These days, most people mark the important dates with PDAs, paper calendars and e-mail “auto alerts.”

In ancient times, many cultures merely looked skyward.

William Iseminger, assistant site manager and public relations director at the Cahokia Mounds State Historic Site in Collinsville, Ill., spoke to participants at the 36th annual Symposium on the American Indian at Northeastern State University Thursday about celestial alignments with ancient mounds, structures and rock circles.

While Iseminger avoided elaborating on the cosmology attached to such sites, but spoke about the mechanisms people once used to track the passing of time and the seasons.

“American Indians are unique in their technique of tracking celestial bodies,” said Iseminger. “Almost all cultures have, in some way, done this since the beginning of time.”

The Cahokia site in Illinois spans five to six square miles and was home to 120 mounds, 80 of which still exist today. Much of the information about the people who once built the city was discovered by accident during excavations in the early 1960s.

Professional archaeologists were trying to save pertinent information, which was to be destroyed by the construction of an interstate highway that was later re-routed.

“After a summer of intense excavation, Dr. Warren Wittry was studying excavation maps when he observed that numerous, large, oval-shaped pits seemed to be arranged in arcs of circles,” said Iseminger. “He theorized that posts set in these pits lined up with the rising sun at certain times of the year, serving as a calendar, which he called Woodhenge.”

After further excavations by Wittry and others, more post pits were discovered in places they had predicted, and evidence pointed to as many as five Woodhenges on the Cahokia site. These calendars had been built over the period of approximately 100 years.

“It appears the posts were red cedar, which is the only native evergreen in the area,” said Iseminger. “It is also known as a sacred tree to the American Indians.”

The first circle would have consisted of 24 posts, said Iseminger; the second, 36; the third and most completely excavated had 48 posts; the fourth, 60; and the fifth and final circle, which was only partially excavated, was only 12 posts, or possibly 13, along the eastern sunrise arc.

Iseminger said had it been a complete circle, it would have had 72 posts, which would continue the 12-increment cycle.

“It’s not known exactly why they increased by 12,” said Iseminger. “It could be attributed to the lunar month.”

Only three posts are crucial as seasonal markers, those marking the first days of winter and summer – the solstices, and the one halfway between marking the first days of spring and fall – the equinoxes. Viewing was from the center of the circle, and several circles had large “observation posts” at that location, where it is likely the head of the group stood on a raised platform.

“Other posts between the solstice posts probably marked special festival dates related to the agricultural cycle,” said Iseminger. “The remaining posts have no known function, other than symbolically forming a circle, which is sacred to many American Indian tribes.”