Measuring Time. Three natural periodicities must have been evident to humans quite early in history: the rising and setting of the sun (day), the waxing and waning of the phases of the moon (lunar month), and the alternation of the stars in the night sky in concert with the changing seasons (solar year). Late in the fourth millennium B.C.E., with the advent of the ability to count abstractly beyond two or three, the period relations among the day, lunar month, and solar year must have become evident; the lunar month consists of either twenty-nine or thirty days, and the solar year consists of slightly more than twelve lunar months.

Naming Months and Years. During the Early Dynastic period (circa 2900 - circa 2334 B.C.E.) each city maintained its own system of naming months and years; the days of the month...