Ancient Arizona rock carvings provide new evidence that Indians in the North American Southwest once used the play of sunlight and shadow to track the seasons.
At these sites abstract patterns are etched into rock along cliff faces and in caves. They are so positioned that distinctive light and shadow patterns, such as a narrow triangle of sunlight, fall on key features of the carvings at the turn of the seasons.
Previously, only one possible observatory site had been reported. This is the so-called ''sun dagger'' at Fajada Butte in Chaco Canyon, N.M. It marks the summer-winter solstices and spring-fall equinoxes. However, at the recent meeting of the American Astronomical Society (AAS), Robert Preston of the NASA Jet Propulsion Laboratory showed slides, movies, and other data on 19 Arizonan sites which he said support the theory.
Eighteen of the sites are attributed to Anasazi Indians, one to the Hohokam. They may date from AD 1450, or even AD 800. This is uncertain. But they clearly are prehistoric. Preston said that, at these sites, he and his wife, Ann, have identified 56 different markers that can be used in 89 different ways to monitor sun positions.
He said that he and his wife are convinced that the petroglyphs served to mark a yearly calendar.
Such conclusions tend to be controversial. They imply a high degree of sophistication on the part of the Indians. Mayan buildings, European stone circles, and other ancient remains have been shown to reflect sophisticated astronomical knowledge. However, archaeologists still demand strong evidence whenever a new case of an ancient ''observatory'' is proposed.
Preston noted that astronomical use of petroglyphs implies patient observation, rather than unusual engineering skill. The Indians had to monitor a rock face for a long time before they began carving. This was not an ''idle thing'' for them, he said. Indeed, he explained, ''It was important to their culture, and widespread in their culture.''
The original Anasazi ''Sun dagger'' at Fajada Butte illustrates this. Discovered in 1977 by artist Anna Sofaer, it consists of two spiral patterns - one large, one small. They lie behind three massive slabs of rock leaning against a cliff face. At noon on the summer solstice, a ''dagger'' of light appears and moves through the center of the large spiral. It is formed by a gap in the rocks. Another noontime light moves through the small spiral's center at the spring and fall equinoxes. At the winter solstice two bands of light frame the large spiral.
The observatory seems an example of exquisite ancient engineering. But the rock slabs are far more massive than anything else the Anasazi are known to have maneuvered. Evelyn B. Newman and Robert K. Mark of the US Geological Survey and R. Gwinn Vivian of the Arizona State Museum have shown that the slabs most likely were placed by a natural rock fall. It now seems likely that careful Indian observers discovered the site and placed the spiral petroglyphs to take advantage of it. This still is a notable feat, but one well within their capabilities.
The newly reported sites support this conclusion. The Prestons say they are intrigued both by the careful observation needed to establish them and by the consistency among them. The same four solar positions - the two solstices and the two equinoxes - are marked by the same four petroglyph symbols and with consistent ways of using light and shadow at these sites. Sun positions 45 days before and after the winter equinox also are marked.
John A. Eddy of the High Altitude Observatory, who has pioneered the study of North American Indian astronomy, said he considers the Prestons' findings to be impressive. Archaeoastronomy specialist Kenneth Brecher of Boston University agrees. But he pointed out that the Prestons still need to do more research to support their claim.
In particular, they need to photograph the play of light and shadow on the petroglyphs at times during the year other than at the seasonal turnings. It is especially important to record the day-by-day sequence for some time before and after an equinox or solstice. This is needed to refute criticism that the seemingly significant patterns marking these events are accidental.
Meanwhile, Eddy noted, the findings appear to be yet more evidence that prehistoric Indians, who had the same intellectual capacity as ''civilized'' people, were using that capacity in sophisticated ways. Birth of the sun
Astrophysicists have speculated that the sun had a 'midwife' at its birth. A nearby supernova star explosion triggered compression of the nebula that condensed to form the solar system, they suggest. Now two University of California chemists say the primordial solar nebula probably got along without such help.
Mark Thiemens and John E. Heidenreich III of the UC San Diego campus note that meteorites, believed to have formed shortly after the sun itself, are enriched by a light form of oxygen called O-16. The nebula would not have had enough O-16 to account for this. Since it could have been contributed by a supernova, that has seemed a likely source.
However, Thiemens explained at a meeting of the American Astronomical Society that he and his colleague have demonstrated a process whereby ultraviolet radiation could have produced the O-16 early in the sun's formation. In this process, molecules made up of O-16 atoms are pulled apart more slowly than are molecules made up of heavier forms of oxygen. No supernova need have been involved.
The discovery may be important for elements other than oxygen. It may, for example, provide a new way to separate deuterium (heavy hydrogen) from the far more abundant light form of hydrogen. Deuterium is a fuel for future fusion power plants.