They got the date wrong by some 3,000 years, but the oldest detailed drawing of Stonehenge, apparently based on first hand observation, has turned up in a 15th century manuscript.

The little sketch is a bird's eye view of the stones, and shows the great trilithons, the biggest stones in the monument, each made of two pillars capped with a third stone lintel, which stand in a horseshoe in the centre of the circle. Only three are now standing, but the drawing, found in Douai, northern France, suggests that in the 15th century four of the original five survived.

In the Scala Mundi, the Chronicle of the World, Merlin is given credit for building Stonehenge between 480 and 486, when the Latin text says he "not by force, but by art, brought and erected the giant's ring from Ireland". Modern science suggests that the stones went up from 2,500 BC, with the bluestone outer circle somehow transported from west Wales, and the double decker bus-size sarsen stones dragged 30 miles across Salisbury plain.

The drawing, recently identified by the art historian Christian Heck, has never been exhibited, but according to the Art Newspaper it will be seen next year in an exhibition at the Royal Academy in London, marking the 300th anniversary of the Society of Antiquaries.

There are two earlier images of Stonehenge, one in the British Library and one in the Parker Library in Cambridge, but the Douai drawing is unique in attempting to show how the monument was built.

It correctly shows tenon joints piercing the lintel, a timber construction technique, although in fact the real Stonehenge tenons only go partly into the top stone.

Stonehenge is rare among prehistoric landscapes, because its sheer bulk meant it was never lost. An Anglo Saxon poet wondered about the origin of the stones and inspired some of the earliest photographs.