Shabbat 34b
שבת ל״ד ב — ד"ה וְאֵיזֶהוּ בֵּין הַשְּׁמָשׁוֹת
Shabbat 34b:3
Defines bein ha-shemashot and discusses its uncertain status between day and night, a key basis for evaluating prayer recited shortly after sunset.
וְאֵיזֶהוּ בֵּין הַשְּׁמָשׁוֹת? — מִשֶּׁתִּשְׁקַע הַחַמָּה כָּל זְמַן שֶׁפְּנֵי מִזְרָח מַאֲדִימִין. הִכְסִיף הַתַּחְתּוֹן וְלֹא הִכְסִיף הָעֶלְיוֹן — בֵּין הַשְּׁמָשׁוֹת. הִכְסִיף הָעֶלְיוֹן וְהִשְׁוָה לַתַּחְתּוֹן — זֶהוּ לַיְלָה, דִּבְרֵי רַבִּי יְהוּדָה. רַבִּי נְחֶמְיָה אוֹמֵר: כְּדֵי שֶׁיְּהַלֵּךְ אָדָם מִשֶּׁתִּשְׁקַע הַחַמָּה חֲצִי מִיל. רַבִּי יוֹסִי אוֹמֵר: בֵּין הַשְּׁמָשׁוֹת כְּהֶרֶף עַיִן, זֶה נִכְנָס וְזֶה יוֹצֵא, וְאִי אֶפְשָׁר לַעֲמוֹד עָלָיו. וְאָזְדוּ לְטַעְמַיְיהוּ, דְּאִיתְּמַר: שִׁיעוּר בֵּין הַשְּׁמָשׁוֹת בְּכַמָּה? אָמַר רַבָּה אָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: שְׁלֹשָׁה חֶלְקֵי מִיל. מַאי שְׁלֹשָׁה חֶלְקֵי מִיל? אִילֵימָא תְּלָתָא פַּלְגֵי מִילָא — נֵימָא מִיל וּמֶחֱצָה. אֶלָּא תְּלָתָא תִּילְתֵי מִילָא — נֵימָא מִיל. אֶלָּא: תְּלָתָא רִיבְעֵי מִילָא. וְרַב יוֹסֵף אָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: שְׁנֵי חֶלְקֵי מִיל. מַאי שְׁנֵי חֶלְקֵי מִיל? אִילֵימָא תְּרֵי פַּלְגֵי מִילָא — לֵימָא מִיל. וְאֶלָּא תְּרֵי רִבְעֵי מִילָא — לֵימָא חֲצִי מִיל. אֶלָּא
Nevertheless, the definition of twilight is uncertain. And what is twilight? From when the sun sets, as long as the eastern face of the sky is reddened by the light of the sun. If the lower segment of the sky has lost its color, and the upper segment has not yet lost its color, that is the twilight period. If the upper segment has lost its color, and its color equals that of the lower one, it is night; this is the statement of Rabbi Yehuda. Rabbi Neḥemya says: The duration of the twilight period is the time it takes for a person to walk half a mil after the sun sets. Rabbi Yosei says: Twilight does not last for a quantifiable period of time; rather, it is like the blink of an eye: This, night, enters and that, day, leaves, and it is impossible to calculate it due to its brevity. And the Gemara remarks: In this dispute over the precise definition of twilight both Rabba and Rav Yosef follow their line of reasoning stated elsewhere. As it was stated: What is the measure of the duration of twilight? Rabba said that Rav Yehuda said that Shmuel said: The time it takes to walk three parts of a mil. The Gemara asks: What is the meaning of three parts of a mil? If you say that it refers to three halves of a mil, let him say a mil and a half. Rather, if you say that it means three-thirds of a mil, let him simply say one mil. Rather, it means three-quarters of a mil. And Rav Yosef said that Rav Yehuda said that Shmuel said: The duration of twilight is two parts of a mil. Again the Gemara asks: What is the meaning of two parts of a mil? If you say that it means two halves of a mil, let him simply say one mil. Rather, if you say that it means two-quarters of a mil, let him say instead: Half of a mil. Rather,
