22 December 2009
The passage and chamber of Newgrange are illuminated by the winter solstice sunrise. A shaft of sunlight shines through the roof box over the entrance and penetrates the passage to light up the chamber. The dramatic event lasts for 17 minutes at dawn on the Winter Solstice and for a few mornings either side of the Winter Solstice.
Read the rest here. The Articles For Further Reading are very good.
Professor O'Kelly excavated and restored Newgrange from 1962 to 1975. He discovered that the builders of Newgrange deliberately oriented the passage so that each year around midwinter, the rays of the rising sun would shine through a special aperture to illuminate the chamber.
His article is here.