Stonehenge is famous for its alignment on the rising midsummer sun. The 5000 year old chambered mound at Newgrange in Ireland is famous for its alignment on the midwinter sunrise. In 1965 Professor Gerald Hawkins captured the public imagination with his theory that Stonehenge had been constructed as a giant computer for the observation of the sun and for the prediction of eclipses. Stonehenge and the many other stone circles throughout the British Isles have intrigued and puzzled generations of scholars, mystics and professional archaeologists. Modern science started to interpret the meaning of these enigmatic structures in the 1960s and the results caused a revolution in the academic view of prehistory and its capabilities. The instigator of this revolution, a Scottish professor of engineering, the late Alexander Thom, has shown, by careful surveys of megalithic sites from Brittany to the Orkneys that these were so arranged as to form stations and markers for accurate observations of the sun, moon and stars, revealing a prehistoric 'megalithic science' whose achievements can only be appreciated in the light of our own. Similar conclusions were reached at the beginning of the 20th century by Sir Norman Lockyer, the eminent astronomer, but he and his followers were largely ignored because their findings conflicted with the current belief in barbaric antiquity. It is now generally accepted that megalithic structures were often sited and constructed with reference to the rising and setting positions of the sun and moon, for reasons about which we are not certain. Ancient man's interest in the movement of the sun, moon and stars may have been for the purposes of establishing an accurate agricultural calendar, or it may have had a symbolic and religious aspect.