for National Geographic News
From December 19 to 23—if the weather cooperates—20 lucky people a day will crowd into an ancient Irish monument's main chamber. There, they'll bathe in 17 minutes of light put off by the rising sun on the shortest days of the year.
This year about 28,000 people applied to take part in the ritual at the Newgrange monument, located in the Irish countryside in County Meath, reports the Brú na Bóinne Visitor Center (Ireland map).
The Stone Age monument dates to around 3200 B.C., making it 500 years older than the Great Pyramid of Giza in Egypt and a thousand years older than England's Stonehenge.
Archaeologists believe the grass-covered mound in Ireland is a "passage tomb." A tunnel runs to a cavelike chamber, where the remains of the dead were placed. (Related video: "Ireland's Mysterious Newgrange Tomb".)
According to Edwin Krupp, director of the Griffith Observatory in Los Angeles, California, the monument also incorporates knowledge that could only have been gained through precise astronomical observations.
"The people who built it knew about the winter solstice—knew when it occurred, knew where the sun would rise—and built a monument that took advantage of that event and incorporated it symbolically into the monument," he said.
The 62-foot-long (19-meter-long) passage faces the winter solstice sunrise.
A little window above the door allows light from the rising solstice sun to reach the depths of the burial chamber from about 8:58 a.m. to 9:15 a.m. local time.
Newgrange is the most elaborate of several passage tombs in the rich agricultural lands along the Boyne River about 30 miles (50 kilometers) north of Dublin.
The number of area monuments "suggests this wasn't a small rural community of a few farmers and herders," Krupp said. "We're seeing something there certainly bordering on chiefdomship, if not actually a chiefdomship."
According to Krupp, the full story behind the purpose of Newgrange and its kin is still shrouded in mystery.
SOURCES AND RELATED WEB SITES