(a) The Gemara 93b states that Rebbi Yochanan said that from "Sheki'ah" to "Tzeis" is 5 Mil but below (94a) a Beraisa is cited that Rebbi Yehudah said that there are 4 Mil from Sheki'ah until Tzeis. The Gemara asks if this means that the Beraisa proves that Rebbi Yochanan is wrong and the Gemara answers that Rebbi Yochanan could defend his position by explaining that he was misinterpreted by Ula and Rava. What Rebbi Yochanan really meant to say was that an average person walks 10 "Parsa'os" (40 Mil) in a day, and Ula and Rava explained that he walks 5 Mil after Sheki'ah but they did not take account of the fact that one of these 5 Mil is walked after Tzeis because most people continue to walk for a short time after Tzeis and he walks 1 Mil then.

Therefore according to the Gemara's answer here even Rebbi Yochanan maintains that there are only 4 Mil between Sheki'ah and Tzeis and the original figure of 5 was only because of 1 extra Mil walked at the beginning of the night.

It takes 18 minutes to walk 1 Mil, so 4 Mil is 72 minutes which is the origin of Rabeinu Tam's famous Shitah that one must wait 72 minutes after Sheki'ah before doing work on Motzei Shabbos. This is the opinion of the Shulchan Aruch OC 261:2 (see Mishnah Berurah #20).

(b) Regarding the issue of Safek:

(1) See the Gemara above (2a) which states, "We rule that until Tzeis it is still daytime". Rashi writes that the source for this is Berachos 2b which cites Nechemyah 4:15 that they did work from "Alos ha'Shachar" until Tzeis. It is also said there (Nechemyah 4:16) "The night was for guarding and the day was for working". This implies that the word "day" means up to Tzeis. So Pesachim 2a implies that the time until Tzeis has a Din of certain day, not of a Safek.

(2) However Tosfos 2a DH v'Ha cites the Gemara (Shabbos 34b) where the Beraisa says "Bein ha'Shemashos is a Safek whether it is day or night". There is an opinion there that the length of Bein ha'Shemashos is 2/3 of a Mil which Tosfos writes would appear to be a long time before Tzeis (because since there are 4 Mil between Sheki'ah and Tzeis, it follows that there are 3 and 1/3 Mil between Bein ha'Shemashos and Tzeis).

(3) Tosfos answers that the problem is that we are not expert enough to know when Tzeis really is, because we do not know what are considered medium-sized stars, about which the Gemara (Shabbos 35b) states that if there are 3 of this size, this is considered Tzeis.

(4) In other words according to the basic Torah law the period between Sheki'ah and Tzeis is really considered certain day, so strictly speaking Bein ha'Shemashos is not a doubt. However our problem is that we do not know exactly when Tzeis is, so we have to be concerned that Bein ha'Shemashos might be considered night because Tzeis might have arrived even though we are not sure if 3 medium stars are visible in the sky. Therefore in practice there is a safek if Bein ha'Shemashos is called night or not.

KOL TUV

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