Stonehenge - stōnˈhĕnjˌ, group of standing stones on Salisbury Plain, Wiltshire, S England. Preeminent among megalithic monuments in the British Isles, it is similar to an older and larger monument at Avebury. The great prehistoric structure is enclosed within a circular ditch 300 ft (91 m) in diameter, with a bank on the inner side, and is approached by a broad roadway called the Avenue. Within the circular trench the stones are arranged in four series: The outermost is a circle of sandstones about 13.5 ft (4.1 m) high connected by lintels; the second is a circle of bluestone menhirs; the third is horseshoe shaped; the innermost, ovoid. Within the ovoid lies the Altar Stone. The Heelstone is a great upright stone in the Avenue, northeast of the circle. It was at one time widely believed that Stonehenge was a druid temple, but this is contradicted by the fact that the druids probably did not arrive in Britain until c.250 b.c. In 1963 the American astronomer Gerald Hawkins theorized that Stonehenge was used as a huge astronomical instrument that could accurately measure solar and lunar movements as well as eclipses. Hawkins used a computer to test his calculations and found definite correlations between his figures and the solar and lunar positions in 1500 b.c. (However, as a result of the development of calibration curves for radiocarbon dates, the main structure at the site, Stonehenge III, is now believed to have been built c.2000 b.c.) Some archaeologists object to Hawkins's theory on the basis that the eclipse prediction system he proposed was much too complex for the Early Bronze Age society of England. Most archaeologists agree, however, that Stonehenge was used to observe the motions of the moon as well as the sun. Research by the archaeologist Alexander Thom, based on the careful mapping of hundreds of megalithic sites, indicates that the megalithic ritual circles were built with a high degree of accuracy, requiring considerable mathematical and geometric sophistication.

See G. S. Hawkins, Stonehenge Decoded (1965); H. Harrison and L. E. Stover, Stonehenge (1972); A. Thom, Megalithic Sites in Britain (1967) and Megalithic Lunar Observations (1973).

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