Molad

Molad (מולד, plural Moladot, מולדות) 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 Traditional Molad Interval

The molad emtza'i (מולד אמצעי, 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 (or, equivalently, 29 days 12 hours and 44+¹/₁₈ minutes) 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, who cited Hipparchus as its source. Although in the era of Hipparchus (2nd century BC) this interval was equal to the average time between lunar conjunctions, mean lunation intervals get progressively shorter due to tidal transfer of angular momentum from Earth to Moon, consequently in the present era the molad interval is about ³/₅ of a second too long.