The Mishnah tells us that the time for reciting the Shema in the evening (Deut.6:4, 11:19) begins when the kohanim can begin to eat their offerings (terumah). If a kohen becomes impure, he is forbidden to eat terumah until he has immersed himself, U-VA HA-SHEMESH (“the sun has come”) VE-TAHER (“and become pure”) (Lev.22:6-7). “The sun has come” means that it has set, and “ve-taher” means that the daylight has been cleared away (and the stars have become visible). “The sun has come” could have meant that it has risen (the next morning), and “ve-taher” could have meant that the man has become pure by bringing a sacrifice to atone for his impurity; but a sacrifice is not required in order to eat terumah.
The Talmud (Shabbos 34b) gives several opinions about the duration of twilight. Here only R.Yosi’s opinion is mentioned: That twilight lasts only for an instant, and cannot be detected.
2. The Watches of the Night (3a-b)
The Mishnah tells us that, according to Rabbi Eliezer, the time for reciting the evening Shema ends at the end of the first watch. R.Eliezer’s view is that the night is divided into three watches of four hours each. He mentions watches rather than hours to teach us that the watches are observed even in Heaven, and at each of them, G-d cries out (see Jer.25:30) “Woe is Me that I destroyed My house and exiled My sons”.
R.Yosi once went into a ruin in Jerusalem to pray. Elijah, the prophet, met him there and rebuked him. He learned from the rebuke that one should not go into a ruin, that one may pray on the road, and that one who prays on the road may recite a short prayer.
While in the ruin, R.Yosi heard an echo (“bas kol”) moaning like a dove “Woe is Me that I destroyed My house and exiled My sons”. Elijah told him that this happens three times every day; and that whenever Israelites gather for prayer or study and say “May His great Name be blessed” (“Y’hei Sh’mei Rabbo…” – the response in the Kaddish), G-d nods His head and says “Fortunate is the King who is praised thus in His house! Why did the Father exile His sons? Woe to the sons who were exiled from their Father’s table!”
A person should not go into a ruin for three reasons: Because he may be suspected of misbehavior, because of danger from falling objects, and because of evil spirits.
R.Yehudah ha-Nasi says that the night is divided into four watches (of three hours each); R.Noson says, into three watches. Gideon’s troops attacked “at the beginning of the middle watch” (Judges 7:19); this suggests that there are only three watches. David sang praises to G-d at midnight, “before watches” (Psalms 119:62,148); this suggests that there are four watches, but it may mean that David got up eight hours before kings usually arise (see Berachos 9b), or it may mean “before 1-1/2 watches”.
In the presence of the dead one should discuss only things that are related to the dead. Some say that this applies only to Torah matters, but others say that it applies even to worldly matters.