Welcome to a new cycle of Mishnah Yomi. I am Rabbi David Spetner, and I look forward to your joining throughout Shas, B'ezrat Hashem. We begin with Berakhot, Perek Alef, Mishnah Alef.
From when may the evening Shema be recited:
The Mishnah opens with a question: from when may one read the evening Shema at night? The answer given is the time that the Kohanim enter to eat their Terumah, that is, the time from which they are permitted to eat Terumah. If a Cohen was tameh and went to mikvah, even after immersing he is not yet tahor for Terumah until erev shamesh, until the sun has set. Only once the sun has set is he permitted to eat Terumah.
Why this means tzeit hakochavim:
The Gemara learns from a pasuk that before the Cohen eats Terumah the sun must set, "v'tahor" - and the Gemara maintains that the word v'tahor does not refer to the Cohen becoming tahor, but to the shamesh, to the sun itself. The sun is completely cleansed away, and that describes the time of tzeit hakochavim, when it is dark and the stars have come out. Accordingly, tzeit hakochavim, when the stars appear, is the earliest point at which one can be yotze kriat shema.
Until when may it be recited:
The Mishnah does not pose this as a formal question, but it continues the same line of thought and presents three opinions:
Rabbi Eliezer - until the end of the first watch.
The Chachamim - until chatzos, the midpoint of the night.
Rabban Gamaliel - until amud ha'shachar, until the light begins to come in the east.
Rabbi Eliezer's opinion: The Gemara explains that the night is divided into three parts, three thirds, and the end of the first watch corresponds to the groups of malachim, of angels in Shamayim, that say praise to Hashem; the first watch of that group is over once one third of the evening has passed. This is how Rabbi Eliezer understands the words that one should read the Shema "when you go to sleep" - the first third of the night is the time when people go to sleep.
Rabban Gamaliel's opinion: Once the light of dawn appears, even before the sun has risen, it is too late; it is no longer the time for Shema. His understanding is that the pasuk means the entire time that people are sleeping - not the moment people go to sleep, but the whole span during which people are asleep. In truth, as we will see, the Chachamim also hold this understanding, and they said ad chatzos only as a migder milsa, a safeguard to keep us from missing the time completely.
The incident with the sons of Rabban Gamaliel:
This is illustrated by the maisa that follows. Rabban Gamaliel's children came back from a party, from a wedding. "Amru lo, lo kari'nu es shema" - they said to him, we have not yet read the Shema, and it was already after chatzos. "Amar lahem, im lo alah amud ha'shachar, chayav adam likros" - he told them, if daybreak has not yet arrived, you are still obligated to read. And he added that this is not limited to Shema: "ela kol ma she'amru Chachamim ad chatzos" - every area where the Chachamim said something may be done only until chatzos, mitzvasan ad sheya'aleh amud ha'shachar, the mitzvah really extends until amud ha'shachar. As the Mishnah will explain, chatzos was stated only as a safeguard, to make sure the time does not pass.
The Gemara's explanation of the exchange:
Rabban Gamaliel himself, though he held that one may read the Shema until daybreak, recognized that his was an individual opinion. His sons, even though the dissenting authority was their own father, were obligated to follow the Chachamim, the majority opinion, which was ad chatzos. Their question to him was therefore this:
Do the Chachamim mean chatzos literally? - that is, they argue with Rabbi Eliezer only about how long "people are going to sleep" lasts: Rabbi Eliezer says a third of the night, they say half the night.
Or do they truly hold like you? - that the pasuk means the entire time that people are asleep, and chatzos was said only as a safeguard. If it is merely a safeguard and we have already passed it, we would still be obligated to read, since their understanding of "uveshachbecha" is all the time that a person is asleep.
Rabban Gamaliel's response was that the Chachamim in fact agree with him in understanding the pasuk as referring to the entire time people are asleep. Chatzos is only a safeguard, and since his sons had already missed the safeguard, the obligation to read remained.
Other mitzvot that extend until amud ha'shachar:
Hekter chalavim ve'evarim - the burning of the fats and the limbs of the korbonos, such as the korban olah offered during the day, mitzvasan ad sheya'aleh amud ha'shachar. Most understand that this is not an example of a chatzos safeguard, since no safeguard was instituted for this halacha; the Rambam, however, is of the opinion that a safeguard was placed here as well.
Korbanos eaten for one day - meaning that day and the night that follows. The mitzvah to eat them really extends until amud ha'shachar, yet the Chachamim said one should not eat them past chatzos.
Why, then, did the Chachamim say "ad chatzos"?
The Mishnah itself raises the question, and the answer applies both to the korbanos and to Shema: to keep a person far from doing an avera.
Regarding the korbanos - one who ate them after amud ha'shachar would be liable for eating nosar, the meat of a korban past its allotted time.
Regarding the Shema - one would have missed the time of kriat shema entirely and lost the mitzvah.
By limiting the time to chatzos, the Chachamim kept a person away from either failure.