Newgrange oldest Observatory in world

By Dick Ahlstrom, Irish Times Science Correspondent - 26 January 1989.

The Newgrange passage grave in Co. Meath constitutes the oldest known astronomical observatory in the world. According to a researcher at the Dublin Institute for Advanced Studies in Dublin. The graves predate the much better known Stonehenge structures by 1,000 years and their alignment with the winter solstice sun is almost certainly not accidental, states Professor Tom Ray of the institute.


The significance of Newgrange in an astronomical sense was ignored for decades, largely because it was assumed the passage grave was only approximately aligned with the rising sun at midwinter. Careful analysis by Professor Ray, published this morning as the cover story in the current issues if Nature, showed, however, that the grave is very accurately aligned towards the sun as it would have risen over Ireland 5,000 years ago.

He dates the Newgrange site to 3150 BC, a full Millennium older than Stonehenge in Britain. At that time, the earth's inclination in relation to the sun was 24 degrees, but the gravitational pull of the moon has over the past 50 centuries reduced this to 23.5 degrees.

Ignoring this fact caused modern Newgrange observers to assume there was a high likelihood of an accidental alignment, but following Professor Ray's analysis this has been boosted to a one in 300 or 400 chance. "I think the case for Newgrange is as good as Stonehenge, if not better" he said.


His calculations adjusted for the change in the earth's tilt showed a very accurate alignment within half a degree of perfect, a remarkable achievement for its megalithic architects.

Light from the mid-winter sun penetrates 18 metres to the back of the passage grave, entering through a gap or "roof box" above the entrance to the tomb.

The sun now enters the chamber four and a half minutes after sunrise, but 5,000 years ago it would have caught its earliest rays, appearing first in the bottom left-hand corner of the roof box. The box size that almost perfectly frames the sun's disc as it rises and the final rays reach into the chamber as the sun tracks away passing the upper right-hand corner of the roof box.

The box height and width add weight to Professor Ray's arguments about the astronomical significance of Newgrange.

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