Modern astronomers keep an eye on the calendar, the clock, and the ephemeris to decide when to observe the sky. Over 700 years ago, native peoples of the Sonoran Desert also observed the sky for their calendar and clock. For these ancient Southwestern people, whom archaeologists call the Hohokam, astronomy was not simply a quest for knowledge. It was a task which likely centered and renewed the Hohokam world.

The Hohokam Culture, the heart of which lies in the Phoenix Basin of Arizona, began around A.D. 1 and lasted until the end of the 15th century. The Hohokam are distinguished by their well developed irrigation farming, having created some of the largest canal systems in the New World. They also built dozens of large earthen mounds and more than 200 semi-subterranean, oval-shaped ballcourts.

In the 1960s it was shown that a 13th century, four-story adobe structure at Casa Grande (near Coolidge, Arizona) served as a Hohokam observatory. Recent study of the Hole-in-The-Rock formation in Phoenix has demonstrated that it also functioned to mark solstice and equinox events.. However, little else has been known about the extent of the Hohokam's astronomical knowledge. For the last 3 years, we have been studying a prehistoric site located on a lower knoll of Shaw Butte in Phoenix. Our study has revealed the site to be a complex Hohokam observatory that marked the solar cycles. The site also may have had ceremonial functions associated with the observation and celebration of the solar and possibly other celestial cycles.

The Shaw Butte site is centered around an oval-shaped, dry-laid masonry compound with multiple interior rooms. The compound is approximately 29 m along the east-west axis and 23 m along the north-south axis (a little smaller than Stonehenge in England). The masonry wall was once at least 1 m in height and as much as 1 m thick. Artifacts are scarce, but prehistoric ceramic types indicate the site was constructed and used during the late 12th and 13th centuries.

Scattered inside and outside the compound are boulders that contain a variety of pecked images, or petroglyphs. We believe some of these petroglyphs relate to the astronomical observations at the site. A large (1.4 x 1.8 x .75 m) boulder stands upright near the compound's center. This rock, which we call the centerstone, has been aligned with winter solstice sunrise and summer solstice sunset. The southwest face is tilted 20 degrees off of vertical and has a series of thirteen circles with dots in their centers. The 13 circle/dots are placed in a swirling pattern, and two sets of two circle/dots are linked by flowing lines. We do not know the meaning of the 13 circle/dots, but it is interesting to note that Hopi and Tohono O'Odham (Papago) Indians once had 13 month lunar calendars. Furthermore, circle/dots symbols have been recorded as Pueblo Indian clan symbols for the moon clan, and concentric circles with dots represent the sun for some Pueblo groups.

The centerstone acts as an instrument to mark the solstices. The flat face of the boulder is in shade as solar noon approaches the time of summer solstice. A notch in the top of the stone allows a tiny shaft of light to touch the middle of one of the circle/dots. As the sun continues to approach zenith, the spot of light grows until the majority of the face is awash in light. Winter solstice sunrise is marked by a shadow which splits the lowermost circle/dot.

Another petroglyph panel adjacent to the center-stone also is significant. This panel has been broken so the full panel is no longer there, although pictures taken in the 1930s enable us to reconstruct this important scene. Its an enigmatic maze of deeply pecked concentric circles, circle/dots, and a spiral. A human figure with upraised arms is present on the right (south) side of the panel. At equinox sunrise, the sun aligns with two of the circle/dots.

Other petroglyph panels inside the compound occur in clusters that lie along distinct axes aligned with specific sunrises and sunsets. The majority of the petroglyphs are within a few degrees of marking the solsticial or equinoctal sunrises or sunsets when viewed from the center of the compound.

Rooms also have certain axes that have astronomical meanings. In the northeast side of the compound is an oval-shaped room that has a doorway which faces northeast. A ray of light shines through this doorway during the summer solstice sunrise.

Another interesting space is a square room in the northwest side of the compound. A pointed stone has been set upright in the west wall of this room. The pointed stone aligns with the summer solstice sunset.

Within the compound are other important features. A small alignment of rocks is present behind the northern wall, and up righted boulder has a single circle/dot petroglyph at the southern wall, and other boulders with petroglyphs align with the winter solstice sunrise and sunset.

Outside the compound is a small, artificially-constructed rockshelter. The exact location of this fragile feature is being kept secret due to the concern for vandalism or unintentional damage to the roof. Hohokam petroglyphs, including a hollow cross image, have been pecked near the rockshelter's entrance. Three grooves have been etched into a flat ledge adjacent to the north wall. The roof has been specifically designed to allow several different light forms to move across the floor/ledge and walls of the rockshelter. These light patterns mark the seasons in a systematic manner. One of the light forms that appears during the summer solstice zenith is a dramatic four-pointed star. Regularities in the patterns of light have lead us to believe that the Hohokam had a formal calendar. Finally, the rockshelter entrance provides a view of the eastern horizon that includes the location of the northernmost sunrise (summer solstice) location and the location of the southernmost sunrise (winter solstice).

The Shaw Butte observatory is relative complex site, with redundant markings of the same solar event. No other Hohokam site has been shown, thus far, to be as complex as the Shaw Butte site.Yet, the architectural features at Shaw Butte have only limited accuracy as astronomical observations go. More importantly, we believe the Hohokam were concerned about their place in the universe and, therefore, the observations and solstice/equinox markings served to initiate and confirm ceremonial cycles important to the Hohokam. Certain group rituals may have been performed in the cleared areas inside the compound before, during or after special solar or lunar events, as determined by a sacred calendar maintained by Hohokam priests. For rest of the year, an individual sun watcher may have been responsible for maintaining the site and making observations. The Tohono O'odham (Papago) Indians assigned sun and star watching to a single individual, who reported his observations to the village chief. Those observations established a seasonal ritual calendar.

With hundreds of years separating us and the Hohokam, we can only speculate about the activities which occurred at Shaw Butte. Nonetheless, our own observations have convinced us that the Hohokam used the site as a solar observatory. Additional study may reveal lunar or celestial importance as well. The Shaw Butte site has taught use that one should never underestimate the level of knowledge of so-called primitive or ancient peoples.

Instruments made of basalt stone are not much of a match in accuracy for today's steel and optics. The instruments made by the Hohokam were aids to remarkably well-trained eyes. The ability to notice small changes in the location in the sun on the horizon is the same as the ability to notice small changes in an image being examined for comets. Over time, the reasons for looking up to the sky have changed, but the patience and skill required of an astronomer are as eternal as the heavens they watch.

The Shaw Butte site is located within a City of Phoenix municipal park which is heavily used. Hundreds of hikers walk by the site every day, although many of them do not know the site exists. Some damage has occurred, partly because citizens do not yet fully appreciate the significance of the site. The City of Phoenix Parks, Recreation and Library Department hopes to eventually develop an interpretive trail into the compound, educating the public about this important Hohokam observatory.