History: An assembly of stone slabs on an isolated butte in Chaco Canyon, New Mexico collimates sunlight onto spiral petroglyphs carved on a cliff face. The light illuminates the spirals in a changing pattern throughout the year and marks the solstices and equinoxes with particular images. The assembly can also be used to observe lunar phenomena. It is unique in archeoastronomy in utilizing the changing height of the midday sun throughout the year rather than its rising and setting points. The construct appears to be the result of deliberate work of the Anasazi Indians, the builders of the great pueblos in the area. Near the top of an isolated butte in Chaco Canyon, New Mexico, three large stone slabs collimate sunlight in vertical patterns of light on two spiral petroglyphs carved on the cliff behind them. The light illuminates the spirals each day near noon in a changing pattern throughout the year and marks the solstices and equinoxes with particular images. At summer solstice a narrow vertical form of light moves downward near noon through the center of the larger spiral. At equinox and winter solstice corresponding forms of light mark the spirals. It was found that the relationship between the shape and orientation of the slabs and the resultant light patterns on the cliff is a complex one and required a sophisticated appreciation of astronomy and geometry for its realization. The site is unique in employing the varying height of the midday sun during the year to provide readings of solar declination. In this respect it is clearly different in concept from the many archeoastronomical sites throughout the ancient New and Old Worlds that tell the passage of the year by marking the rising and setting points of the sun and moon. The Anasazi Indians occupied Chaco Canyon from about A.D. 400 to 1300. In this arid and unproductive region, these early inhabitants left evidence of a skilled and highly organized society. They constructed multistory pueblos and large ceremonial centers, and developed extensive systems of roads, irrigation, communication, and trade. The Anasazi had established an accurate calendar for agricultural and ceremonial purposes. To do this they determined the recurrence of the solstices and equinoxes. This astronomical knowledge was commemorated in the design and alignment of major buildings. The precision of the historic Pueblo calendar has been described in a recent study of its synchronization of the monthly lunar cycles with the annual solar cycle. | 
