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When Anna Sofaer visited Los Alamos Feb. 21, her “Mystery of Chaco Canyon” film played to an overflow crowd in the Bradbury Science Museum. Sofaer will return to the Bradbury Thursday with computer modeler Alan Price, whose new simulation may shine even more light on the mystery of the Sun Dagger.Based on studies of the archaeological record of structures and petroglyphs Sofaer has studied for more than 30 years, the film demonstrates how the Chacoan culture of 1,000 years ago had knowledge of the sun’s solstices and equinoxes, as well as of the moon’s 18.5 year cycle of rising and setting at specific points on the horizon.Sofaer first discovered the “Sun Dagger” during the summer solstice of 1977. The phenomenon consists of a dagger shaped point of light that shines on a deeply etched spiral petroglyph on a butte above Chaco Canyon. Shadows cast on the petroglyph by rock slabs placed in front of the petroglyph mark the track of the moon as well.Alan Price has constructed a computer model to demonstrate the workings of the Sun Dagger construction, which not only tracks the sun’s movements across the sky, but also marks the shadow cast by the moon at significant times of its cycle.“The demonstration will be interactive with the audience,” Sofaer said in an interview last week. “It will give people insight into all the variables.” She said that people will have the opportunity to shape each of the slabs of rock, and move each of the slabs to show the effects.The site, located on Fajada Butte, is now protected, making it difficult for even recognized researchers to enter, so Price’s model is essential in analyzing the configuration of slabs and petroglyph spiral that make up the Sun Dagger.“We needed to create a research model, and we needed to create an educational model,” Sofaer said.Sofaer’s film and subsequent writing have gone a long way toward explaining the relationship of the buildings in Chaco Canyon to astronomical events. She has demonstrated how lines can be drawn between the different sites, often miles apart, which exactly demonstrate the maximum and minimum sunrise and moonrise.The film reveals that between 850 and 1150 AD, the Chacoan people designed and constructed massive ceremonial buildings in a complex celestial pattern throughout a vast desert region. The structures may have had more ceremonial than practical uses, as the rooms in the massive buildings had no light, very little ventilation and no evidence of cooking.In 1989, Sofaer found that the rock slabs had shifted and the way that the light shines on the petroglyph had changed.“We are particularly excited that, in addition, the interactive capability of this computer model offers dynamic opportunities for scientific exploration of the site,” she said. “One can navigate around the 3-D model, observing it from any angle, set the calendar date and time of day for positioning the sun and moon, projecting shadows of the stone slabs onto the cliff and spiral patterns in real time.”The slabs and the spirals can be adjusted in their positions and shapes. The student or scholar is in essence in the role of a Chacoan astronomer, testing the sensitivity of the elements of the site and assessing what actions may have been taken by the Chacoans to precisely mark the sun and the moon. In these images the model accurately replicates the summer solstice Sun Dagger. Copies of Sofaer’s latest book, titled “Chaco Astronomy: An Ancient American Cosmology” are available in Los Alamos at Otowi Station bookstore.For more about The Solstice Project, visit www.project.org. For more information about Thursday’s presentation, call the Bradbury Science Museum at 667-4444 or see the museum’s webpage atwww.lanl.gov/ museum.