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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.

Molad Emtzai (Average Molad)

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 Jerusalemmarker 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 Qandaharmarker, Afghanistan!

Although the moment of the traditional Hebrew calendar molad is announced in synagogues on the Shabbat prior to each month (except before Tishrei), its only relevance to the present day fixed arithmetic lunisolar Hebrew calendar is that the molad of the month of Tishrei determines the date of the New Year Day (Rosh Hashanah), subject to possible postponements of 0, 1 or 2 days (depending on certain postponement rules).

Traditionally the announced or printed molad moment is quoted in terms of the hours, minutes, and 18ths of a minute elapsed from mean sunset, because Hebrew calendar days begin at sunset. Some printed sources subtract 6 hours to convert the molad moment to "civil" time, but doing so causes the Hebrew weekday to be wrong 25% of the time (whenever the molad moment is between sunset and midnight). Also, some printed sources even add an hour during the summertime for "daylight saving", but that is also a mistake because that would affect the molad of Tishrei and occasionally imply an erroneous date for Rosh Hashanah.

Molad Amiti (True Molad)

The molad amiti (true molad), which has no relevance to the Hebrew calendar, is the time at which the actual astronomical lunar conjunction occurs, often expressed either as the mean solar time in Jerusalemmarker (Universal Time + 2h 20m 56s or simply + 2h 21m) or as the clock time in Israelmarker. If the moment is desired for ritual or social purposes then it may be best to express it in terms of the local clock time.

On average the traditional molad of the Hebrew calendar is currently >2 hours late, and there are substantial periodic variations in the astronomical lunar cycle length, such that in the present era it varies over a 28-hour span ranging from 12 hours early to 16 hours late, compared to the Jerusalem mean solar time molad amiti, if all months are included in the evaluation. If the evaluation is limited to a single Hebrew month, however, for example Tishrei, then the portion of the variations that are due to Earth orbital eccentricity are for the most part eliminated and the average has an offset that is month-specific, such that presently the molad of Tishrei varies over about a 20-hour span ranging from 4 hours early to 16 hours late.

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