The winter solstice—more accurately known in the northern hemisphere as the December solstice since in the southern hemisphere it is the summer solstice—occurs on December 20, 21, 22, or 23. The 2015 December solstice was on 22 December at 0448 GMT. At that point, the sun was directly over the Tropic of Capricorn, 23.43721° south of the equator. The North Pole, therefore, was tilted 23.43721° away from the sun, its greatest distance from the sun and the reason the days are short and the temperatures (usually) low.
At Newgrange, which is closest to the town of Donore, Ireland, the sun rises over Red Mountain, which is 100 meters above sea level, at 0854. At 0858, sunlight beings to appear on the floor of the passage. It widens to a band of 17 cm by 0909. At 0925, the band of light disappears. The whole things lasts 17 minutes.
How it happens is that the sunlight shines through gap between two lintel stones 2.5 meters from the entrance. The gap, 1 meter by 60 cm, is known as the roofbox. This is what it looks like from outside the chamber:
The floor of the chamber is on the side of a hill. When we entered the chamber, we made our way up a slope to a point two meters higher than the entrance. The passage was so narrow that we had to squeeze past the orthostats and to keep our heads down so we didn’t whack them on the lintels. In five days in Ireland, I knocked my head more than once, in Newgrange and other passage tombs, and even on the replica of the Newgrange passage in the Brú na Bóinne Visitors Center.
The roofbox acts as a kind of lens, narrowing the light of the rising stone. The placement of orthostats L18 and L20 in the passage further narrow the beam of light. The target of the solstice sunlight is the innermost recess of the burial chamber, which is decorated with a triskele, or triple spiral, the same shape inscribed on the entrance kerbstone.
I have no doubt that I was lucky to see the sun inside the chamber. It’s Ireland. It’s winter—or nearly—it rains a lot and is often overcast. But O’Kelly wrote in 1982: “It might be thought that sunrise would rarely be clearly seen at this time of year in the Irish climate, but we have seen the phenomenon at Newgrange every year for the last eleven years on one or more of the three or four days centering on 21 December.”