The best known feature of Newgrange today is the orientation of the passage and chamber to the winter
solstice sunrise, using a special feature nowadays called a Roofbox. The alignment was noted by the astronomer Locklear in 1909, and the symbolism of the rising sun was used as an image in the poetry of the mystical writer A. E. or George Russell.
The phenomenon was first recorded in modern times by excavator Michael
O' Kelly in 1967. The O'Kelly's had heard many reports of the sun lighting up the chamber at midsummer, but upon investigation was found to be midwinter sun rise.
The entrance faces south-east and as the sun rises over the days around December
21st, its rays enter the specially contrived roof-box structure over the
door and penetrate 20 meters into the mound to illuminate the chamber.
The only other roof box currently known is at Cairn G, Carrowkeel in Co. Sligo, which was probably built two or three hundred years before Newgrange.
The Newgrange roof box is a sophisticated structure which took a great deal
of planning and engineering. Since the passage slopes gently up hill,
the floor of the chamber is at the same level as the roof box. It allows
the sun to enter for a maximum of 22 days over the winter solstice, or
about 11 days on either side of the solstice proper.
On solstice mornings given a clear sky, as the sun clears the horizon,
it's rays flood into the roofbox and flash up the passage, penetrating
almost to the end of the cruciform chamber. The beam of light is quite
narrow, and stays in the chamber for approximately 17 minutes, before
moving out and down the passage again. The light is so strong and bright
that it illuminates the whole chamber, and the capstone, 6 meters above
the floor can be seen. The engraved art is thrown into high relief.
During the excavation a quartz block was found in the roofbox; this was
one of two used to close the skylight in ancient times when complete darkness
was required in the chamber. It must have been well used, since it's opening
and closing had worn a groove on the slab it sat on.
large flat slab was used to seal the entrance to the mound; this was found
lying flat behind the Entrance Stone, worn smooth by many years of people
stepping on it as they entered the cairn. Today the door slab stands bolted
in place to the right of the entrance.
For years people had to book a place to enter the chamber and witness
the winter solstice sunbeam; in more recent years visitors get to fill
out a card for a lottery when they purchase a ticket in the visitor centre.
However, the sun can shine into the chamber for a number of days on either
side of the shortest day, so there is no harm turning up near the solstice.
The display that takes place outside the entrance to the mound is quite spectacular.
The quartz wall lights up with a bright yellow golden colour, which could
surely be seen from many miles away. Martin Brennan was the first to note
that the standing stones near the entrance cast shadows on the kerbstones.