Dear Rabbi Kornfeld,
How does the Gemara's explanation of the shiur of Rebbi Nechemiah's bein hashmashos help us? It seems to confuse matters more-it is not clear from the Rishonim which "yam" he is going to tovel in, nor if he is on top of the mountain when the shiur starts, nor if he has to return to the top of the mountain. What was wrong with the plain statement of half a mil?
Gershon
We wrote the following in the Background to the Daf. It should answer your question.
12*) [line 19] HA'ROTZEH LEIDA SHI'URO SHEL REBBI NECHEMYAH - That is, although Rebbi Nechemyah gives a very clear Shiur in the Mishnah, the Gemara wants to describe the Shiur in terms of the time it takes to perform a Tevilah, so that people will be able to time their Tevilah to finish before Bein ha'Shemashos (TOSFOS -- according to this explanation, the Gemara is easier to understand if the end of the Tevilah coincides with the beginning of Bein ha'Shemashos, as Tosfos indeed explains. RASHI explains the Gemara according to its simple reading, though, that the end of the Tevilah coincides with the end of Bein ha'Shemashos, and its beginning coincides with the beginning of Bein ha'Shemashos. Nevertheless, the Gemara may be describing the Shiur of Rebbi Nechemyah in terms of Tevilah, for those that want to time their Tevilah to finish before Bein ha'Shemashos begins -- M.Kornfeld)
*13*) [line 19] YANI'ACH CHAMAH B'ROSH HA'KARMEL - (The name "Karmel" in this Sugya clearly does not refer to the Har ha'Karmel which lies along the Haifa coast, since the person in our Sugya appears to be watching the sun set to the West of Karmel, and then going to the Mikvah to the East of the mountain. Rather, it is a mountain to the west of Teveryah, from which one can see Lake Kineret to the East, as Rabeinu Chananel explains. This is equally evident from the fact that the well of Miriam is visible from the top of the Karmel, since we know that the well of Miriam is hidden in Lake Kineret.)