolithic village was discovered in 1982, yielding a number of interesting artifacts. Among them was a piece of bone 3.4 centimeters long with notches between 4.9 and 5.1 millimeters apart, which might have served as a ruler for megalithic inch measures (Ponting 1988).

The results of Thorn's surveys can be summarized as follows:

1. Megalithic builders had a complete and somewhat sophisticated knowledge of solar and lunar astronomy, and their monuments included observational purposes; in many cases Thom found monuments built on sites chosen specifically for their relation to pronounced irregularities of the horizon line, which facilitated astronomical observation (astronomical use did not, of course, exclude other uses and meanings—religious ones, for example—ofwhich, alas, we are unaware).

2. The megalithic builders' understanding of geometry and mathematics was enough to enable them to build quite sophisticated geometrical constructions, such as oval rings plotted with intersecting circular arcs. Often the starting point for these constructions was a triangle with all integer legs (the reasons for this remain unknown).

3. There is a remarkable uniformity in the planning of megalithic sites; there are also hints at the widespread use of a common unit of measure that Thom called the megalithic yard.

Let us look at how astronomy was used, according to Thom, in the places we have visited thus far. In Thom's view, Carnac was an astoundingly large and ambitious grouping of lunar observatories. The heart of the Kermario complex was the Le Manio Menhir, the departure point for a number of long alignments. The Grand Menhir also served as a reference point for long, precise alignments that parted from Locmariaquer, passing among various tumuli and menhirs to indicate the limits of the lunar stations. It is possible that this great interest in the moon exhibited at Carnac, apart from the likely purpose of predicting the eclipses, arose from its builders having made the connection between the moon and the tides, which in Brittany are an extremely significant natural phenomenon (recently, the Thoms' astronomical interpretation of the site has been strongly criticized, particularly with the thesis that the Grand Menhir could have toppled while it was being erected; however, there is no other available reasonable explanation today for having transported tens of thousands of tons of enormous stone blocks to Carnac, and the whole question fully merits a complete reexamination from scratch).

In Ireland, every element of the sacred landscape of the Boyne Valley was designed to serve some astronomical end. At Newgrange, in addition to the winter solstice alignment we have already seen, there are numerous aspects of the carved inscriptions that suggest the study of both the solar and lunar calendars. One of the corridors at Dowth is oriented toward the winter solstice, while the presence of 18 tumuli in the Knowth group seems to indicate a connection with the lunar cycle; in fact, Knowth 2 and 4 are aligned with the northern major lunar standstill, and several of the figures carved into the stones can be interpreted as lunar calendars. Finally, at Loughcrew, ""Tumulus T'' bears a striking structural resemblance to Newgrange, though its east-west orientation suggests that its meaning was probably different.

In the Orkneys, the main focus was on the solar cycle. Maeshowe, for

Figure 2.15: Alignments in the Carnac landscape according to Thom

example, is aligned with the setting of the winter solstice. The last rays of the setting sun filter through the complex to illuminate the central niche, immediately after which the sun disappears behind the Barnhouse Stone, in alignment with a tumulus several hundred meters away. Structure 2 at Barnhouse is aligned with the rising of the sun on that same day, while structure 8 is aligned with the summer solstice. As we have already seen, these two structures are probably contemporary and played complementary roles, the exact nature of which continues to elude us. We find at Maeshowe a phenomenon that I personally have always found exceptionally intriguing— as certainly did the neolithic astronomers who built it: looking from Maeshowe toward the western horizon, for some 20 days on either side of the winter solstice, the setting sun disappears behind the crest of Ward Hill, and then rises again for several minutes at the base of the hill. This extraordinarily beautiful event occurs because the sun's trajectory takes it behind the hill's shoulder-like protuberance, interrupting the line of sight from Maeshowe. (Plates 3 and 4)

As far as Avebury is concerned, oceans of ink have been spilled in its name, not to mention rivers of inanity that run from "telluric currents'' to the "representation of the human egg cell.'' In truth, very little is certain about this monument. The Thoms, discouraged by the incompleteness of the remains (Avebury had been used for centuries as a ready-made deposit of quarried blocks), preferred to concentrate on how the complex geometry of the three circles was determined, one of their conclusions being that it had been necessary to inscribe a circular arc fully 750 megalithic yards long— that is, 723 meters. This is a colossal measurement to plot when all you have to work with is rope and poles, if only for the fact that it would have been impossible for the persons on opposite ends of the arc to communicate by voice. Finally, the purpose of Silbury Hill remains utterly mysterious, although the prospect of reading it as an astronomical observatory is rather tempting.

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