Each person who is able, has a mitzvah to be familiar with the workings
of the Jewish Calendar (Shabbos, 75a). There are several points of view as
to the extent of the obligation, including the following:
To Rashi and Maharsha, each person is required -- according to ability
-- to study astronomy in connection with the calendar, but that the calculations
required to determine the months are only the duty of the High Court. The
Yerei'im taught that each person is required to be able to calculate the
months, not only the Beis Din!
According to the Rambam, the Talmud required a basic knowledge of astronomy
(see Magid Harakiah pp. 17-21).
Determining the Calendar: Review of Kiddush
We began our discussion regarding Kiddush Hachodesh last year (Outline
Vol. 1, # 12):
In determining the Jewish lunar month, three elements must
be taken into account: The "molad" (calculated conjunction), the actual
conjunction and the visual sighting of the witnesses (corresponding to the
moon's first phase).
The "molad" is the conjunction between the sun and the moon,
when the moon is centered between the sun and the earth, and cannot be seen.
However, the Torah says that... the moon... [must be] spotted by witnesses,
and this only occurs hours later, at the beginning of the moon's first phase.
This stage is referred to as "chiduso shel lavana" -- the renewal of the
One of our readers asked whether it is the crescent moon which determines
the months, or the molad.
The Calculation in Ancient Times
Today, the calculation determines the (average) molad, not the actual
However, there is a great dispute as to how the calendar was fixed in
Previously, we cited Rebbenu Bachaye and the Rashba, who held that the
determination was not based on the observation of witnesses, but was entirely
based on the calculation.
However, Magid Harakiah concludes that the two authorities -- Rebbenu
Bachaye and the Rashba -- have differing views. Although both agree that
a calculation is the sole determining factor, they have slightly different
calculations: Rebbenu Bachaye understood that the calculated molad determines
the new month. According to the Rashba, however, additional hours must be
added to insure that witnesses could possibly see the moon. In other words,
although it was not necessary for witnesses to see the moon, the calculation
must yield the time of the moon's first phase (the crescent), rather than
the molad (conjunction).
The opinion of the Rashba fits in well with another discussion of ours.
Use of Telescopes in Kiddush Hachodesh (From Outline
Moadim Uzmanim relates a contemporary debate: The visual
sighting in ancient days was performed, of course, without aid of instruments.
If the power of the court were resumed today, would the witnesses use telescopes
to arrive at a closer determination of the conjunction?
1. The Chazon Ish would allow the telescope.
2. The Brisker Rav, however, would not: From Rashi's commentary
to the Torah the six hours between the actual molad and the sighting of the
witnesses is an eternal "shiur" (legally binding measurement).
The meaning of this obscure passage [from the Brisker Rav]
would seem to be the following: The sanctification of the moon is not determined
by the actual molad (conjunction), but by the appearance of the moon's first
phase. Therefore, the fact that by use of telescope the molad can be detected
earlier, has no affect on the determination of the month. It is still several
hours before the first phase of the moon is detected here on earth...
The Brisker Rav's thinking coincides with the Rashba's: the determining
factor should be the first phase of the moon, not the conjunction.
Rabbi Yaakov Bernstein Kollel of Kiryas Radin 11 Kiryas Radin
Spring Valley, NY 10977 Phone: (914) 362-5156 E-mail: