The Torah tells us that we must make sure that Passover always falls in the springtime. Likewise, we know that a Jewish month is calculated according to the moon’s movement. Now, in order for Passover, Sukkot, and our other holidays to fall at the correct times, the calculations must be exact, taking into account the difference between the lunar and solar calendars. (That’s why we have leap years, where a thirteenth month is added to the year.)

This presents a very serious problem if the calendar is to be a product of human intellect. The Muslims, for instance, set their holidays according to the lunar calendar, with each new moon signifying the beginning of a new month. Since the lunar year (354 days) is approximately eleven days shorter than the solar year (365 days), their festivals fall out in entirely different seasons every few decades. Over time, a given Muslim festival will fall out during each season of the year and then begin cycling throughout the seasons again. The Torah, however, says that Passover must always be in the spring and Sukkot in the late summer/early autumn. The lunar month must be calculated precisely, and coordinated with the periodic addition of a leap month, in order for this to happen.

The Sages tell us that the moment when Moses was told about making the calculations of the months by the lunar calendar, God told him the precise rules of how to calculate the new moon. These calculations were handed down generation to generation, to the greatest Sages of Israel, but weren’t revealed to the multitude. The new-moon date was determined each month by the testimony of witnesses. However, after the destruction of the Second Temple the Torah leaders felt that these teachings had to be taught to the multitude or else they would be forgotten.

The Talmud records (Tractate Rosh Hashanah 25a) that Rabban Gamliel told the Great Court that he had a tradition going back to his grandfather’s house that the renewal of the moon took place not before 29 days and 12 hours, plus two-thirds of an hour and 73 parts. (The hour is divided into 1080 parts — each second has 18 parts to it. Two-thirds of an hour is 720 parts plus 73 parts which equals 793 parts. Thus we find that the new moon occurs every 29 days, 12 hours, and 793 parts.)
[According to this tradition, every 29.530594 days the moon is renewed. Only recently did NASA (National Aeronautics and Space Agency of the United States) come up with their computerized calculations of the time between one new moon and the next. And their calculation is almost exactly as ours! (Just for the record, they say it’s every 29.530588.)1


How is it possible? How could a human being have come up with such an accurate calculation without the benefit of modern, scientific equipment? The answer is that the traditions of the Sages go all the way back to Moses, who received them directly from the Divine source.

  1. Ridpath, Dictionary of Astronomy, Oxford Press, 1997.copyright Rabbi Shmuel Waldman
    From the Book “Beyond a Reasonable Doubt” available for an unbelievable price at

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