A roofbox is a specially contrived opening above a doorway, usually built for some astronomical significant event.
The roofbox was built directly above the entrance door to the passage. The passage and chamber were specifically aligned with the rising sun of the mornings around the winter solstice in December. The 17 metre passage rises in elevation as it leads to the chamber.
The chamber floor, the roofbox and the local horizon are on a single plane and so as the sun's morning rays penetrate the roofbox, they illuminate the chamber floor for a maximum of 17 minutes (weather depending).
A roofbox is also found in Cairn G of the Carrowkeel Megalithic Cemetery.
O'Kelly, M. J., 1983, Newgrange: Archaeology, Art and Legend, Thames & Hudson