Moon Calendars

One hallmark of ancient civilizations is the development of a calendar to regulate activities with respect to the seasons. Many ancient lunar calendars are often called “lunisolar” because they keep up with the annual solar year by adding a thirteenth lunar month every two or three years. This solar element is not present in purely lunar calendars such as the Muslim calendar.

One synodic lunar month is equal to 29.53 days, and twelve lunar months is slightly over 354 1/3 days, whereas the solar year is just under 365 ¼ days; hence a purely lunar calendar will have a discrepancy of almost 11 days every year. In order for the lunar year to keep up with the solar year, i.e. for lunar months to recur in or around the same seasons, this discrepancy must be compensated every three (and sometimes two) years with the intercalation of an additional lunar month.

In many lunar calendars, the beginning of the month can be affected by non-lunar factors. These factors may be political, as in ancient Greek calendars, or religious, as in the Jewish calendar. Nevertheless, the monthly cycle of the moon remains the most essential criterion.

In almost all ancient lunar calendars, the month begins around the time of the new moon. Precisely when this occurs, however, varies considerably from one calendar to the next. 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.

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 will generally begin on the first morning when the old moon is no longer visible. This is the case, for instance, in the Egyptian cultic calendar.

In some other calendars, finally, the new moon is given a more scientific definition: it is the moment of the conjunction; this is the case, for instance, in the Chinese calendar. “Conjunction” is the moment when the moon, along its orbit, passes between the sun and the earth. At this point, the moon is at minimal illumination; to the naked eye, it is completely invisible. In such calendars, the month will generally begin at some point (e.g., morning or evening) of the day when conjunction occurs.

The time interval between conjunction and first visibility of the new moon is quite considerable, and hence significant to the calendar. Because of its small size and its proximity to the sun, the moon sliver is not visible to the naked eye immediately before or after the conjunction. The new crescent only becomes visible after it has reached a certain size, and after it has become sufficiently distant from the sun. This interval can be between 15 hours minimum and over two days, maximum. Thus in any lunar calendar, it makes a significant difference, typically of two calendar days, whether the month begins at the conjunction or at the first sighting of the new moon.