Professor Alexander Thom puts forward his theory that Stonehenge and other megalithic sites were used to record time and predict solar and lunar eclipses. Magnus Magnusson looks at Thom's evidence and hears what different archaeologists think of the suggestion that Stone Age Britons could make such elaborate calculations. If Thom's theory is correct, previous archaeological certainties about the knowledge and ability of people in the Stone Age would be overturned.