ON THIS year's midsummer solstice morning, as usual, a great crowd - nearly 30,000 people - gathered at Stonehenge to watch the disc of the dawn sunrise exactly over the Heel Stone. This annual observation, if cloud or mist does not obscure, is seen to prove that the most famous of all prehistoric monuments is astronomically oriented.
A solar element in Stonehenge's geometry had been known for many decades, but to the English-born and American-based astronomer Gerald Hawkins is due the stronger proposition: Stonehenge was built as an astronomical observatory-cum- computer. Its many upright stones and other features were placed to capture alignments to a host of solar and lunar events, set so as to record or to observe the many points around the horizon where sun and moon rise or set on significant days. This idea, first set out by Hawkins in two Nature papers in 1963 and 1964, was then stated at book length in his Stonehenge Decoded (co-authored with John B. White, 1965). Ever since, the million people a year who go to Stonehenge know what was the ancient purpose of the place they see.
Hawkins's celebrated study prompted others to explore Stonehenge astronomy, and the wider field of "archaeo-astronomy" or "astro- archaeology" of which it would be part. By 1965 Hawkins himself had already worked out a broadly comparable astronomical scheme for prehistoric Callanish, the singular and complex array of standing stones in the Outer Hebrides which he called "a Scottish Stonehenge". (Perhaps it was in the Scottish rain he began his habit of wearing a deerstalker.)
He also studied astronomical aspects of Egyptian, Mayan and Aztec archaeology, moon markings in ancient cave art, and the enormous ground figures of Nasca in the Peruvian desert. Varied insights emerged there, but none so clear and compelling as his vision of Stonehenge, which swept the field.
For Stonehenge, the Cambridge astronomer Sir Fred Hoyle produced his own scheme of what was observed there and how eclipses could be predicted by manipulating features of it. By degrees, a profound problem of method emerged which to this day undermines, perhaps fatally, studies of prehistoric astronomy: the fact that a skilled and knowledgeable astronomer today, equipped with all elaborations of modern understanding, can devise a way to use Stonehenge as an observatory or calculator does not in itself prove that was its original use and purpose. Stonehenge, if you made the right observations or moved the stones about in the right way, could be used nowadays to predict the opening hours at Salisbury Museum, or of Sainsbury's in Swindon.
If one follows the evidence of ambiguous archaeological traces and of anthropological analogy, even the basic orientation of Stonehenge is falsely understood: the monument is not oriented north- east towards the midsummer sunrise but in the other direction, south- west towards the midwinter sunset - in the view of the present writer, an archaeologist unpersuaded by the more ambitious archaeo- astronomical schemes. The crowd ought to go on a December afternoon, not …