Winter Solstice at Newgrange

Deep inside the world's oldest known building, every year, for only as much as 17 minutes, the sun -- at the exact moment of the winter solstice -- shines directly down a long corridor of stone and illuminates the inner chamber at Newgrange.

Newgrange was built 1,000 years before Stonehenge and also predates the pyramids by more than 500 years.

Lost and forgotten along with the civilization that built it, the site was been rediscovered in 1699. Excavation began in the late 1800s and continued in fits and starts, until it was undertaken in earnest in 1962. It was completed in 1975.
Seen as a tomb, the function of Newgrange in regards to the solstice wasn't known until 1967 -- and then by happenstance acting on a hunch. It was in December of 1967 that the astronomical alignment was witnessed and understood:

Michael O'Kelly drove from his home in Cork to Newgrange. Before the sun came up he was at the tomb, ready to test his theory.

'I was there entirely alone. Not a soul stood even on the road below. When I came into the tomb I knew there was a possibility of seeing the sunrise because the sky had been clear during the morning.'

He was, however, quite unprepared for what followed. As the first rays of the sun appeared above the ridge on the far bank of the River Boyne, a bright shaft of orange light struck directly through the roofbox into the heart of the tomb...

Read more here.