One of the essential preparations performed prior to immersion in a mikva is washing one’s hair to ensure that there are no ḥatzitzot – foreign objects that would serve as obstructions to proper immersion. When must a woman wash her hair?
Rava teaches that a woman can wash her hair on Friday afternoon in preparation for immersion on Saturday night. Rav Pappa objected to this teaching, referring Rava to a letter that had been received from Ravin that clearly forbade this since the halakha requires that immersion should closely follow the washing of the head. The Gemara concludes:
Rava subsequently appointed an in connection with this matter and he taught: The statement I made to you is an erroneous one, but in fact it was this that was reported in the name of Rabbi Yoḥanan, ‘A woman may not wash her hair on the eve of and immerse after the conclusion of.’
Letters, such as the one from Ravin that Rav Pappa referred to, attest to a common means of communication that kept the Jewish community in Israel and the Diaspora united even after travel became difficult and often dangerous. Letters from the Sages in Israel to their counterparts in Babylonia most often referred to such matters as establishing the calendar, but dealt with other issues of Jewish law, as well.
When the Gemara notes that Rava appointed an amora to correct his earlier ruling, it refers to an individual who served in the capacity of a meturgeman – a speaker/ translator, whose job it was to present the words of the Rabbinic Sage to the students or to other members of the audience. Oftentimes the amora would not only repeat the words of the Sage or translate them from Hebrew to Aramaic, but he would also interpret and clarify the teachings. It is for this reason that the Rabbinic Sages of the Gemara referred to themselves as amoraim, since they viewed their role as interpreters of the words of the Sages of the Mishna who preceded them.