Presentation #105.05 in the session Daytime and Dark Sky Heritage in American Southwestern Archaeoastronomy.
Worldwide, cardinality is exhibited in early public architecture. This seems to be an unambiguous reference to the celestial hemisphere. Rather than following the alignments suggested by natural features of rivers and canyons, alignments to east-west and north-south are common. Such alignments are seen at such diverse cultures and horizons as at the pyramids of the Giza Plateau, Egypt, and Taj Mahal on the southern bank of the Yamuna River in Agra, India. One such structure is Pueblo Bonito in Chaco Canyon, New Mexico is the “D-shaped” great house, in which about half of the south wall is aligned east-west with remarkable precision. Our question is not WHY such alignments were imposed on the architecture, but HOW this could be done to high precision. We report on a simple accurate method to determine east-west using only the shadow of a gnomon, where the trajectory of its tip over the course of the day of equinox constitutes a straight line, oriented accurately east-west. This method is demonstrated with a simple gnomon placed just south of the southwest wall at Pueblo Bonito, with the results presented here. We suggest that the ancient Chacoans who constructed this remarkable structure may have used a gnomon technique to help them align their structures to the observable cosmos.