How did ancient civilizations develop calendars, or ways for tracking days, months, and years?
Celestial bodies, such as the Sun, Moon, planets, and stars, provided peoples of ancient civilizations a reference for measuring the passage of time. Ancient civilizations relied upon the apparent motion of these bodies through the sky to determine seasons, months, and years. Historians know little about the details of timekeeping in prehistoric eras, but wherever archaeologists dig up records and artifacts, they usually discover that in every culture some people were preoccupied with measuring and recording the passage of time. Ice-age hunters in Europe over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the Moon. Five thousand years ago, Sumerians in the Tigris-Euphrates valley (in present-day Iraq) developed a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to two of our hours), and divided these periods into 30 parts (each like four of our minutes). Historians have no written records of Stonehenge, built over 4,000 years ago in England, but its alignments show one of its reasons for existence was to determine seasonal or celestial events, such as lunar eclipses and solstices.