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even partly successful would have been formidable.

Hawkins explains this by supposing that the Late Neolithic priests used the 56 Aubrey holes as a device for predicting many more eclipses. The basis of this suggestion is that 56 is almost exactly the number of years in three of the lunar cycles of 18.61 years. Such a 56 year cycle can be used to record the azimuthal swings of moonrise and moonset over several centuries before becoming inaccurate. Eclipses of course only occur at full and new moon and when the moon is near the ecliptic at those times and is thus in line with the earth and the sun. By moving up to six stones at set intervals apart regularly around the Aubrey circle at the rate of a hole a year many important eclipses can be foreseen. The details do not matter here: the fact is that it seems to be workable. The only objection, apart from a general disbelief that anything so advanced was possible at so early a date, is an archaeological one. Those Aubrey holes which have been excavated were apparently refilled with their own chalk rubble soon after they were dug: on some cases they were then emptied and filled again. There were cremation burials in most, sometimes in the primary refilling but more often in the later, and objects found with the bones were of Secondary (Late) Neolithic type. No sign of the holes having ever held stones was noted though presumably large posts would have served almost as well as markers of the sort required by Hawkins' theory.

There seems to be no doubt that a circle of 56 holes could be used for eclipse prediction in the way Hawkins suggests. Whether the Aubrey holes at Stonehenge I were so used is another matter. It depends on what the sum total of the evidence about the cultural level of Late Neolithic Britain suggests was possible at the time. If a circle of 56 holes had been found in the floor of a contemporary Mesopotamian temple it would be quite reasonable to accept that the astronomers there, who are known to have been greatly concerned with eclipses, used it for prediction. Indeed a practical method for such prediction suitable for early times was not known until Hawkins set out his ideas. However Late Neolithic Britain seems at first sight an unlikely environment for such spectacular intellectual advances and this is indeed the crucial question. Are Hawkins' theories about Stonehenge I plausible against the background of our knowledge of the rest of Britain at that time?

500 stone circles have been accurately surveyed

Here we turn to Megalithic Sites in Britain and in this we find the essential background to the problems of Stonehenge just discussed for Thom—an emeritus professor of Engineering Science at Oxford—has spent many years visiting and accurately surveying some 500 stone circles and allied sites in England, Wales and Scotland. The problems of Stonehenge when

Plan of the Dinnever Hill, Cornwall, stone circle (NGR: SX/126800), reproduced with the kind permission of Professor A. Thom. This is one of Thorn's type A flattened circles, made of very small stones and 130 feet across, and the dotted lines show its probable geometrical construction. A large part is a true circle inscribed from the centre point. Part of the north side is a segment of a circle of twice the radius of the foregoing, drawn from the point on the circumference opposite. The flat northern section is joined to the rest by two sharp 'corners' formed of short segments of circles of half the radius of the main one. These were drawn from the intersections of the main and long radii at the extreme points of their respective arcs.

discussed in isolation inevitably assume a certain air of unreality: too much hinges on too little evidence. However the formidable mass of accurate data on many comparable circles assembled by Thom has allowed him to extract from it by statistical analysis a startling amount of information which, considering the qualifications of the author, is scarcely likely to be challenged. The conclusions he draws about the circles are twofold and concern the geometry of their construction and their astronomical alignments and function.

Many of the rings are true circles and were obviously laid out with a length of rope tied to a central peg. Many others however are clearly not circular and Thom shows, with a wealth of examples, how they must have been constructed. Many are laid out with circles of different diameter superimposed, whose centres are

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