section_path: Accuracy/Molad interval
section_name: Molad interval
paragraph_text: Molad interval
  
  A " new moon " (astronomically called a lunar conjunction and, in Hebrew, a molad) is the moment at which the sun and moon have the same ecliptic longitude (i.e. they are aligned horizontally with respect to a north–south line). The period between two new moons is a synodic month. The actual length of a synodic month varies from about 29 days 6 hours and 30 minutes (29.27 days) to about 29 days and 20 hours (29.83 days), a variation range of about 13 hours and 30 minutes. Accordingly, for convenience, the Hebrew calendar uses a long-term average month length, known as the molad interval, which equals the mean synodic month of ancient times. The molad interval is 29 days, 12 hours, and 793 "parts" (1 "part" = 1 / minute = 3 1 / seconds) (i.e., 29.530594 days), and is the same value determined by the Babylonians in their System B about 300 BCE and was adopted by Hipparchus (2nd century BCE) and by Ptolemy in the Almagest (2nd century CE). Its remarkable accuracy (less than one second from the current true value) is thought to have been achieved using records of lunar eclipses from the 8th to 5th centuries BCE. In the Talmudic era, when the mean synodic month was slightly shorter than at present, the molad interval was even more accurate, being "essentially a perfect fit" for the mean synodic month at the time.
  
  Currently, the accumulated drift in the moladot since the Talmudic era has reached a total of approximately 97 minutes. This means that the molad of Tishrei lands one day later than it ought to in (97 minutes) ÷ (1440 minutes per day) = nearly 7% of years. Therefore, the seemingly small drift of the moladot is already significant enough to affect the date of Rosh Hashanah, which then cascades to many other dates in the calendar year, and sometimes (due to the Rosh Hashanah postponement rules) also interacts with the dates of the prior or next year.
  
  The rate of calendar drift is increasing with time, since the mean synodic month is progressively shortening due to gravitational tidal effects. Measured on a strictly uniform time scale (such as that provided by an atomic clock) the mean synodic month is becoming gradually longer, but since the tides slow Earth's rotation rate even more, the mean synodic month is becoming gradually shorter in terms of mean solar time.