Newgrange is an ancient structure in Ireland so constructed that the sun, at the exact time of the winter solstice–which is today, December 21–shines directly down a long corridor and illuminates the inner chamber.
More about Newgrange from Gerard Vanderleun and from Professor Michael O’Kelly, who rediscovered the Solstice illumination at Newgrange in 1967
If you visit Newgrange, they have lights set up to simulate the effect of dawn at the solstice. The beam of light is slightly off-kilter, since in 5000 years the tilt of the Earth’s axis has shifted partway through a 40,000 year Milankovitch cycle. In 35,000 years it will line up again.
Cool information!