Explore Jewish time through astronomy.
Molad: Mean Month and True Conjunction
Understand the traditional mean molad, true lunar conjunction and crescent visibility in Jewish calendar calculations.
The term
Molad names the traditional calculated lunar renewal used in fixed Hebrew calendar arithmetic. A mean lunar month is 29 days, 12 hours and 793 chalakim; an hour contains 1,080 chalakim. These are units of an arithmetic model, not a claim that every physical lunation has that length.
Astronomical meaning
A true conjunction is calculated from the relative positions of the Sun and Moon. The interval between successive conjunctions varies. Conjunction does not mean that a crescent is visible: elongation, altitude, light and the observer’s horizon all matter.
Use in Jewish Cosmology
The Moon tools distinguish the standard calendar from observational experiments. Experimental-year molad comparisons show traditional and calculated events together with elapsed time. Read the event type and timezone before comparing local clock times; the traditional molad count and a civil clock use different conventions.
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About Jewish Cosmology
Jewish Cosmology is an interactive research and educational web application connecting Hebrew dates, traditional Jewish calendar calculations and halachic concepts of time with the observable motions of the Sun, Moon and planets, astronomical seasons and eclipses.
Models, terminology and limitationsHalachic times are unverified, have no rabbinic approval, and must not be relied upon for practical halachic decisions unless and until an appropriate authority approves them.