Thousands of prehistoric petroglyph sites are scattered throughout the southwestern U.S. on cliffs, boulders and cave walls. These images are composed of petroglyphs (rock carvings) and pictographs (rock paintings). For the vast majority of sites, the meanings and functions of these drawings have been lost. During the last 16 years, Bob Preston working largely in Petrified Forest National Park, has shown convincingly that many of these sites were used as "solar calendars" to track the yearly movement of the sun across the sky through the interplay of sunlight on the petroglyph. As long as time has not altered the alignments or surfaces of the rocks involved, these solar calendars function the same today as they did when they were created almost a thousand years ago.
When the Spanish explorer Coronado first came through this part of Arizona in 1540, it was essentially uninhabited. However only a few hundred years earlier, subsistence farmers, belonging to principally the Ancestral Pueblo People culture, occupied 600 known sites in the park. By 1450 A.D. they had moved away. Descendants of these early residents of Petrified Forest might be found in the historic pueblos in the surrounding region, the Hopi pueblos about 100 km to the northwest or the Zuni pueblos about 70 km to the east in western New Mexico.
Most petroglyph sites in the park date from about 1000-1350 A.D. The petroglyphs are primarily geometric designs with several basic design elements appearing frequently, especially spirals and circles. Petroglyphs were made by pecking away the dark surface (desert varnish) of the rock to reveal the lighter underlying rock, generally sandstone. Petroglyphs in Petrified Forest National Park are some of the best preserved in the Southwest, and provide an excellent source for scientific study.
Since the first European encounters with the indigenous Southwest pueblo cultures in the sixteenth century, there is no historic record to indicate that the descendants of the early pueblo cultures monitored the motion of the sun, moon and stars to regulate ceremonial and agricultural calendars, as is common in many pretechnological cultures.
The earth's equator is tilted with respect to its orbit about the sun by an angle of 23.4 degrees. Hence, on one side of the orbit the sun is over the northern hemisphere, reaching maximum latitude of 23.4 degrees on the summer solstice, while on the other side of the orbit the sun is over the southern hemisphere, reaching minimum latitude of 23.4 degrees. Therefore, the sun follows different paths in the sky throughout the year. In the summer the sun follows a very high path in the sky, with the highest track reached on summer solstice, usually June 21st (or December 21st in the summer hemisphere). In winter the sun follows a very low path, with the lowest track reached on winter solstice, usually December 21st (or June 21st in the southern hemisphere). Since the sun is in different positions during the year, the sunlit images (or shadows) it casts on rock faces are also in different positions at different times of the year. At the two solstices the sun reaches the highest and lowest points in the sky, and the sunlit images on the rock faces also reach their maximum positions on these dates. In March and September the sun is over the equator and its track is midway between the solstice tracks; these dates are known as eqinoxes.
Most people know that there are longer days in June than in December, and some may notice that the sun's sunrise and sunset positions are changing. In June the sun rises in the northeast and sets in the northwest, and in December the sun rises in the southeast and sets in the southwest. Since the angle between the earth's equator and its orbit changes slowly (about a tenth of a degree per 1000 years), the path of the sun on the solstices (or any other day of the year) is almost precisely the same as it was on that day the previous year (or even 1000 years ago). The tracks of shadows cast by the sun on rocks on a particular day also remain the same year after year (as long as the rocks don't move or erode).
In 1977 a spiral petroglyph at Chaco Canyon National Monument was discovered which displayed a precise interaction with sunlight at the time of summer solstice by means of a narrow shaft of sunlight that moved down a shadowed rock face to bisect the center of a large spiral petroglyph. Subsequent observations found that on winter solstice and equinoxes there were intriguing interactions of sunlit shafts with the large spiral and a smaller spiral nearby. No other example of a sunlight interaction with prehistoric or historic petroglyphs was known at this time. However, there was a tradition of Pueblo sun watching in historic times, particularly of the varying sunrise and sunset positions throughout the year, to set the dates for ceremonies. Another exmaple of an elaborate calendar and map is located at Parowan Gap, Utah.
As a result of the Chaco Canyon find, Bob Preston initiated a research project to determine whether other petroglyph sites in the Southwest functioned as solar "observatories." Over the last 16 years he has identified about 120 examples of similar solstice events at more than 50 petroglyph sites in Arizona, New Mexico and southern Utah. Evidence indicates that the phenomenon may have been spread over as much as a 1000-km region. These findings show clearly that certain petroglyphs were used by early pueblo cultures to function as calendrical markers for the winter and summer solstices. Petrified Forest National Park contains the greatest known concentration of solar calendars, with 16 of the sites being in or immediately adjacent to the park, and has been key to understanding their nature.
Shadows and sunlit images are found to move across petroglyphs due to other rocks being in the path of the sun's rays. As the sun's path across the sky changes throughout the year, the positions of the shadows and sunlit images change on the petroglyph panels. In many cases the petroglyphs have been placed on the rock faces in just the right position so that specific interactions occur on the solstices. The most common types of petroglyphs on which solsitial interactions have been identified are spirals and circles. The key to determining that these were intended and not by chance is that interactions are seen from site to site, and occur on the solstices more frequently than on other days of the year. These consistent interactions may involve a point of sunlight or shadow piercing the center or tracing the edge of a spiral or circular petroglyph; or shadow lines may suddenly appear or disappear at the center or edges of the petroglyph; or they may move up to the center or edge and then retreat. It is not uncommon for a single petroglyph to display multiple interactions of this type, either on the same solstice or on each of the solstices. In fact, at one site, there are five circular and spiral petroglyphs that show 15 interactions on the both solstices.
An intriguing question is whether types of petroglyph images were involved with specific dates. In several cases similar sunlight and shadow interactions occur on spiral and circular petroglyphs on the equinox, and distinctive interactions occur with other petroglyphs on the solstices and other dates. Clearly much of the puzzle remains to be unraveled.
This interdisciplinary research, linking the disparate fields of astronomy, archaeology and art, opens up a previously untapped source of archaeological data in the Southwest. The observed interplay of sunlight and petroglyphs are "slow motion movies" that have survived for a thousand years, providing a window into the ceremonial lives of these early people. Visitors to Petrified Forest National Park can visit Puerco Pueblo and experience the wonder of a functioning solar calendar during the ten-day periods proceeding or following the summer solstice.
The above information was adapted from a National Park Service fact sheet.