Stonehenge: Neolithic Man and the Cosmos

It is hard to avoid the feeling that the near-perpendicularity of the two Deneb directions was the result of an attempt to make them precisely so. Had they been so then each would have been precisely 45° from north, that is, yielding azimuths of 315° and 45°, rather than 314.2° and 45.8°. The discrepancy might have been adjusted by a slight difference in viewing altitudes. It is not likely to be entirely due to our own error in the azimuths, since the need to bring Betelgeuse to the same altitude was surely enough to thwart any ambition to produce a Deneb right angle. What is certainly of great interest is the similarity between the Skendleby 2A’s use of perpendicular sighting lines to Deneb and the presence of perpendicular sighting lines to Sirius over the natural horizon at the Horslip long barrow. The date assigned above to that barrow was only three quarters of a century later than the date obtained here for Skendleby 2A.

The façade held thirty-one posts, and showed signs that at some stage posts had been removed by burning. This is of course one of the easiest ways of removing an old heavy post, but it should not be assumed that removal was once and for all. The façade functioned, if the present analysis is accepted, for many centuries, and might have been replaced many times. Traces of the posts of the façade show that they were by no means of constant girth, and some posts—especially towards the middle—were no doubt appreciably taller than others. How they were arranged it is impossible to say, but the excavation produced fragments of charcoal from small branches that the excavator interpreted as possibly indicating a wattle and daub arrangement. This fits very well with the idea of a curtain of timber through a gap in which observation of the first glint of the rising midwinter Sun was seen, in a way that will be explained.

FIG. 41. Plan of the area around the façade of the Skendleby 2 long barrow. An observer in the front ditch, looking over the façade in a direction originally marked with four stakes, saw the setting of Deneb. Note the north–south and east–west directions to the corners of the façade from the observer. An observer at the gap could observe the setting Sun at its midsummer extreme, along line 1. Line 2 is the sight-line towards the midwinter rising Sun from the ditch at the northwest (see Fig. 40 (#ulink_1af62872-f102-5cbb-8e4a-18ba3d231b06)).

The line of four posts meets the backfilled ditch at its very centre, and there are two equally spaced parallels to it that graze the holes for the large D-posts in the burial area and bring us to the corner of the (original) end-ditch. Over a natural treeless horizon, midwinter sunrise was at altitude 0.57° and azimuth 132.7°, and midsummer sunset at an altitude 1.44° and azimuth 311.6°. From a point near the northwest corner of the original back ditch (see Fig. 40 (#ulink_1af62872-f102-5cbb-8e4a-18ba3d231b06)), the Sun would have been observable at its midwinter extreme along the line marked 2 in the figure, grazing one of the burial split trunks and one side of the gap in the façade. The burial post would have covered the solar image as it rose, being about 10 per cent larger in angular diameter. The first glint of the Sun’s image would thus have been trapped between two posts, almost (but not quite) in a manner that was to become classic over the following two millennia. Midsummer sunset was similarly observable by an observer looking along the line of sight marked 1. The posts flanking the gap in the façade were unusual, as it happens, in that they had flattened faces to the inside. It was easy to follow the rising Sun, by shifting one’s own position slightly, opening and closing the slit at will. The angular width of the D-post was well matched to that of the Sun, as seen from the end-ditch, being about 10 per cent larger. From the façade, of course, the post was nearer and the margin much greater.

While we do not have a ditch section at the observer’s position—a high priority for any future excavation—we do have one not far from the centre that offers support for the claim that the end ditch was used for solar observation. As shown in Fig. 42 (#ulink_795cc65b-47d3-5cf7-862a-99b9a4352cb0), a ledge there—had it been used for viewing—would have brought the observer’s eye to the same level as the top of the posts over which the setting of Deneb was observed. Given no other obstacle, in other words, the observer can see perfectly well down to the natural horizon. This suggests solar viewing, since no star, not even Sirius, is visible down to that level (0.57° without trees). Observations would have been made by someone standing at the point described, but level with the step and at the inside edge of the ditch—in fact more or less at the old ground level.

FIG. 42. The front ditch (right-hand side) and (original) rear ditch of the long barrow Skendleby 2A. The observer stands in the front ditch on the upper step in the chalk, with eye at E, and views the setting Deneb at altitude of nearly 14°. The same observer at ground level would just have been able to see over the posts at F. The lower step was cut for the later barrow (Skendleby 2B). At the left of the figure a section is reproduced probably resembling others from which midwinter observations of the Sun could have been made with Skendleby 2A (and possibly later). The observer’s eye (0) is at this end evidently also on a level with the top of F. The places actually used no doubt offered a more comfortable place to stand than the one shown. At the optimal point for the natural horizon the observer stood virtually at the old ground level, but with eye at the same level as shown here.

This all leads to the suspicion that here are signs of the very first activity of this sort on the site. The old ditch at this end could have been cut only after the ditches for crosswise viewing of Deneb and Betelgeuse were added. If the unsymmetrical solar arrangement ever made it seem desirable to view the Sun from a more central place, this would have demanded a higher altitude, easily arranged by building up the relevant façade posts. (For viewing along any of the set of three parallels, for instance, they would have been 2.9 m above ground level at mid-façade.)

One cannot be certain that there was no tree cover on the horizons needed for solar observations of the sort described, but both horizons are set by land fairly close by. It was shown from an analysis of molluscs sampled during the excavation that the immediate neighbourhood of the barrow was grassland, and that the mixed deciduous woodland that had covered the area had been cleared not long before. There are good reasons for thinking that the site was chosen for its solar properties: to bring the sighting lines to the solstices as near to parallels as possible, a higher altitude was needed towards midsummer setting than towards midwinter rising. The idea of perfectly reversible solstice alignments will be met with repeatedly elsewhere, but only at a much later date. Skendleby 2A is remarkable chiefly because it is such an early example of a carefully designed solar structure.

What of the implications of the mound, our Skendleby 2B? It should be plain enough that it brings alternative azimuths into the reckoning. The mound edges mentioned earlier provide much the same azimuth as the second crucial figure, and it seems that an effort was made to keep it, presumably not only for continuity’s sake in some abstract sense but because of its implications for observation of the Sun (the change in which is so slow that we can here ignore it). This, however, does not explain the direction of the very well defined line of stake posts and larger rubble running down the spine of the barrow.

Viewing across this would have provided azimuth 228.9°, which replaces the previous 233.9°, in our conjectural reconstruction of observing practice. Each of the options may be combined with a direction (48.9°) perpendicular to the spine of the barrow and the other related lines. In view of the barrow’s imperfections, it is unlikely that much more reliable data than this will ever emerge. Applying the usual method then yields a rather surprising result. The first option suggests viewing of the rising of Deneb and the setting of Aldebaran at an altitude of 15.76° around 3750 BC. The second option suggests viewing of the rising of Deneb and the setting of Betelgeuse at an altitude of 15.75° around 3730 BC. What is surprising is that the same mound could have been used to observe the setting of Aldebaran and Betelgeuse at more or less the same dates, as long as the directions of viewing were not too stringently set. (Aldebaran and Betelgeuse could have been seen setting across Skendleby 2A at points 5° apart in azimuth.) We have always assumed—for want of any sign of permanent direction markers—that observers after the initial architectural act had no precise means of knowing that the direction of view was strictly at right angles to any particular edge. It is for this very reason that a long barrow, used as explained, could have avoided becoming plainly outdated, in the minds of those who used it, for decades, even centuries. From the moment of foundation of a barrow, the stars were slowly shifting their places of rising and setting, but it would have been long before any but the expert realized the extent of the drift.

This is to assume that the barrow was used for ritual observation after its foundation, and that it was not just a question of ‘building the stars into the monument’ and then paying little attention to its properties. That the front ditch was still being used at Skendleby 2B to observe the setting Deneb, in exactly the same way as before, seems very likely, and the evidence is beautifully simple. The upper step on the inside wall of the ditch (Fig. 42 (#ulink_795cc65b-47d3-5cf7-862a-99b9a4352cb0)) is just 32 cm above the lower, and this is exactly what would have been needed to change the viewing angle from 14.00° to 15.75°, at the distances in question, assuming that the property was to be retained whereby the relevant façade post came to the height of the observer’s eye. As for the character of what are here being referred to as steps, it is not to be supposed that they resemble those at St Paul’s. They were rough-hewn. They do seem to have been worn with some use, although very frequent use would surely have worn them more. These things have not been properly studied, since ditches have usually been regarded only as quarries for mound building. Either observation was done relatively infrequently, say by a single person at selected seasons, or the step was protected, perhaps by a wooden platform that it was meant to support. The second seems in any case not unlikely, just as ladders allowing entry to ditches in general would have been almost essential.

Skendleby 2B, with its long mound, perhaps built in more stages than suggested here, was thus brought into existence to redress the changes of the centuries. Some centuries later, and perhaps because of dissatisfaction with yet further changes nearby, a completely new barrow was built along similar lines, namely Skendleby 1. Very briefly, starting from azimuths 36.3° and 218.6°, one may conclude that the rising of Vega was seen from the southern side and the setting of Bellatrix from the other, both at altitude 17.4° around 3120 BC. It has to be said that there is another possibility, and that around the year 3550 BC the rising of Vega could have been combined with the setting of Betelgeuse, at altitude 19.0°. Other things being equal, the (later) option with lower altitude seems more fitting to the known scale and architecture of the ditches. Perhaps the two phases were both known; and it might even be prudent to assume that there was a still earlier ‘façade’ phase, as at the barrow nearby—in which case the labels Skendleby 1A, 1B, and 1C ought to be reserved. The chief reason for favouring the year 3120 BC (with the usual uncertainties) is that Skendleby 1 has provided two radiocarbon dates, equivalent to 3125 ± 225 and 3000 ± 300 BC (2460 and 2370, each ± 150 bc). The first of these could hardly be closer to our figure, although it would be unwise to claim better than an accuracy of a century or so in our own dating.

It seems that there were three centuries between Skendleby 2A and 2B. In a third phase, or even in a tail appended to 2B, it might have been used with Deneb still, but with Bellatrix in place of Betelgeuse—at the mound head around 3420 BC, say. Deneb would eventually have become unusable there. It is just conceivable that in due course Skendleby 2 passed into yet other phases. It would in this case have been built up slightly higher, making the viewing angles around 20°, and it could then have been used successively with Vega and Aldebaran (around 3310 and 3070 BC, in different ways) and Vega and Betelgeuse (around 3030 BC). On the whole it seems safer to put aside all these options, and to associate a highly specific structure only with a first probable use, that is, the use for which it seems to have been deliberately designed.

Did Skendleby 1 have any solar use? Its axis is less than 2° away from the required direction. There is an almost perfect alignment set by the revetting posts running from the northwest corner of the mound towards the right-hand (southwest) edge of the entire façade, but this part of the façade has been lost. The burial area might have been implicated in some alignment using the middle of the façade, but it is difficult to see how, and much less is on record concerning the fine structure of the façade here than at the other barrow.

Most of the fourteen radiocarbon dates associated with Skendleby 2 provide support for the gamut of astronomical dates offered here. The oldest, for a piece of façade charcoal, was equivalent to about 4250 ± 100 BC (1965 ± 80 bc). This was presumably large and very old timber, and is recognized as anachronistic. There followed three highly consistent specimens, of oak charcoal and antler, the oldest being 3910 ± 120 BC (3155 bc), and so belonging to phase 2A. One item of charcoal from the north burial post pit (3790 ± 150 BC), a solar observing post, shows this phase continuing in use. Another specimen from the same post was well over a century earlier—supporting the idea of post renewal. The mound and burials produced more, hovering around 3500 BC, all with similar ranges of uncertainty; and there were others still later. These dates, however, are less interesting by far than the simple fact that the Sun’s turning points were being accurately observed at a Lincolnshire long barrow in all probability just before the beginning of the fourth millennium.

As a postscript to this section, it is worth putting on record a curious fact about the nearest stretch of the A1028 road, marked on the Ordnance Survey as a ‘Roman Road’. A consistently straight tract of this is still in use, and covers nearly 4 km. The now disused part of the road continued in exactly the same direction for 7km more, taking it south to the village of Burgh le Marsh. The section still in use passes within 400 m of both Skendleby barrows, and its azimuth is 133.7°—differing therefore by only a fifth of a degree from the line of stake holes in front of the façade of Skendleby 2. This does not imply any immediate connection with the barrows, of course, but it does seem highly probable that the road embodies an alignment on the Sun, and that this was pre-Roman. The gradient is very slight (averaging 0.23° upwards in the direction 313.7°). The direction of the road never matched that of the extreme of midwinter sunrise, but it did once perfectly match that of sunset (last glint) at the summer solstice in the Neolithic period. Using the figures quoted here implies a date in the neighbourhood of the twenty-second century BC, but there are slight uncertainties in the direction and altitude, and these are far more critical than in the case of dating by the stars. (The direction of midsummer sunrise changed by only two-fifths of a degree over the previous two millennia, for the stated altitude.) This is our first example of a trackway aligned on the Sun, but as will be shown in the following chapter, it was by no means an isolated instance.

The Radley Parallelogram (#ulink_f0de815f-a611-5d5b-b24a-af55cd78774e)

A barrow nearer to those of Wessex that seems to have shared some of the contrivances of Skendleby 2 has a most unusual shape, for its mound was in the form of a parallelogram, and not especially elongated. It was excavated at Barrow Hills, Radley, Oxfordshire, in a rescue operation led by Richard Bradley in 1984—as one of many prehistoric monuments on a site destined to be submerged under a housing estate on the northeastern edge of the town of Abingdon. Lying on a gravel terrace a little more than a kilometre north of the River Thames there, this site had seen Neolithic, Bronze Age, Romano-British and Anglo-Saxon settlement. (The tradition of refusing to treat the land as a museum is clearly an ancient one in Abingdon.) The nearby Abingdon ditched enclosure was separated from the barrow by a very shallow valley, and it is assumed that the two were set up at much the same time.

The long barrow is difficult to analyse not only because most of its surface detail has been lost but because it had ditches that were superimposed on it, in some places cutting through an older trench, assumed to have been a post trench. Fig. 43 (#ulink_35597d30-01d4-5b9c-8deb-48f371dcd898) shows the ditches in rough outline. It was suggested by Bradley that there were four phases:

(1) A fenced enclosure, the posts of which were later removed. Our outline figure hints at the existence of the narrow trench in which these were taken to have been placed. He thought that the grave might have belonged to this phase, or more probably to the next. It was very shallow and held two crouched burials of adult males laid along the axis of the site with heads at opposite ends of the grave and legs laid across one another.

(2) A three-sided ditch (surrounding the first trench to the north of points x and y on our figure) and large post holes E and F, the former seeming to have held a split trunk. (For the notation, see Fig. 46 (#ulink_57953086-9e28-555c-bffa-fbff5e022697).)

FIG. 43. Outlines of the ditches surrounding the long barrow at Barrows Hills, Radley.

(3) The open end (below x and y, where a causeway was left) was closed with another length of ditch avoiding one of the large posts.

(4) The outer ditch was cut in a series of segments between 2m and 5 m long, with a causeway at the southernmost corner. There were deposits of pottery and flint scrapers near both causeways.

Stretches of the sides of the outermost ditch were clearly deliberately curved, in contrast to the inner ditches, which contain some quite passable straight lengths, for example on the ditch floor. Averaging over three of these in the short direction and four in the long, the plan can be provisionally taken to have been meant as a parallelogram with sides at azimuths 136.0° and 216.0°.

It is very unfortunate that all detail of the structure of the mound has been lost, but one thing is clear: the trench and inner ditches were intimately related in shape. The material thrown up from the trench would not have been enough to build a mound worthy of the name. Even the material from the inner ditches would not have created an artificial horizon to a person standing anywhere but in the ditch. This presents a difficult problem. Why did they cut into the trench at all?

There are many possible answers, but it is not easy to find one that carries conviction. The first phase might have begun with the outer ditch, perhaps starting from the south side, and looking north to Deneb’s rising. By the usual procedures—possibly using E and F as scaling posts—the required angle of view would have been known. The mound area might then have been marked out and surrounded with short revetting posts to retain the soil. But the outer ditch alone (about 90 cu m) would not have provided enough material to create a revetted mound setting an angle of 12.3°. Perhaps labour was in short supply, or for some other reason it was decided to build a less ambitious mound. For the same viewing angle they would have needed to move closer. Since the mound was so much lower, the revetting posts were no longer needed, so that they and the trench would have been dispensed with.

A first impression of the ditches at Radley is that they are a rather chaotic patchwork, but this is not so. There are surprisingly many points of agreement between the excavation outline plan and a geometrical scheme shown in Fig. 44 (#ulink_fef0f5bf-a14c-5777-9a8d-9a33727f8ad9), tending to support—rather than prove—the idea that the outer ditch was planned in relation to the mound limits, the positions of the southern posts, and the grave area (ill-defined to the excavators). It should be noted that all circles in the scheme are centred on the parallelogram, and that the centre in all probability was originally at the head of the grave. The scheme otherwise speaks for itself. Only those with the experience of making geometrical constructions on the open landscape, with antler pick and ox-bone spade, have a perfect right to say that agreement could have been closer.

Another explanation of the discarded trench might be that an attempt was made to build a mound with high edge—say half a metre—from a shallow inner quarry ditch, but that quantities were misjudged, and the revetting, growing increasingly unstable, was abandoned in favour of a low mound. The inner ditch (about 75 cu m) would have provided just enough material for a 12° mound tapered to the ground, without posts (Fig. 45 (#ulink_78c82f1c-6693-56d7-ab6b-075839a54139)).

No matter what the explanation, it can make little difference to the astronomical geometry of the situation that has come down to us, which has interesting echoes of Skendlebury 2. There, matters were so arranged that Deneb could be seen rising across the barrow and setting down its axis. Here, at Radley, conversely, Deneb was to be seen rising along the length of the barrow (by the observer at C), and setting across the mound (by observers at points marked D, or between them). More details will be given shortly. At Radley, however, there was exactly the same doubling of function with respect to Bellatrix too: it would have been seen rising across the mound by observers between points marked A, and setting along the barrow’s length by an observer at C.

There is some latitude in the placing of observers in the inner ditches, since all that we require of them is that the lines of sight pass over points of maximum altitude of the mound. In fact each of the viewing positions marked on the figure has something more to recommend it, however insignificant it may seem (B was at mid-ditch to E, B was off centre at a point where the ditch was cut slightly deeper, and so on).

Ignoring for the time being the slight discrepancies between those ditch lines and azimuths that are exactly multiples of 45°, the following solution illustrates principles found from earlier barrows. Ensuring the same viewing altitude in the two directions would have guaranteed that Deneb—observed at right angles to a short edge and parallel to a short edge—was to be seen rising and setting. Neither here nor at Skendleby is viewing strictly along an axis. The same goes for Bellatrix with respect to the long sides. All four possibilities, however, would have held simultaneously only at one particular time in history. To find that time and the shared viewing altitude for the latitude of Barrows Hills (51;40, 45°), our usual procedure is followed. Given the ideal as explained, the year turns out to be 3700 BC and the viewing altitude 12.3°.

FIG. 44. Potential geometrical construction lines for the entire original system of ditch and mound at Radley. Only the surviving ditch edges are shown. The short sides of the parallelogram in particular are based largely on the ditch structure and not only on its outline as drawn here.

FIG. 45. The probable overall shape of the Radley mound, in idealized form.

FIG. 46. The mound area and inner ditches of the Radley long barrow, in the form of a parallelogram. The mound was limited by a narrow trench, probably before these ditches were cut. The broken lines show what is here taken to have been the ideal aimed at. The short sides are drawn here at exactly 45° to the cardinal points, differing only very slightly from what would be suggested by internal features of the ditches. The lines of sight are drawn parallel or perpendicular to the sides. Another ditch, omitted, but more or less surrounding what is shown here, was about 2 metres distant from it. Points A, B, C, D, and R are potential viewing positions for the stars Bellatrix, Deneb, and Rigel.

The date is of course very sensitive to the chosen directions, and in all of them there are uncertainties attendant on the relatively small dimensions of the barrow. The measured azimuths quoted earlier produce a year of 3740 BC and a viewing altitude of 11.6°, with various qualifications that are hardly worth explaining. It would not be unreasonable to quote the date 3700 BC and to set the range of uncertainty as a century and a half either way. A reason will be given shortly for preferring a slightly later date.

The broad scheme explained here, with two pairs of perpendicular sight-lines to Deneb and Bellatrix, seems to be the only viable one. (Betelgeuse with Deneb offers the only other significant solution with any plausible lines of sight, but it is in the fifth millennium.) Lines along the axis, for observers in the ditches north and south, seem to offer no alternatives, and viewing at right-angles to ditches is in any case something we have found used often enough elsewhere. But why, under these circumstances, did they not simply build a square barrow, or at least a rectangular one? Why did they produce a skewed parallelogram?

The answer is that the axis had an importance of its own, for it was aligned on the setting of Rigel over the natural horizon. The barrow mound, following the altitudes derived here, would have been no more than 1.36 m above the eye of the ditch observers, and so no obstacle to an observer standing at ground level. As for the natural horizon, the town of Abingdon has long stood in the line of sight, but it seems likely that in Neolithic times the passage to the river in this direction would have been clear of trees, and that it is reasonable to take the extinction angle for Rigel. But what should that be? The site was not on high ground with good viewing conditions, but looked across the Thames valley. Using the norm for favourable viewing, the extinction angle would have been 1.53°. The derived declination in this case fits a year around 3600 BC. The extinction angle here could well be half a degree higher, however, which would bring the date a century later.

The Abingdon ditched (‘causewayed’) enclosure produced radiocarbon dates in the first half of the third millennium bc, quite consistent with all estimates made here, but why are these spread over a century and a half? The most probable error is in the azimuths, which are probably all a degree too high. On this assumption, all dates fall within three decades of 3700 BC. Whether or not this is so, it seems likely that the Radley barrow is half a century or more later than Skendleby 2. This is not certain, but what does seem certain is that both made use of the same clever device for introducing the rising and setting of the same star.

What of parallels with Skendleby’s alignments on the Sun? At Radley, there is no sign whatsoever of a solar alignment. The southern extreme of the Moon’s setting might have been observed in the same direction as Rigel’s, but this idea presupposes a level of sophistication for which we have no other evidence, and it must be abandoned. An isolated direction is no evidence at all. What is needed is evidence that other extremes were also observed, whether here or in the same general context, and this is lacking.

The Grendon Square Barrow (#ulink_01e37ad7-02bb-58ce-848a-1f6a9a765880)

The last earthen long barrow to be considered here in detail was intermediate in date and place between the solar barrows at Skendleby and South Street. It was revealed in another rescue excavation, this time in the Nene Valley near Peterborough in the seasons 1974 and 1975. This ‘short long barrow’ at Grendon, Northamptonshire, has an importance out of all proportion to the quality of our knowledge of it, for it seems to signal a transition between two different astronomical traditions of barrow construction. At first it was thought to be a round barrow, like others on the same site (which also included an Iron Age farm), for it was enclosed within a double ring ditch. It was eventually found to enclose a square structure with a façade strongly reminiscent of Skendleby 1 (Skendleby 2 was excavated only in 1975–6), and the ring ditch was seen to be a later addition. In advance of gravel quarrying on the site—which had been discovered from aerial photography—the barrow was excavated, but not completely, and an ‘overzealous removal of topsoil’ meant the loss of much information as to the sequence of construction. But in this case the very geometry of the barrow comes to the rescue, and it is in some ways reminiscent of Radley’s.

The ‘long barrow’ components of Grendon (strictly area C, barrow V) are shown in Fig. 47 (#ulink_e0ce928f-7790-5e11-83a3-359a42abad49), while the surrounding ring ditches are added to Fig. 48 (#ulink_77fc90a7-ecc8-5e47-bd10-437bdca894ca). All signs of the mound have been lost, but there is one good and one passable line from the façade, and one excellent straight inner edge to the ditch. What is more, the curved ditch opposite the façade fits closely to two arcs of circles of different radii, and the resulting ensemble of construction lines has some remarkable geometrical properties, quite independently of any assumption we might choose to make about potential astronomical lines of sight. Briefly, the angles marked x in Fig. 47 (#ulink_e0ce928f-7790-5e11-83a3-359a42abad49) radiate from the centre of the circle bounding the inner edge of the ditch and are all equal to 22.5° (a sixteenth of a circle), while the angles marked y are at the centre for the outer edge, and are within half a degree of 60°. The quadrant made up of the four x-angles is not set to the cardinal points of the compass, but in any case these are not sight lines.

Looking for lines of sight one may begin by taking parallels to the façade. (Perpendiculars to edges, which differ only slightly, will be considered shortly.) The azimuths are 314.5° and 125.4° (each of course might be reversed) for crosswise viewing. These at first make for uncertainty as to which stars were involved. Looking northwest, there can be little doubt that the setting of Deneb was observed, but in the other direction one might choose between Aldebaran (beginning of the fourth millennium), Bellatrix (mid-third millennium) and Betelgeuse. Following the standard procedure, the rising of Betelgeuse is indicated, with the setting of Deneb, both around 3450 BC and at altitude 12.7°. This fits well with stylistic considerations, but it is ultimately the radiocarbon dates obtained from the site that confirm this preference.

Mature oak from the façade trench gave a date equivalent to only 2900 ± 120 BC (2330 ± 70 bc), but there is now good reason to expect the renewal of posts, so this is no embarrassment, when taken in conjunction with a date of 3490 ± 140 BC (2750 ± 130 bc) for a sample of mixed oak and blackthorn from the ditch. One of the post holes—that nearest the bottom corner of the figure—contained ‘fairly mature oak’ datable to 3690 ± 140 BC (3000 ± 80 bc), which it was thought might have come from old charcoal or be from a first use of the site. Whilst we can bring our date forward by a century, by assuming half a degree error in the Deneb direction, it falls within the stated range even without doing so, if old timber is assumed. The pair of post holes at the lower corner is so similar in position to the split posts at Skendleby 2—lying across and to one side of the sight-lines though the gap in the façade—and makes for such a consistent pattern of star viewing, that the mid thirty-fifth century seems highly probable.

FIG. 47. The Grendon square barrow. Potential lines of sight are added as continuous lines, and possible construction lines are shown as broken lines, some of them being placed—to avoid confusion—on the right-hand figure.

FIG. 48. The two Grendon ring ditches surrounding the earlier square barrow. The broken circles are the suggested aims of those responsible for the ring ditches. Their centres are marked (near the upper crossing of lines of sight), and they are shown in the inset figure in more detail.

The similarities do not end here, for despite their utterly different orientations—one is almost at right-angles to the other—the two barrows were clearly designed for observing Deneb twice over. The rising of Deneb would have been seen at Grendon by an observer in the ditch at D, looking over the mound along a line parallel to the inner edge of the southeast ditch. The azimuth of this is taken to be 45.5°, so that it would have fitted perfectly with the angle assumed for cross-viewing. They are not perfectly at right angles, but only a degree removed, and if the error is ours, rather than theirs, then the date—as alluded to earlier—moves to about 3550 BC. Like Skendleby 2, therefore, Grendon was aligned twice over on Deneb and once on Betelgeuse; but now, four or five centuries on, the last alignment is not on the setting but the rising of Betelgeuse.

The other important similarity of function involves solar observation. To decide this question it is first necessary to make an estimate of the size of the mound. One cannot say exactly where were the preferred positions for crosswise viewing, but those marked A and D (for Deneb’s setting and rising, respectively, parallel to the lines a and d on Fig. 47 (#ulink_e0ce928f-7790-5e11-83a3-359a42abad49)) and B (for Betelgeuse’s rising, parallel to b) are the most likely, in view of the large post positions, and the lines crossing over the centre of the square seem the most significant of all. The proximity of the crossing point to the centres of the ring ditches adds credence to the idea that the mound was at its highest there. Even though there might originally have been a ridge, it would in any case have passed through this point. The height needed here, to set equal angles of 12.7° to observers in the three viewing positions, is about 1.9 m above eye-level. Perhaps the ditches were in fact 1.9 m deep at the appropriate places, so that by standing at ground level an observer could have seen the natural horizon over the mound from any position around it. If the eye of a normal male observer was at ground level—our previous findings fitted perfectly an assumption that the height of a man’s eye was critical—then the mound would have been about 23 cm above his eye were he to have stood at ground level. It is not even necessary to make this assumption, however, to make a claim for solar viewing at the site: the southernmost pair of posts would have been usable in connection with the natural horizon, using the line (at azimuth 132.4°/312.4°) marked on the figure, in order to observe the rising midwinter Sun in one direction and the setting midsummer Sun in the other. This is so because at a conservative estimate the height of the mound here would have been no higher than about a metre, and it might have been less. (The placement of such a sight line with respect to the posts in the manner shown was to become standard, as will be discovered in Chapter 7 (#litres_trial_promo).)

What is surprising here is the fact that, at a mere glance at the Grendon plan, midsummer sunrise or midwinter sunset would have been thought the likeliest candidates, either of them viewed along the axis of the barrow, just as at Skendleby 2 we found midwinter sunrise and midsummer sunset along its axis. At Grendon, this method is impossible; but by taking a line across the barrow, it is clear that what was observed could have been exactly the same as at Skendleby 2, namely midwinter sunrise or midsummer sunset or both. (It is also possible that these phenomena were observed by looking along the façade in some way.)