In the late 19th century, a number of cuneiform astronomical texts were uncovered in excavations at Babylon and shipped to the British Museum. Among these texts were the Babylonian Diaries. The first astronomical texts were transcribed by the Assyriologist Johann Strassmaier. The mathematician and astronomer Joseph Epping began the decipherment of their astronomical contents.

The Babylonian Diaries are astronomical observations recorded daily by Babylonian priest-scholars for some 800 years, from about the reign of the Babylonian king Nabonassar (747–733 B.C.) until the first century A.D., when cuneiform writing became extinct.

The priests recorded the positions of the sun, moon, planets and stars with respect to one another. They also made note of such celestial events as eclipses, solstices and equinoxes. Their purpose was to extract from the divine heavens helpful omens regarding political, economic and climatic events—such as the outcome of a battle, the fluctuations of commodities prices, or the flooding of the Euphrates River. One Diary entry goes: “In the night Saturn came near to the moon. Saturn is the ‘star’ of the sun. This is the solution: it is favorable to the king [for] the sun is the king’s star.”

A number of these ancient events can be dated by modern astronomers, either because the events were unique or because they occur in cycles. If a lunar eclipse is said to have occurred at a certain time on a certain date, for example, then astronomers can count backwards through all past lunar eclipses to determine which one—or ones—fit the description. Just such a lunar eclipse is recorded (in a tablet called Strassmaier Cambyses 400) as taking place before midnight on Month 4, Day 14 of Year 7 of Babylon’s Persian ruler Cambyses. Interestingly, the second-century A.D. Alexandrian astronomer Ptolemy recorded that a lunar eclipse took place in Egypt before midnight on Month 7, Day 17 of year 7 (according to the Egyptian calendar) of Cambyses (the Persians had conquered Egypt as well as Babylonia). We know that these two dates are both July 16, 523 B.C., in the modern calendar.

This means that all three pieces of evidence line up perfectly: the astronomical event (the eclipse), the record of the event from Babylon, and the record of the event from Egypt. Such matching dates give scholars of ancient history a good deal of confidence in the reliability of the current chronology.