At dawn on Winter Solstice every year, just after 9am, the sun begins to rise across the Boyne Valley from Newgrange over a hill known locally as Red Mountain. Given the right weather conditions, the event is spectacular. At four and a half minutes past nine, the light from the rising sun strikes the front of Newgrange, and enters into the passage through the roofbox which was specially designed to capture the rays of the sun.
For the following fourteen minutes, the beam of light stretches into the passage of Newgrange and on into the central chamber, where, in Neolithic times, it illuminated the rear stone of the central recess of the chamber. With simple stone technology, these wonderful people captured a very significant astronomical and calendrical moment in the most spectacular way. [here]