Hebrew calendar (astronomically called a lunar conjunction and, in Hebrew, a molad) is the moment at which the sun and moon are aligned horizontally with respect to a north-south line (technically, they have the same ecliptical longitude). 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, a long-term average length, identical to the mean synodic month of ancient times (also
Surya Siddhanta in South and Southeast Asia are rooted in this text, but the regional calendars adapted and modified them over time. The "Surya Siddhanta" calculates the solar year to be 365 days 6 hours 12 minutes and 36.56 seconds. On average, according to the text, the lunar month equals 27 days 7 hours 39 minutes 12.63 seconds. It states that the lunar month varies over time, and this needs to be factored in for accurate time keeping. According to Whitney, the Surya Siddhanta calculations were tolerably accurate and achieved predictive usefulness. In Chapter 1 of "Surya Siddhanta", states Whitney, "the Hindu