A new book claims the megalithic tombs of the Boyne Valley were sophisticated and exact calendars of celestial movement built by astronomers.
The authors of Island of the Setting Sun, Anthony Murphy and Richard Moore, assert that the stone age builders were influenced by Irish folklore and astronomy. Their theories are supported by astronomical data from the night sky of the time.
They believe the wall of white quartz on the exterior of Newgrange was probably inspired by the Milky Way.
"Five thousand years ago, the Milky Way would have been visible as a complete ring of light on the horizon at certain times of the year. This is something which does not happen now."
The authors believe Newgrange would have accepted light not only from the sun at the winter solstice but also from the moon and possibly Venus. These celestial bodies could also be behind the triple spirals cut into the entrance kerbstone.
They assert that the cross shape of the chamber at Newgrange and the smaller passage grave of Fourknocks some 15km (9.3 miles) away "echoes the cruciform outline of the swan constellation". The book reveals that the Newgrange passage points towards Fourknocks, and its chamber structure is orientated towards where Deneb - the brightest star in the constellation of Cygnus the swan - would have been rising about 5,000 years ago.
Newgrange is an important wintering ground for the whooper swan. In the Irish myth of Aonghus and Caer, the couple went "into" Newgrange after taking the form of swans.
"Our ancestors had developed a very complicated calendar based on sophisticated astronomical observations . . . They were able to predict eclipses of the moon."
The book is published by Liffey Press.