1) BEIN HA'SHEMASHOS
QUESTION: The Gemara quotes a Beraisa in which Rebbi Yehudah states that an average person can walk 10 Parsa'os (40 Mil) in one day. He adds that a person can walk 4 Mil in the time between Amud ha'Shachar (dawn) and Hanetz ha'Chamah (sunrise), as well as another 4 mil in the time between Sheki'ah (sunset) and Tzeis ha'Kochavim (nightfall).
This implies that, according to Rebbi Yehudah, Bein ha'Shemashos (the period of time between sunset and nightfall) is the time that it takes to walk 4 Mil. However, in Shabbos (34b-35a), Rebbi Yehudah himself states that Bein ha'Shemashos lasts as long as it takes to walk 3/4 of a Mil (according to Rabah's interpretation of Rebbi Yehudah's opinion there, or 2/3 of a Mil according to Rav Yosef's interpretation).
How can Rebbi Yehudah say that the duration of Bein ha'Shemashos is 4 Mil, when in Shabbos he says that it is only 2/3 or 3/4 of a Mil? (TOSFOS DH Rebbi Yehudah)
(a) TOSFOS (DH Rebbi Yehudah, and in Shabbos 35a, DH Trei) explains in the name of RABEINU TAM that the Gemara here says "mi'Sheki'as ha'Chamah," while the Gemara in Shabbos uses the phrase, "mi'Shetishka ha'Chamah." The Gemara here refers to the moment that the sun disappears from our eyes. That point marks the beginning of sunset ("mi'Sheki'as ha'Chamah"). At that moment, however, the sun is still on its journey through the firmament (away from us). Once the sun has passed through the entire thickness of the firmament, the "end of the sunset" occurs. This is referred to as "mi'Shetishka ha'Chamah." Shortly afterwards (3/4 Mil), the sun's remaining light disappears altogether and three medium-size stars can be seen.
In summary, the order of events is as follows:
1. The sun disappears from view, at which time it begins to set.
2. A period of 3 1/4 Mil (or 3 1/3 according to Rav Yosef) passes, at the end of which the sun completely sets (but some of its light is still visible). This moment marks the start of the Bein ha'Shemashos under discussion in Shabbos.
3. Finally, after a period of 3/4 Mil (or 2/3 according to Rav Yosef), the light of the sun completely disappears, and the stars appear. Accordingly, Halachic nightfall does not occur until long after sunset, after the time it takes to walk 4 Mil. This is the view of RABEINU TAM.
(b) The VILNA GA'ON (in SHENOS ELIYAHU, beginning of Maseches Berachos, and in BI'UR HA'GRA OC 235 and 261) explains that both here and in Shabbos, the times mentioned (4 Mil and 3/4 Mil) begin only once the sun completely disappears from view. However, the Gemara here discusses a different Tzeis ha'Kochavim than the Gemara in Shabbos (and not a different Sheki'as ha'Chamah, as Rabeinu Tam suggests). In Pesachim, "Tzeis ha'Kochavim" refers to the time at which every last ray of light disappears from the sky (which occurs four Mil after sunset), and all of the stars can be seen. In Shabbos, "Tzeis ha'Kochavim" refers to the time at which three medium-size stars can be seen, which is the Halachic definition of nightfall. (The Gemara here in Pesachim is a discussion of Agadah and does not express the Halachic definition of Tzeis ha'Kochavim.)
According to the VILNA GA'ON, therefore, nightfall occurs shortly after sunset -- the time that it takes to walk 3/4 of a Mil.
(c) The RE'EM (SEFER YER'EI'IM) offers a third opinion. He suggests that the Gemara here refers to both a different Sheki'ah and a different Tzeis ha'Kochavim than the Gemara in Shabbos. The Gemara here focuses on the period of time from when the sun sets until the last ray of sunlight disappears. The Gemara in Shabbos, when it discusses "Bein ha'Shemashos," refers to a period of 3/4 of a Mil beginning from before the sun has completely set, while the sun is still visible. Nightfall occurs when the sun can no longer be seen. This moment comes at the end of Bein ha'Shemashos. The Gemara here is a discussion of Agadah and does not discuss the Halachic definitions of sunset and nightfall.
2) HALACHAH: THE TIME THAT IT TAKES TO WALK ONE "MIL"
OPINIONS: The Chachamim often express a value of time in terms of how long it takes the average person to walk one Mil. For example, in Pesachim (46a), the Gemara says that dough becomes Chametz if left without being baked or handled for the amount of time that it takes a person to walk one Mil. The RAMBAM (Hilchos Ma'achalos Asuros 6:10) writes that in order to remove blood from meat, it must be salted for the amount of time that it takes a person to walk one Mil (see also SHULCHAN ARUCH YD 69:6). How long is this time period?
The Gemara here in Pesachim is the source for this measurement. Ula (93b) states that in one day (the day of the equinox, when daytime and nighttime are of equal duration), a person can walk 10 Parsa'os, or 40 Mil. However, 5 of these Mil are walked between Amud ha'Shachar and Hanetz ha'Chamah (dawn and sunrise), and 5 more are walked between Sheki'as ha'Chamah and Tzeis ha'Kochavim (sunset and nightfall). Therefore, a person can walk 30 Mil from Hanetz (sunrise) until Sheki'ah (sunset). If that period is 12 hours long (720 minutes), then the time that it takes to walk 1 Mil is 24 minutes (720 / 30 = 24).
The Gemara challenges Ula's opinion from the view of Rebbi Yehudah in a Beraisa. Rebbi Yehudah says, like Ula, that a person can walk 40 Mil in one day. However, as Rashi explains, according to Rebbi Yehudah a person can walk only 4 Mil, and not 5, between Amud ha'Shachar and Hanetz and between Sheki'ah and Tzeis ha'Kochavim. Therefore, a person walks 32 Mil between sunrise and sunset. Accordingly, the time that it takes to walk 1 Mil is 22 1/2 minutes (720 / 32 = 22.5).
However, the Yerushalmi (Berachos 1:1, as cited by the SHENOS ELIYAHU to Berachos 1:1 and BI'UR HA'GRA OC 459) understands Rebbi Yehudah's statement differently. Rebbi Yehudah means that a person can walk 40 Mil in one day, from sunrise to sunset. The distance one can walk from dawn to sunrise (and from sunset to nightfall) is an additional 4 Mil, and is not part of the total 40 Mil. Consequently, the time it takes to walk one Mil is 18 minutes (720 / 40 = 18). While this interpretation allows for a much greater discrepancy between the opinions of Ula and Rebbi Yehudah, it is much more consistent with Rebbi Yehudah's contention that the time between Alos and Hanetz is "1/10 of the day" (i.e., of the 12 hours between Hanetz and Sheki'ah). This may be what the Gemara means when it says that "[Ula] made a mistake in counting the time from Hanetz to Sheki'ah" when he heard Rebbi Yehudah's statement. Ula thought that the 10 Mil is included in that period, when it actually is besides that period (Vilna Ga'on, ibid.).
What is the Halachah?
(a) The BEIS YOSEF (OC 459, and in SHULCHAN ARUCH 459:2) cites the opinion of the TERUMAS HA'DESHEN who says, based on the Gemara here, that it takes 18 minutes to walk one Mil. However, he reaches this figure through a different calculation than the Yerushalmi (see Bi'ur ha'Gra, ibid.). This is also the opinion of the RAMBAM in Perush ha'Mishnayos to Berachos (1:1).
(b) The RAMBAM (Perush ha'Mishnayos, Pesachim 3:2) says that the time it takes to walk one Mil is 24 minutes. The Rambam seems to rule like Ula, as is clear from his words in Hilchos Korban Pesach (5:8) where he defines "Derech Rechokah" as 15 Mil outside of Yerushalayim (which is the opinion of Ula). Even though the Gemara refutes Ula's opinion, apparently the Rambam had a different text in his Gemara (Bi'ur ha'Gra OC 459). TOSFOS (Pesachim 11b, DH Echad Omer) and the ROSH (Ta'anis 1:12) also cite the opinion of Ula.
(c) The BI'UR HA'GRA (OC 459) points out that according to the way Rashi and Tosfos seem to understand the conclusion of the Gemara (i.e., not like the Yerushalmi), the time that it takes to walk one Mil should be 22 1/2 minutes.
HALACHAH: The BI'UR HALACHAH (OC 459:2) writes that one should always use the more stringent measure. Therefore, dough that sat still for only 18 minutes is considered Chametz. However, one must salt his meat for at least 24 minutes to remove the blood (l'Chatchilah, one should leave the meat in salt for at least an hour).