The latest theories on how Stonehenge was built were aired at a Royal Society conference this week. John Davies reports. This issue of The THES went to press just before the astronomical milestone of the spring equinox. A time that was also chosen this year for a two-day discussion conference in London on one of antiquity's most famous monuments with astronomical meaning: Stonehenge.

Geoffrey Wainwright, chief archaeologist for English Heritage, says the conference's timing was a happy coincidence. "Over the past 18 months we have been collecting the results of work this century," Wainwright explains, "and we published it in a major volume last October - surprisingly, there's never been a proper publication about Stonehenge". The Royal Society was so impressed it proposed a conference about themes not discussed in depth in the volume.

The result: this week's headline-grabber where, under the title "Science and Stonehenge", aspects of the monument's archaeology were discussed, with the full panoply of present-day science - from virtual reality to carbon dating - being marshalled to investigate this wonder of neolithic technology.

Not that there are definite answers to the major puzzles of Stonehenge. The popular idea, for instance, of the monument as an "astronomical computer" for its day is, Wainwright thinks, fanciful. "It makes a good story, but apart from the broad orientation of the monument towards sunrise on the summer solstice, I don't think there's a lot in it."

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All the same, the people who built Stonehenge were part of what George Eogan, professor of archaeology at University College, Dublin, calls a "sophisticated and you could almost say scientifically conscious society".

Evidence from other sites of the same period such as Newgrange in Ireland and Maes Howe in the Orkneys shows monuments aligned with the sun's position at solstices.

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So was Stonehenge primarily a religious site? Alastair Whittle of the University of Wales, Cardiff, thinks the monument "belonged to a sacral landscape, not to a major settlement concentration". But Michael Allen, an archaeologist with English Heritage, has a "less popular" view. From 3000bc onwards the Salisbury Plain area was, he says, "a very rich high-status farming economy". Stonehenge and its associated monuments were "dots within a landscape that was fully utilised. People don't like to think of the area as farms and fields, but it was then the breadbasket of England". The thin chalky soil of today is due, he explains, to "prehistoric man ploughing it to buggery."

In other words, although Stonehenge "undoubtedly has great importance" as a sacred site, Allen's view is that "it doesn't mean the whole landscape was an exclusion zone. People were living there - there are neolithic sites all over the place. And someone must have had a lot of power to get say 600 people at a time working to build a monument". But to extend the evidence of strong political control and organisation to argue that the area was simply a "ritual landscape" is, Allen thinks, wrong.

As for the vexed question of when it was built, English Heritage experts, using radio-carbon dating techniques think construction began in about 3050bc and was finished 50 years later, by 2300bc; about 700 years earlier than previously thought.

As someone whose major interest is in other sites contemporary with Stonehenge, the Kilmartin area of Argyll and Ireland's Brugh na Boinne - Eogan says "the extraordinary thing about Stonehenge is its architecture. Contemporary work in other areas is by no means as spectacular.

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"The society living in the Stonehenge area was one that must have had a surplus of wealth. And it was a very innovative society - they were experimenting in wood and then translating that into stone, which is a great creative achievement."

Which brings us back to the actual engineering feats involved in building Stonehenge - namely how did the stones get to be transported to the site and put up?

Here there are at least two puzzles. Some of the stones - not the biggest ones - seem to have come from the Preseli Hills in Pembrokeshire, a long distance for that period. Could they have reached their Wiltshire site by glacier rather than by human means? That hypothesis, it seems, is losing ground, and the ingenuity of neolithic man has to be credited with the transportation of the Preseli bluestones, some of which weighed as much as four tons.

But there were heavier sarsen stones, and even though they had shorter distances to travel - 20 or so miles, from the Marlborough Downs - they too would have been an enormous challenge. In the words of Julian Richards, like Allen an archaeologist with English Heritage, "how do you move and erect a 40-ton stone and 10-ton lintel - the largest single element in the great trilithon?"

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Richards now has one answer. Along with civil engineer Mark Whitby, he devised a full-scale experiment using only stone-age technology that achieved the feat of pulling the equivalent of the largest sarsen up a one in 20 slope - the kind of hill that would have faced the stones' movers 4,500 years ago - and then setting it upright. What's more, he did it with just 120 people.

But, says Richards: "I'm not arrogant enough to say that this was the way it was done. It's just one way of doing it."

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All the same, it's likely to make a popular impact later in the year. The experiment was filmed by the BBC, and will be shown in the series The Secrets of the Lost Empires, in August. Too late for the summer solstice, but never mind.