Around 4,500 years ago, the famous silhouette of Stonehenge would have looked very different. Writer and archaeologist Mike Pitts digs up clues to the mystery of the circle’s long-lost stones.

If you are fortunate enough to be at Stonehenge on midwinter day, 21 December, and also for the sky to be clear near the horizon as the Sun sets you can experience a striking event. Position yourself between the tall, outlying Heel Stone and the stone circle, and look south-west through the megaliths. In the closing darkness they appear like a huge crumbling wall, orange light slanting through vertical fissures. In the last quick moments, the Sun disappears from a window formed by two great vertical stones and the horizontal lintel they support. It’s dark and cold. Stonehenge, it feels, has swallowed the Sun.

My archaeological colleagues and I are convinced that this alignment is no coincidence: it was designed by the monument’s builders. But were you able to see this annual drama 4,500 years ago, the spectacle would be yet more impressive. The solstice sightline was marked by as many as six futher upright pairs. Of the greatest of these – the tallest and the most finely carved stones on the site – now just a single megalith known as Stone 56 is left. A projecting bulge on the top of this stone once fitted into a giant lintel. Now that tenon rises exposed and useless.

And many more upright stones have gone. What happened to these missing stones? Who took them down and where did they go? How do we know they were once there? Can we picture what the completed Stonehenge looked like? Indeed, was it ever finished at all?