Newgrange was built by our neolithic ancestors 5,000 years ago, 500 years before the great pyramid of Giza and 1,000 years before Stonehenge. Its significance lies in the fact that this ancient passage grave is aligned with the winter solstice, December 21st, the shortest day of the year.
The passage forms a 19-metrelong underground tunnel cut into a mound of earth and walled with carved stones. The tunnel runs slightly uphill into a six-metrehigh chamber, which, throughout the year, remains in complete darkness.
At dawn on the morning of the solstice, however, light from the rising sun passes through a small 25 cm-high opening above the passage entrance and reaches far into the tunnel, almost to the large chamber at its end. The light appears precisely at dawn, 8.58 a.m., reaches its maximum at 9.04 a.m. and declines until 9.15 a.m. when it finally disappears until the next winter solstice.
This remarkable alignment, re discovered by Prof M.J. O'Kelly in 1967, showed that Newgrange was designed and built with the solstice in mind, making it one of the world's oldest astronomically aligned monuments.
Later research found that in fact the alignment was perfect when Newgrange was built in 3,200 BC. Prof Tom Ray of the Dublin Institute for Advanced Studies discovered that the sunlight would have illuminated the large chamber at the back, where the ashes of the dead were kept.
The tilt of the Earth, which gives us our changing seasons, can vary over time due to a "wobble" in its orbit. The tilt was 24 when Newgrange was constructed but is currently 23.5. When Prof Ray adjusted for this .5 per cent change, he found the roof box would indeed have allowed the sun to reach all the way to the chamber for a few minutes each year.