Once a year, at the Winter Solstice, the rising sun shines directly along the long passage, illuminating the inner chamber and revealing the carvings inside, notably the triple spiral on the front wall of the chamber. This illumination lasts for approximately 17 minutes. Michael J. O'Kelly was the first person in modern times to observe this event on 21 December 1967. The sunlight enters the passage through a specially contrived opening, known as a roofbox, directly above the main entrance. Although solar alignments are not uncommon among passage graves, Newgrange is one of few to contain the additional roofbox feature. (Cairn G at Carrowkeel Megalithic Cemetery is another, and it has been suggested that one can be found at Bryn Celli Ddu.) The alignment is such that although the roofbox is above the passage entrance, the light hits the floor of the inner chamber. Today the first light enters about four minutes after sunrise and strikes the middle of the chamber, but calculations based on the precession of the Earth show that 5,000 years ago, first light would have entered exactly at sunrise and shone on the chamber's back wall. The solar alignment at Newgrange is very precise compared to similar phenomena at other passage graves such as Dowth or Maes Howe in the Orkney Islands, off the coast of Scotland.