Prehistoric CalendarsProbably the most famous example of a prehistoric site said to have been used for astronomical observations is Stone Henge, the stone megalith site on the Salisbury Plain in England. The alignment of the 35 ton heelstone, probably erected between 3100 and 2300 BC, is said to allow a person standing in the center of the circle to watch the sun rise on the Summer Solstice directly above it. Approximately 900 such megaliths are said to be located in Great Britain. Of course, Stone Henge is just one of thousands of similar sites in Great Britain and Europe. Sites likely related to astronomy in the North American continent include Cahokia's Wood Henge, a circle of large wooden posts with a calendrical alignment; the Observatory at Chichen Itza, and the Big Horn Medicine Wheel of Wyoming. South America has Nazca lines, which may be in part calendrical. Huge physical structures on the landscape may have produced the data for the first calendars, but the calendars themselves, that is the written record of what day is what, were not developed until at least the rudiments of writing were established. Written calendars which survive today include the Aztec and Mayan calendars. | 
| 