Stonehenge Solved Its Three Observatories, Who built them, How and Why

The previous chapter is concerned with biological evidence of the prevailing climatic conditions but now we turn to possible physical, evidence of Earth's axis of spin having moved. It is thought that when the turn of the Moon's orbit plane had settled to where it is today, and Earth's axis of spin had not far to go before taking up today's location(s), people lost interest in observing the Moon's apparent movements. They were no longer fearful of what these might portend. The time came when no more new henges and circles, or rings, were built for this purpose, and old ones were abandoned.

But this did not mean there was no longer curiosity about how the rises and sets of the Moon moved back and forth on the horizon; on the contrary, people in the vicinity of Stonehenge found them intriguing. The third, or Final installation was created to assuage curiosity, as the aberrant paths of the Moon were settling into those with which we are familiar. As is well known, the Moon's arcs in the sky vary, smallest in summer, largest in winter. Less well understood are its rises and sets moving back and forth along the horizon: they cycle thus every 18.61 years. (For an explanation see Fig 75 and (i) below.) But at, say c1300BC, there also continued a third movement, as we have seen in chapter 21, and the consequences of all three in combination excited the interest of people at Stonehenge.

The methodology they employed for observations was of sighting across the tops of pairs uprights, the rise and set positions of the Moon; the sightlines clearing the crown of the surrounding bank, Fig 73. This was done in two phases, Fig 74. The first was concerned with sighting winter Moonset on its largest standstill arc (A) and (A₁), and summer Moonrise and set on their smallest standstill arc (a) and (a₁), Fig 75 (i). For maximum accuracy, those of the first of these phases were to have the longest possible sightlines within the confines of the chalk spoil bank. For this reason they were arranged to just clear the vestiges of the stone circle, on each side. Two pairs of stones, 94 from 91, and 92 from 93 were used, and in each case the foresight was encircled by a ditch, this to ensure that newcomers could not mistakenly think it was a backsight. As with all waymarking ditches its message was 'do not pass over me'. (An example of this practice is to be found at the broken upright of the erstwhile arch, the Heel Stone. See chapters 15 and 20.) A third sight line was 92 from stone G. A fourth sightline was also added, with a stone backsight F placed at the foot of the bank and adjacent to the Slaughter Stone 95; its fore sight was stone 94. This must have been aimed at a winter Moonset of particular interest to the observers, but why this was so is not obvious.

The second phase was notionally 9.3 years later, at the other extreme of the cycle, when concentration was focussed on summer Moonset on its largest standstill arc (b) and (b1). For this, the first phase foresight stone 92 was utilised as the foresight for twelve backsights, notated h, all timber posts except one, stone H. All of these were positioned around the foot of the bank anticlockwise of stone 92. (To avoid confusion only five of these are depicted in Fig 74, but all backsight post holes are clear in Fig 76, where they are seen hosting some of the many cremations in the vicinity (ii).

What is to be made of all this? Interpretation must take account of small distortions, or shifts in the orientations of sightlines orientated generally to south and southwest, these caused by the horizon being higher in altitude than the installation. This has the effect of making further apart the apparent conjunctions of the smallest standstill arc with the horizon in summer. Summarising: the maximum winter Moonset (A₁) was the same as today; the minimum arc of summer Moonrise and Moonset (a) and (a₁) display equal foreshortening, while compared with today's (b₁): sightlines 92 from backsight h1, to 92 from backsight h9, straddle (b₁), the maximum standstill arc of today's summer Moonsets.

If the raised horizon accounts for the foreshortened (a) and (a₁), then their symmetry, taken together with the accurate alignment of (A₁), would appear to indicate the Moon had settled into its paths of today. This leaves the sightings straddling (b₁). These may reflect the possibility of Earth's axis of spin, aka its core, not having fully settled down at this juncture. It is difficult, otherwise, to account for the intensity of interest in the vicinity of Moon's largest standstill arc of summer sets.

It is thought an earth tremor preceded the Final henge, perhaps centred off the Channel Isles, this arising from Earth's crust being affected by the gravitational pulls of the Moon's numerous unusual passages. It is noteworthy that the general pattern of mounting severity of collapse round the sarsen circle, reflects the progressive deterioration in construction workmanship clockwise from the main entrance. In addition to reducing much of the Stone henge, such a tremor may also have razed some of the stones of Avebury, and in Brittany, Le Grande Menhir Brise, completely.