The drift in question is actually a consequence of the instability of the Earth, which in relation to the Sun provides the frame of reference against which star positions are measured. Just as the longitudes of places on the Earth are measured from the Greenwich meridian, so star longitudes are measured from a point on the celestial equator (a great circle on the celestial sphere directly above the Earth’s equator). By convention, this is the point where the Sun, moving along the ecliptic, crosses the equator in spring, one of the two ‘equinoctial points’. What was earlier described as a drift in star positions, amounting to a slow increase in their ‘celestial longitudes’, is really a drift of the zero point from which their positions are measured. The drift is for this reason now called the ‘precession of the equinoxes’.
The increasing longitude of a star changes its apparent (angular) distance from the north pole of the sky, and in turn affects the point on the horizon over which it rises and sets. To take just one example: the Pleiades rose 10° south of east at Stonehenge in the early forty-second century BC, but twelve centuries later they rose due east.
This brings us back to a serious problem, hinted at previously. How are we to know whether an orientation indicates a concern with the Sun rather than with a star? east–west directions in long barrows, for example, are relatively uncommon but not unknown. Like alignments that seem to be to the rising and setting Sun at the solstices (midsummer and midwinter), they might well have been directed to a particular star. The change in the direction of sunrise and sunset at the solstices is very slow—say a fifth of a degree of the horizon in a thousand years. Over the same period of time, the point of rising of the Pleiades would have moved by twelve degrees. Continuity of custom is less obvious with the stars than when the Sun was involved. Prehistoric peoples might have aligned all their barrows on the rising or setting of a particular star or group of stars and have done so with great deliberation and precision at different periods of history, and yet have left us with a scattered set of compass directions, in short, with an impression of carelessness and imprecision. Fortunately there is a way of cutting through this problem, for it will soon emerge that the custom was to align long barrows on two or more stars simultaneously, and to consider mostly very bright stars. This greatly reduces the number of ways of interpreting individual cases, and from a study of well excavated examples a coherent picture gradually emerges, as clear in its way as that of the later monuments with their more stable solar orientations.
In brief, it turns out that the Wessex long barrows were mostly stellar, while the later circular monuments were solar and lunar. The later long barrows already show signs of change. There are two different criteria here—alignment and illumination. Matters were sometimes so arranged that at one of the Sun’s extremes it could illuminate the end of a long gallery in the tomb, through a suitable entrance slot. Even this will prove to have followed a similar arrangement with stars, from an earlier period.
FIG. 6. Four different examples of taper in long barrows, one from Poland, one from northern Germany, one from Lincolnshire, and one from Wessex. All have left interesting traces of mortuary houses (mh), the sides of all of which are approximately parallel to the sides of barrows erected later on the same site, or perpendicular to them. Note the letters (a, b) marking these highly significant properties. The scales and orientations of the different barrows are approximately correct, with Wayland’s Smithy, the smallest, about 55 m long. The outlines of barrow/mortuary house were formed by either timber posts (t) or stones (s). From left to right, the materials were: s/t, s/s, t/t, s/s. A mortuary house at Wayland’s Smithy, hinted at in the figure but redrawn in Fig. 19 was of timber.
First Thoughts on the Taper of Long Barrows
While some of the longer and earlier examples of earthen long barrows were parallel-sided, or nearly so, and just possibly of constant height over their length, most were tapered in height and width. (An idea of the plans of some of the different styles may be had from Fig. 6. The rationale of their three-dimensional forms is the subject of this chapter, and the differences are too subtle to be readily illustrated at this stage, but Fig. 53 should give an idea of what taper in height and breadth entails.) It is doubtful whether the property of taper originally had any astronomical purpose, since it is found in many early post-built houses throughout Europe, but the precision with which those houses were built—for instance in the alignment of their side walls—is usually greatly inferior to that of most large tombs. Precision might have signified respect, a wish to give something perfect to the dead, but there are good reasons for thinking that it served also to direct the eye towards significant risings or settings over the horizon. Even supposing that the trapezoidal form was first adopted for the housing of the dead by analogy with the housing of the living, support for this other explanation is strong. It is buttressed by the fact that time and again, on an astronomical reading, different sites seem to indicate an allegiance to a relatively small number of select bright stars.
Accepting the astronomical idea provisionally, two or three potential explanations of the taper in plan offer themselves. (Taper in height will turn out to be another question.) The first of these is easily appreciated in terms of what seems to have been a strong desire to find lines that point to the rising of an important star in one direction and the setting of another in precisely the opposite direction. This is easier said than done, but by taking two lines at a fairly small angle, it can always be done. Using those two lines as the orientations of the sides of a barrow, the assumption that stars were viewed along the barrow’s sides provides one possible explanation of taper.
It is not difficult to imagine others. As illustrated in Fig. 1 (Chapter 1), the direction of the rising midsummer Sun at an arbitrary place on the Earth’s surface is usually near to, but not precisely the same as, the reversed direction of the setting midwinter Sun. Since the (angular) height of the horizon helps to determine the directions of rising or setting over it, if there were hills of different heights in the two directions, the angle between the lines might be several degrees. This difference might have been built into a long barrow in various ways. The broad (interment) end of the barrow might have been made to point to midsummer sunrise, and another part, say a side or axis at the other end, to midwinter sunset. (There is an instance fitting this description in the Winterbourne Stoke group in Wiltshire.) On level parts of Salisbury Plain the difference between the ideal directions would have been only about 3°, so that there an edifice with only a 3° taper could have served both directions at once. It would be wrong to place much emphasis on a solar explanation at this stage, however. It is certainly not relevant to the early history of the taper of long barrows, the alignments of which will prove to have been primarily stellar.
At first sight it might seem that a single building alignment—say that of a central ridge along it—cannot cater for two opposed solar extremes—for both midsummer rising and midwinter setting, for example. This is where taper