An ancient astronomical observatory
Located about 850 meters west of the monumental Monks Mound, the Cahokia Woodhenge is a large timber circle that functioned as a solar calendar. Its discovery in the early 1960s revealed that the Mississippian people who built the city of Cahokia had a complex understanding of the solar cycle. Archaeologist Warren Wittry first identified the structure during salvage excavations ahead of highway construction. He noticed a repeating pattern of large, oval-shaped post pits and hypothesized they formed a circle used for astronomical observation, a name inspired by similar structures in England.
Further excavations confirmed Wittry's theory, revealing not one, but five distinct Woodhenges built in sequence between 900 and 1100 CE. Four of these circles were built on the same spot, each one larger and with more posts than the last. The most completely studied of these is Woodhenge III, which is believed to have been constructed around 1000 CE. This circle measured 410 feet (125 meters) in diameter and consisted of 48 posts. A reconstruction of Woodhenge III was built in 1985, with posts placed in the original excavated holes.
The posts of the original structures were massive, estimated to be 20 feet (6.1 meters) tall and up to 20 inches (51 cm) in diameter. They were made of red cedar, a wood considered sacred by many Native American groups. The builders dug long, sloping trenches to help maneuver and raise the heavy timbers into their deep footings.
Alignments and cultural use
The Woodhenge was precisely aligned to track the sun's annual cycle. From an observation post near the center of the circle, a viewer could watch the sunrise align with specific perimeter posts on important days. These alignments marked the winter and summer solstices, as well as the spring and autumn equinoxes. On the equinoxes, the sun rises due east, and from the Woodhenge's central post, it appears to emerge from the front of Monks Mound, the city's largest earthwork. This alignment physically links the calendar to the main ceremonial feature of the city.
The solar calendar allowed the people of Cahokia to determine the correct times for planting, harvesting, and important community festivals. The structure shows the sophisticated integration of architecture and astronomy within the Mississippian culture.
Archaeologists believe Woodhenge III had 48 posts, but other versions had 24, 36, and 60 posts respectively. The final and largest circle, Woodhenge V, may have had 72 posts, though only a partial arc was ever completed. This might have been because the large trees needed for construction were becoming scarce in the area. The area was used for housing before the Woodhenges were built and appears to have returned to a residential area after the last circle was no longer in use.