Molad (plural
Moladot) is a
Hebrew word meaning "birth" that also generically refers to the time at which the New Moon is "born". The word is ambiguous, however, because depending on the context it could refer to the
actual or
mean astronomical lunar conjunction (calculated by a specified method, for a specified time zone), or the
molad of the traditional Hebrew calendar (or another specified calendar), or at a specified locale the first visibility of the new lunar crescent after a lunar conjunction.
The molad emtzai (average molad, used for the traditional Hebrew calendar) is based on a constant interval cycle that is widely but incorrectly regarded as an approximation of the time in Jerusalem of the mean lunar conjunction. Each molad moment occurs exactly 29 days 12 hours 44 minutes and 3+1/3 seconds after the previous molad moment. This interval is numerically exactly the same as the length of the mean synodic month that was published by Ptolemy in the Almagest. Although this interval quite closely approximates the average time between lunar conjunctions, in the present era it is about 3/5 second too long.
The traditional epoch of the cycle was 5 hours 11 minutes and 20 seconds after the mean sunset (considered to be 6 hours before midnight) at the epoch of the Hebrew calendar (first eve of Tishrei of Hebrew year 1).
Historically, the original molad reference meridian of longitude was halfway between the Nile river and the end of the Euphrates river, but the excess length of the molad interval causes it to drift progressively eastward at an accelerating rate — in the present era it is at a meridian that passes near Qandahar, Afghanistan!
