The Venus Tablets
One strand of evidence allowing scholars to date ancient Mesopotamian kings is provided by the so-called Venus Tablets.
The 9-inch-high fragment at right was excavated at the Assyrian site of Nineveh in 1850 by the British explorer Austen Henry Layard. Although the tablet dates to the seventh century B.C., its cuneiform text refers to the Old Babylonian king Ammisaduqa (1646–1626 B.C.), who reigned about a century after Hammurabi (1792–1750 B.C.). Thus scholars believe that the Nineveh tablet—along with several other identical tablets that have also been found—is a copy of a tablet originally produced a thousand years earlier.
These tablets are called the Venus Tablets because they record observations of the risings and settings of the planet Venus (as both morning and evening star). It turns out that the appearance of Venus in the sky forms a pattern that repeats itself every 60 years. This means that the reigns of Ammisaduqa and other Old Babylonian kings (whom we know from king lists telling us how long each king reigned and who reigned before and after him) can be dated to 60-year intervals, as astronomers count backwards in time. Drawing on other evidence, scholars can narrow this chronology down to two 60-year cycles—that is, to a specific 120-year period. There remain, therefore, three possible dates for Ammisaduqa’s reign, each 60 years apart: the high chronology (1706–1686 B.C.), the middle chronology (1646–1626 B.C.) and the low chronology (1586–1566 B.C.). Most scholars opt for the middle chronology, as we have above in providing dates for Ammisaduqa and Hammurabi.