Stonehenge: Neolithic Man and the Cosmos. John North

FIG. 8. The letters B show where the collections of bones were found. The central pit might originally have been used for a central support to a massive beam—or double beam—spanning the two main uprights. Note that the palisade trench interferes with the socket for one of these, showing that it is a later and independent structure.

      The planning of the barrow must have been no less taxing, for there can be little doubt that it was done with reference to the peculiarities of the landscape in regard to the risings and settings of certain bright stars. Even more interesting is the case of the original mortuary house. At a cursory glance the posts bounding this are badly skewed, but it will be seen to have pointed within two or three degrees of 22° north of east. Since 20° is the approximate mean direction of the splayed sides of the later barrow, this is not very surprising.

      To progress any further, we must make reference to certain striking resemblances between its plan and that of the first mortuary house on the site of the barrow at Wayland’s Smithy. As will be described shortly, the Wayland’s barrow had split trunks with the flat of the D-shaped section to the inside, and the trunks were of diameter 1.2 m. The pits were of much the same dimensions as at Fussell’s Lodge. The areas on which the bones were placed were at both sites about a metre across, although the length of the area at Fussell’s Lodge was greater. Lines of sight from the posts of what has been interpreted as a porch would have skimmed the two sides of the western trunk at Fussell’s Lodge had it also been about 1.2 m across, and the other is presumed to have been of much the same size. The stacked bones were found to lie in a neat line not very different from 22° to the east–west line. This can reasonably be taken as a first approximation to the direction of the axis.

      Now it so happens that another long barrow, today virtually obliterated, was on the skyline at a distance of 1.85 km and in a direction just 21.7° south of west, as seen from the Fussell’s Lodge barrow. The sighting of one barrow from another will later prove to be so common that this line must be treated as important, and doubly so when we discover that it indicates the setting of the star Aldebaran over the neighbouring barrow at about the right period of prehistory. (As for the precise date, some of the deciding factors must first be explained.) Working from this assumption, we are no longer, therefore, entirely dependent on the edifice itself in establishing at least this property of its orientation. The observed altitude of the barrow (not the star) would have been about 1.32°, depending on the height and position of the observer. This raises an important point: the quoted altitude is based on the assumption that the observer is standing at ground level. A study of the relatively extensive and important collection of human remains from the Fussell’s Lodge barrow, by D. R. Brothwell and M. L. Blake, led to the conclusion that the average adult male stood 170 cm (5 ft 7 in) and the average adult female 157.5 cm (5 ft 2 in).

      At a given date, any star has a particular position on the celestial sphere, that is, can be assigned a given set of sky coordinates comparable to latitude and longitude on the Earth’s surface. There are two particularly useful reference planes comparable to that of the Earth’s equator: one is the celestial equator above it, and the other is the ecliptic (see Chapter 1 for a simple account, and Appendix 2 for more detail). Taking the equatorial system for the time being, declination is the coordinate corresponding to terrestrial latitude. If its value is known, then using methods outlined in Appendix 2 one can say exactly where on the horizon the star would have been seen rising and setting at any date in the past from any given place—and conversely, what was the date at which it rose or set in such and such a position. A star of declination in the neighbourhood of –12.67° can be said categorically to have been seen setting over the now defunct barrow, but only if the star was visible down to the horizon. Aldebaran had this declination in 4365 BC, but for reasons to be explained, the star would not have been seen as low as the true (natural) horizon.

      Whenever a distant natural horizon enters into the calculation it is necessary to take possible tree cover into account, since a star descending into even distant trees leaves one unable to decide precisely when it disappears. In the present example the horizon was topped by a barrow, however, so that tree cover can there be ignored. (Trees seen against the sky do not necessarily modify the effective horizon altitude when the Sun and Moon are being observed. If the trees are not in leaf, as at winter solstice and spring equinox, and the horizon is defined by a fairly sharp ridge, the Sun and Moon are often easily visible through the trees down to ground level.)

      In the reverse direction of the axis at Fussell’s Lodge, 21.7° north of east, the ground above and below the tomb slopes at about 3° to the horizontal, this being effectively also the altitude of the eastern horizon. The ground is near at hand, and was no doubt cleared of trees. It is no accident that the slope of the final earthen barrow, according to a plausible reconstruction, was also of about 3°. One may see relatively faint stars at such an altitude. Over the barrow to the west, however, where the horizon altitude is lower, few stars are bright enough to be visible at the horizon proper. This difference needs to be explained.

      How low in the sky one may detect a star depends on weather conditions, of course, but even under the most favourable conditions there is a limit to what is possible that depends on the star’s brightness. Astronomers classify the luminosity of stars by their so-called magnitude. The brighter the star, the smaller the magnitude. Sirius, the brightest star in the sky, has now a magnitude of –1.46. Only one other star then visible from Britain, Arcturus, has (and had) a markedly negative visual magnitude. Our concern here will be with only the brightest stars, of which about 45 have magnitudes less than 2, although not all of those are visible from northern latitudes.

      Typical minimum altitudes for viewing the very brightest stars under favourable viewing conditions from the downs of southern England, are these: Aldebaran 2.00°, alpha Crucis 2.42°, Altair 2.01°, Antares 2.32°, Arcturus 1.43°, beta Centauri 1.86°, beta Crucis 2.35°, Betelgeuse 2.06°, Capella 1.48°, Deneb 2.35°, Pollux 2.24°, Procyon 1.71°, Regulus 2.44°, Rigel 1.53°, Rigel Centauri 1.58°, Sirius 0.74°, Spica 2.12°, Vega 1.48°. The cluster of stars known as the Pleiades is a difficult case, but it was probably rarely seen below 4°, with 4.4° more typical.

      These ‘extinction angles’ are for specific viewing conditions, and the accuracy with which they are quoted here is rather excessive, although useful for purposes of comparison. Actual values are as uncertain as the weather itself. Climatic conditions were not identical to those of modern times, and even local air currents may affect the issue. Fortunately it will soon be discovered that in very many cases precise values are unimportant—that is, when the height of the actual (natural or artificial) horizon is appreciably greater than the extinction angle.

      To return to Fussell’s Lodge. Since Aldebaran might have been seen only down to about 2°, it is necessary to review the earlier statement of what was seen over the neighbouring barrow. The declination, recalculated for this slightly higher altitude, turns out to be –12.07° and the corresponding date 4245 BC. The shift of more than a century from such a seemingly insignificant change of altitude draws attention to the importance of working with precise data.

      If we are to believe that the star Aldebaran was observed setting along the axis of the mortuary house, how would it have been sighted? It is wise to consider the possibility that a distinction has to be drawn between an initial act of design and later acts of ritual observation. At the foundation of the mortuary house, however, the alignment would have been the same on both occasions: just as the designer had done, so others would have been able to look in the direction defined