From Ohio History Central
The Octagon Earthworks consist of a circular earthen enclosure connected to an octagonal enclosure by a short segment of parallel walls. The Octagon Earthworks formed one part of the Newark Earthworks, the largest set of geometric earthworks built by the Hopewell culture (100 B.C. to 500 A.D.) of prehistoric Native American people. The circle encloses about 20 acres and the octagon about 50 acres.
Ray Hively and Robert Horn, of Earlham College, believe that the Octagon Earthworks served as a sort of astronomical observatory. A line extended through the center of the circle and octagon points toward the place on the horizon where the moon rises at its northernmost point in a cycle of moonrises that takes 18.6 years to complete. Other moonrise and moonset alignments also are present in the architecture.
The Octagon Earthworks, as part of the Newark Earthworks, are a National Historic Landmark. The Octagon Earthworks site is located at North 33rd Street and Parkview Road on the west side of Newark, in Licking County, Ohio.