Important Dates in the Lives of Jesus and Mary 

holy days (on which Jews could not work) from interfering with the Preparation Day of the Sabbath (on which 

work is done to prepare for the Sabbath day of rest from work). The decision as to whether a month had 29 or 

30 days was a human decision. In the modern Jewish calendar, the months generally have set lengths. For 

example, Av is always 30 days long and Elul is always 29 days. But some months will vary in length, even in 

the modern calendar. For example, Heshvan and Kislev can each be either 29 or 30 days in length, depending 

on the year; and AdarI is usually 29 days, but has 30 days during any leap year. 

New Moons 

    In the modern Jewish calendar, the start of each month is determined mainly by the date and time of the 

new moon, as determined by astronomical calculation. One cannot view the moon at the time of the 

astronomical new moon (except during a solar eclipse) because the moon is too close to the sun. Calculation, 

rather than observation, is used to determine the start of the months in the modern Jewish calendar. 

    In the ancient Jewish calendar, the start of each month was determined mainly by observation of the new 

crescent moon. This calendar system is called the observation based calendar. Sometime after the astronomical 

new moon, usually one or two days later, the new moon has moved far enough away from the sun to be 

visible just after sunset. The new moon at this time has a thin crescent shape. 

    At some point in time, the Jewish people made a transition from an observation based calendar to a 

calculation based calendar. There is no clear agreement among scholars as to when this transition occurred. 

However, most scholars believe that in the first century 

A.D.

 the calendar was still observation based. My 

belief is that the transition to a calculation based calendar occurred in the first century 

B.C.

, during the reign of 

king Herod the Great. 

Transition to a Calculation based Calendar 

    When Herod the Great captured Jerusalem (43 

B.C.

, my revised date), the Roman conquest of the Jewish 

people was completed. Roman politics, culture, and science then became more influential in the land of Israel, 

because the Romans controlled the country. 

    Roman scholars could calculate lunar eclipses and solar eclipses. Pliny the Elder explains that, as far back as 

the 6th century 

B.C.

, scholars could predict solar and lunar eclipses. He also states that, 200 years before Pliny 

the Elder's time, Hipparchus also made significant contributions to the study and prediction of eclipses.

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Hipparchus (a Greek astronomer) lived during the 2nd century 

B.C.

 Thus, Roman scholars had knowledge of 

eclipses and how to predict them, well before Herod's capture of Jerusalem in the first century 

B.C.

    The Romans knew that solar eclipses always coincided with new moons. Pliny mentions his understanding 

of this fact.

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 And Cassius Dio states the same:  for it [the moon] is eclipsed only at full moon, just as the sun 

is eclipsed at the time of the new moon. 

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 If one can calculate solar eclipses, one can also calculate new 

moons. To know when a solar eclipse will occur, one must be able to determine the future positions of the sun, 

moon and earth relative to one another. This is the same information needed to determine new moons. 

    Also, solar eclipses offer a way to verify calculations of new moons. The astronomical new moon is not 

generally visible because of its proximity to the sun. But during a solar eclipse the astronomical new moon is 

visible. In this way, the Roman's calculation of astronomical new moons could be periodically verified. Thus 

Roman scholars could calculate new moons as well as eclipses. 

    Once Herod captured Jerusalem, Roman and Jewish societies coexisted. Jewish scholars and Roman 

scholars became part of the same Roman empire. Jewish scholars were almost exclusively men of religion, 

either priests or Levites, or else scribes who were closely connected to the leaders among the priests. Jewish 

scholars could now more easily obtain knowledge from Roman scholars. (While the war was going on, this 

would have been much more difficult.) 

    Sometime within a few years after the capture of Jerusalem by Herod, Jewish religious leaders would have 

been able to calculate new moons. The Romans could calculate the relative positions of sun, moon, and earth 

closely enough to know if the moon would eclipse the sun, or merely pass nearby. Thus, the Jewish leaders, 

using the same knowledge, would be able to calculate new moons with a fairly high degree of precision. The 

Jewish religious leaders valued knowledge about new moons, as this was needed to make decisions about their 

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