Talk at the Jewish Calendar Seminar on
Wednesday, 2 Shvat (Jan. 12, 05), 4-5 pm

Speaker: Eran Raviv (Revava)

Title: "A Comment in `Yesod Olam' by Rabbi Yitzhaq Hayisreeli"

Abstract:
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Rabbi Yitzhaq ben Yosef Hayisreeli wrote `Yesod Olam', one of the most 
comprehensive books on Jewish astronomy and the Jewish calendar. In this
talk we shall review the man and his work, concentrating on one comment  
regarding the date of Passover in the year 5094 (1334 CE).

As an appendix to `Yesod Olam' there is a letter from Yitzhaq ben Yisrael,
quoting a critical comment by "one Paritz" on the dating of that year's 
Passover, and providing a detailed answer refuting all his claims.

We shall analyze the comment and the refutation in light of astronomical 
data of the feasiblity of moon sighting at the relevant dates, using 
contemporary tools.   

Some open problems will be presented for further discussion.