First light on an Irish tomb.

First light at an Irish tomb

When the sun rises on the day of the winter solstice (Dec. 21), a narrow shaft of light slips through a carefully positioned slit in the roof near the entrance of a prehistoric underground Irish tomb. Shooting down a narrow passageway about 60 feet long, the sun's light illuminates the floor of the tomb's main chamber. Such a scenario more often fits the stuff of legend than the result of a precise survey, but astrophysicist Tom P. Ray of the Dublin Institute for Advanced Studies in Ireland contends his measurements show that an ancient tomb at Newgrange, about 30 miles from Dublin, may have been deliberately designed to catch the sun's rays at the solstice.

"The evidence . . . supports the theory that the orientation of Newgrange was deliberate, which would make it . . . the oldest megalithic structure known for certain to have an astronomical function," Ray reports in the Jan. 26 NATURE. The tomb, erected about 5,150 years ago, is centuries older than Stonehenge and the Egyptian pyramids.

Ray's calculations indicate someone designed the gap in the roof to catch the first rays of the midwinter sun, as it appeared 5,150 years ago. As seen from the main chamber, the gap would frame the rising sun, almost perfectly matching the sun's apparent width. The first beam of sunlight would create a patch of light, initially about 6 feet long and a few inches wide, on the inner chamber's floor, bisecting the chamber and illuminating designs carved in the rock. Then the beam would broaden before narrowing again and finally disappearing.

Ray's results lend credence to a longstanding local belief that the sun illuminates the tomb chamber at certain times of the year. Although earlier scholars suggested the possibility of astronomical alignments, it took Ray's careful measurements and calculations to show the tomb's alignment with the midwinter rising sun was much more likely deliberate than an accident, as some skeptics argue.