The closing daf in Massekhet Berakhot tells of the succession of leadership in the yeshiva in Pumbedita.
Rabba and Rav Yosef: Rav Yosef was “Sinai,” extremely erudite, and Rabba was “oker harim” – one who uproots mountains, i.e., extremely sharp. The moment arrived when they were needed; one of them was to be chosen as head of the yeshiva.
They sent the following question there, to the Sages of: Which takes precedence, Sinai or one who uproots mountains?
They sent to them in response: Sinai takes precedence, for everyone needs the owner of the wheat, one who is expert in the sources.
Nevertheless, Rav Yosef did not accept the appointment, as the Chaldean astrologers told him: You will preside as head of the yeshiva for two years.
Rabba presided as head of the yeshiva for twenty-two years. After he died, Rav Yosef presided for two and a half years.
When Rav Yehuda, who was the head of the yeshiva in Pumbedita, died, there were two qualified candidates to replace him: Rabba and Rav Yosef. Rabba, who was younger than Rav Yosef, was renowned for his sharp intellect, while Rav Yosef was renowned for his encyclopedic knowledge. Since there was uncertainty with regard to which of them should be chosen, they posed a fundamental question to the Sages of: Which takes precedence, “Sinai” or one who uproots mountains? The answer that was received was that Sinai takes precedence. However, Rav Yosef, for reasons described in the Gemara, deferred, and during the twenty-two years that Rabba served as head of the yeshiva, Rav Yosef did not assume even the slightest air of authority. Only after Rabba’s death, Rav Yosef assumed the position at the head of the yeshiva.
Regarding the Chaldean astrologers, it is apparent from several places in the Talmud that the Chaldeans, or, as they are known in the Book of Daniel (2:4), Kasdim, were sorcerers and magicians with whom the Torah prohibits consulting. However, the Chaldeans were the scientists of that era and their primary area of expertise was astrology, i.e., foretelling a person’s future based on the stars. Although not everyone approved of consulting the Chaldeans (see Tosafot, Shabbat 156b), there is no real transgression in doing so, and it was not uncommon for Jewish men and women to seek their advice.