An assembly of huge stone slabs unearthed in Egypt and dated 6,500 years old has been confirmed by scientists as the oldest known example of a stone monument aligned with the sky.

Nabta, located west of the Nile in southern Egypt, consists of a stone circle and five lines of standing and toppled megaliths. At 6,500 years old, Nabta predates Stonehenge by more than 1,000 years. One of the megalith lines is aligned east-west.

Aligned with the sky

"This is the oldest documented astronomical alignment of megaliths in the world," says professor J.McKim Malville of the University of Colorado at Boulder.

The ruins lie on the shoreline of an ancient lake that began filling with water 11,000 years ago when the African summer monsoon shifted north.

It was used by nomads until 4,800 years ago when the monsoon shifted south again.

The stone circle at Nabta contains four sets of upright stone slabs. Two sets were aligned in a north-south direction while the second pair provides a line of sight towards the summer solstice horizon.