The Denial of History

Velikovsky, Worlds In Collision, MacMillan Page 210:

In -747 a new calendar was introduced in the Middle East, and that year is known as "the beginning of the era of Nabonassar." It is asserted that some astronomical event gave birth to this new calendar, but the nature of the event is not known. The beginning of the age of Nabonassar, otherwise an obscure Babylonian king, was an astronomical date used as late as the second Christian century by the great mathematician and astronomer of the Alexandrian school, Ptolemy, and also by other scholars. It was employed as a point of departure of ancient astronomical tables.

"This was not a political or religious era.... Farther back there was no certainty in regard to the calculation of time. It is from that moment that the records of the eclipses begin which Ptolemy used." [Cumont, F., 1912] What was the astronomical event that closed the previous era and gave birth to a new era?