In many lunar calendars, the beginning of the month can be affected by extraneous, i.e. non-lunar, factors. These factors may be political, as in ancient Greek calendars, 1 or religious, as in the rabbinic calendar. 2 Nevertheless, the monthly cycle of the moon remains the most essential criterion of any calendar that is defined as 'lunar'.
In almost all ancient lunar calendars, the month begins around the time of the new moon. Precisely when this is, however, varies considerably from one calendar to the next. 3 The most common definition of the 'new moon' is when the new crescent becomes visible for the first time. This occurs invariably in the evening, not long after sunset, and close to the point on the horizon where the sun has set. The first day of the month will generally begin, therefore, on that evening. This is the case, for instance, in the Babylonian calendar. 4
In other calendars, the lunar month begins at an earlier time: when the old moon ceases to be visible. Since the old moon is visible for the last time in the morning, shortly before sunrise, and close to the point on the horizon where the sun will rise, the first day of the new month