Cahokia Mounds State Historic Site is planning a special event to mark this year’s winter solstice.  The Southern Illinois historic site was once home to thousands of Native Americans, who built a wide variety of structures.  Sun watchers will gather at 7 AM on Sunday at “Woodhenge,” a huge circle of posts that is a reconstruction of the solar calendar used by Native Americans.  An archaeologist will explain how Woodhenge was discovered, as well as its form and function.  Illinois Historic Preservation Agency officials say that no ceremonies or rituals will be conducted, out of respect for American Indian beliefs.  The solstice marks the start of winter, which is also the shortest day and longest night in terms of sunlight.