  urlLink The Endangered Night Sky Darkness Project at CCNHP : "Near the east end of Chaco Culture NHP lies a rock with several Navajo sun symbols etched into its surface. Viewed from a particular flat stone 'seat', on the winter solstice, the sun rises in a notch created where the stone meets the farther horizon. The edge of the stone lies at an angle against the sky. This angle is the same as that of the sun's rising, so that as the sun climbs into the sky it follows the edge of the stone for about 40 minutes. Clouds may come and go, but at anytime during the sun's ascent in that 40 minutes, if there is even a moment of clear sky, a watcher can determine the winter solstice has arrived. " 
