High atop the imposing butte at the entrance of the canyon is the most famous of Chaco’s sites. There, a set of spiral petroglyphs pecked into a cliff face behind three giant slabs of rock functions as a solar marker. At summer solstice, a vertical shaft of light pierces the main spiral exactly at its center. On the winter solstice, two shafts of light perfectly bracket the same spiral. Light shafts strike the center of a smaller spiral nearby on the spring and fall equinoxes.
site, now known as the Sun Dagger, was discovered by Anna Sofaer in
1977. After extensive investigation, Sofaer and her colleagues published
data that showed that the spirals may also track the 18.6-year lunar