Many rocky sites in the southwestern United States are adorned with petroglyphs, pictures scratched in the rock centuries ago by Native Americans. Some of the figures are abstractions; some are symbolic representations of human or animal figures. Archaeologically and anthropologically the usual assumption is that such pictures are not made purely for decorative or aesthetic reasons. A social function is looked for. A few years ago, an observer named Anna Sofaer found that certain petroglyphs at Fajada Butte in Chaco Canyon, N.M., seemed to be markers for important dates in the sun's annual cycle (SN: 8/ 26/78, p. 148). On the solstices or the equinoxes of sunlight formed by the shadows of adjacent rocks would fall on the centers or other prominent points of certain figures. Sofaer and collaborators found enough such instances at Fajada Butte to convince them that one of the purposes of the place was to serve as a kind of solar observatory. One such site might constitute some kind of anomaly. Now it is joined by 19 others scattered around Arizona. Eighteen of these newly found sites are, like Fajada Butte, apparently the work of the Anasazi people, who inhabited the region from about the year 700 to the year 1300. The nineteenth site, at Painted Rocks State Park near Gila Bend, in southwestern Arizona, probably belonged to the Hohokam people, who were more or less contemporary with the Anasazi. Evidence for sun-petroglyph interactions at these new sites has been gathered by a husband-and-wife team, Robert A. Preston, supervisor of the Astronomical Measurements Group of the Jet Propulsion Laboratory in Pasadena, Calif., and Ann L. Preston of the California College of Arts and Crafts in Oakland. They have spent a good deal of their spare time in the last few years gathering data in the Arizona desert. Eighteen of their sites, all except the Painted Rocks one, are in or near the Petrified Forest National Park. The Prestons presented their findings at the meeting in Boston of the American Astronomical Society and have submitted them for publication in SCIENCE. At the meeting they showed both still pictures and time-lapse motion pictures of the sunlight-petroglyph interactions. The motion pictures are particularly dramatic, showing shadows or daggers of light sweeping across the rock figures until, on the appropriate day, they touch certain points on the figures. The figures are mostly spirals and circles. Human and lizard-like figures make up 16 percent, and four percent are crosses. In the circles, spirals and crosses, the reference points be the centers or the edges. In the lizards they are usually the tip of the tail; in the humans the tip of the penis. Tail, says Robert Preston, may be a euphemism. In addition to situations in which shadow or light moves across a figure, there are those where the observer places an eye against the figure, and then, at sunrise on the relevant day, a spot of light can be seen through a notch or hole in another rock. The dates marked are the solstices, the equinoxes and a day 45 days (one eighth of a year) before and after winter solstice. The Prestons say they were inspired to look for a multitude of sites to determine whether sunwatching was a purposeful and widespread activity of Americans of that time and place. Only when multiple occurrences of a particular solar interaction are found can one be sure the interaction is purposeful, they write. To this end we studied every principal petroglyph site of Anasazi origin we could locate or conveniently work within a specified geographical region. Using a computer they calculated precision azimuths and elevations for both the sun and the moon at the location of their base sample, an area of northeastern Arizona 10 kilometers by 25 in extent, and made plots of the sun's daily path through the sky on five relevant dates, the solstices, equinoxes and the 45th day after winter solstice. This way they could predict petroglyphs likely to have the proper interactions. Not all the drawings function as astronomical markers. Some seem to have been related to fertility ceremonies. Ann Preston says they had to be careful making pictures for public display, as -many of the solar markers are surrounded by drawings of a pronounced erotic character. Within a few years it is difficult temporally and logistically to be at so many sites on just the proper days, so certain substitutions in technique were made. Observations can be made within 3.5 days around the date of a solstice with an error of no more han 0.05? in the declination of the sun. Such observations are good enough for the purpose, since the sun's
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Jul 15, 1999
Paul E Olsen + 1
