How did Babylonians determine the beginning of a month?

The Babylonians determined the beginning of a month based on the first visibility of the new moon. Each month began at sunset on the evening when the new crescent moon was first observed. This observation was considered significant enough that, as late as the 8th century BCE, court astronomers were responsible for reporting the sighting to the Assyrian kings.1

The Babylonian calendar was a lunisolar system consisting of 12 synodic months, which are cycles of the Moon phases. Because this lunar year of approximately 354 days is shorter than the solar year, the Babylonians reconciled the two by occasionally adding an extra month through intercalation. By about 380 BCE, this process was regularized using a 19-year cycle, ensuring the first month of the year, Nisanu, remained near the onset of spring.21