cosmic calendar

Carl Sagan covers a wide range of scientific subjects, including the origin of life and a perspective of our place in the universe.

Chaco Canyon, located in northwest New Mexico, is perhaps the only site in the world constructed in an elaborate pattern that mirrors the yearly cycle of the sun and the 19-year cycle of the moon. How did an ancient civilization, with no known written language, arrange its buildings into a virtual celestial calendar, spanning an area roughly the size of Ireland?