Stonehenge: Neolithic Man and the Cosmos. John North

FIG. 37. The dating of the South Street barrow. On the right, the graphs for the setting of Sirius as seen in line with the first bay from the north ditch and the rising of Vega as seen similarly from the south, meet opposite the year 3260 and altitude 9.42°. On the left is the evidence for the sighting of Regulus and Bellatrix along the line of the diagonal row of stake posts at about the same time, but now at altitudes differing by about 0.92°. On the case of Capella, see the text.

      The date of 3260 BC, probably good to within half a century, fits well with two out of four radiocarbon dates obtained from the site. The four, briefly, are these: from oak under the mound (2810 ± 130 bc, equivalent to 3535 ± 165 BC); from an ox vertebra (2750 ± 135 bc, 3495 ± 150 BC); from a red deer antler, north ditch (2670 ± 140 bc, 3360 ± 260 BC); from another, second bay of the mound (2580 ± 110 bc, 3190 ± 150 BC).

      Without a knowledge of precisely where the observer stood, it is impossible to give a precise height for the barrow at the eastern end, but it was very close indeed to the height of a man’s eye above the carefully levelled ground on which the mound was erected (see sections at the right hand side of Fig. 36). This means that it was just possible to look over the barrow from its façade, for instance, in ways that might have been indulged in before the barrow was built.

      The calculated height in question is so close to the length of the sarsen marked S on the figure that it is tempting to suppose that it originally stood upright. It needed no significant stone hole, since it would have been held up by the chalk of the mound. This was not the opinion of the excavator, J. G. Evans, however, who noted that the mound material gave no evidence of collapse. He noted too that the three largest had been prepared for the positions in which they were found by having a slice of stone removed. We might have been inclined to see in S not a sighting stone but a stone that was meant to remain only just covered by the mound. Oddly enough, although it is too low for the mound in its prone position, it could have served this very purpose at an earlier stage, when a still lower angle had to be set by the mound. (This lower angle will be discussed later.) Attrition of such carefully prepared mounds by rain, wind, and invading plants must have been a serious problem, and regular shaping must surely have taken place.

      Other sarsens shown on our figure seem to have had a part to play in the planning of the diagonal D, and the spine as far as the third and fourth bays. The idiosyncratic line of stakes marked E in the diagram was very probably a simple alignment on the rising Deneb—but this explanation must be abandoned if the southern ditch eventually turns out not to be unusually shallow.

      Had there been only the transverse rows and the ditches, South Street would have added nothing to what has already been discovered about long-barrow astronomical practice, but a most unusual aspect of this barrow lies precisely in that very straight row (D) of stake holes that ran diagonally across it. As emphasized by the placing of the compass rose in the figure, this line has a most beautiful symmetry with the axial line of stake holes, for the angle between them is almost perfectly bisected by a north–south line.

      There are two possible explanations for this diagonal, one of them almost certainly correct, the other perhaps true of an earlier period of history. If, before the barrow was built, observations had been made at the same horizon altitudes along the two lines (axis and diagonal), both to the south or both to the north, then the symmetry implies that the same star would have been seen rising along one line and setting along the other. In the case of the South Street barrow, horizon altitudes were far from being equal in any pair of the four key directions, but this offers no problem, for the stars to be seen rising and setting in this way would have been seen at their extinction angles—Pollux to the north around 3600 BC or Bellatrix to the south around 3920 BC. In either case, therefore, one could have expected perfect symmetry with respect to the north–south line. As it happens, the Pollux date falls comfortably within the range of radiocarbon dates obtained from the piece of oak found under the mound. This is surely not the date of the mound as it is now known, or of the diagonal row of stakes, but it is quite possible that the direction was embodied in some nearby material structure—say a pair of stones or another barrow. In fact at a distance of only 300 m the line runs along the southern edge of another long barrow (one of three parallel barrows northwest of the Beckhampton roundabout).

      Whatever the truth of this explanation, why the diagonal line was incorporated into the barrow itself is of more immediate interest, and can be explained in a different way, not necessarily ruling out the first. It is difficult to give a precise argument for want of data from the ditches, but the rising of Regulus could have been observed along it to the northeast and the setting of Bellatrix to the southwest. Assuming that the line was regarded as very important, the entire mound could have been designed around it, with these new stars in mind. Perhaps somewhere else in the neighbourhood, or perhaps on this very site, these two stars were regularly observed along this line at equal altitudes (just over 6°) in the thirty-fifth century (point T on Fig. 37). Had this been done at South Street—and the radiocarbon dates tell us that this was not impossible—the mound would have been lower then, or the ditches shallower, or both; and then as the years passed, the direction no longer functioned as it had done, and it became necessary to look south at a higher angle than north in order to keep the old direction.

      By 3260 BC the difference was about 0.92°. Had Capella and Bellatrix been taken as the pair of stars, it would have been found that there was never a time when they could be seen at equal altitudes along the diagonal. As the figure shows, they were much closer, however. The reason for opting for the larger separation is to be found in the ditches. It is not difficult to calculate that no matter what the height of the mound, within reason, the required difference in viewing angles would be provided if the observer looking north were standing about 33 cm higher than the observer looking south. The precise value depends on where in the ditches the observers stood, but on the evidence at present available it seems that there was probably a difference of levels somewhere between 25 and 40 cm. The required viewing altitudes for the year 3260 BC (see the figure) imply a barrow that would have been about 33 cm lower at the crossing point of the diagonal than at the barrow’s eastern end. This is precisely a quarter of the way along its length of 36.8 m as judged by the bays—note that it comes after five out of twenty bays. If it sloped regularly, therefore, the barrow would have fallen from