Pesachim 94a
פסחים צ״ד א — ד"ה סָבַר לַהּ כִּי הָא דְּאָמַר רַבָּה
Pesachim 94a:5
Analyzes the duration and astronomical character of twilight, including disagreement about the path of the sun beneath the horizon; this is a primary source for defining nightfall.
סָבַר לַהּ כִּי הָא דְּאָמַר רַבָּה בַּר בַּר חָנָה אָמַר רַבִּי יוֹחָנָן: כַּמָּה מַהֲלַךְ אָדָם בֵּינוֹנִי בַּיּוֹם — עֶשֶׂר פַּרְסָאוֹת: מֵעֲלוֹת הַשַּׁחַר וְעַד הָנֵץ הַחַמָּה חֲמִשָּׁה מִילִין, מִשְּׁקִיעַת הַחַמָּה עַד צֵאת הַכּוֹכָבִים חֲמִשָּׁה מִילִין, נִמְצָא עוֹבְיוֹ שֶׁל רָקִיעַ אֶחָד מִשִּׁשָּׁה בַּיּוֹם. מֵיתִיבִי, רַבִּי יְהוּדָה אוֹמֵר: עוֹבְיוֹ שֶׁל רָקִיעַ אֶחָד מֵעֲשָׂרָה בַּיּוֹם. תֵּדַע, כַּמָּה מַהֲלַךְ אָדָם בֵּינוֹנִי בַּיּוֹם — עֶשֶׂר פַּרְסָאוֹת, וּמֵעֲלוֹת הַשַּׁחַר עַד הָנֵץ הַחַמָּה אַרְבַּעַת מִילִין, מִשְּׁקִיעַת הַחַמָּה וְעַד צֵאת הַכּוֹכָבִים אַרְבַּעַת מִילִין, נִמְצֵאת עוֹבְיוֹ שֶׁל רָקִיעַ אֶחָד מֵעֲשָׂרָה בַּיּוֹם. תְּיוּבְתָּא דְרָבָא, תְּיוּבְתָּא דְעוּלָּא. תְּיוּבְתָּא.
The Gemara explains: Rava holds like that statement which Rabba bar bar Ḥana said that Rabbi Yoḥanan said: How far can an average person walk in a day? One can walk ten parasangs, which are forty mil. This is explained: From dawn until sunrise one can walk a distance of five mil; from sunset until the emergence of the stars one can walk another five mil. Therefore, it is found that the thickness of the firmament is one-sixth of the distance that the sun travels during the day. Between sunrise and sunset, the sun travels a distance of six thousand parasangs during the amount of time an average person can walk thirty mil. During the dawn and twilight periods in the morning and evening, when the sun passes through the thickness of the firmament, an average person can walk five mil, which is one-sixth of the distance he can walk between sunrise and sunset. Consequently, during dawn and twilight the sun must also travel one-sixth of the distance it travels between sunrise and sunset, i.e., one thousand parasangs, which is the thickness of the firmament. The Gemara raises an objection from the following baraita: Rabbi Yehuda says: The thickness of the firmament is only one-tenth of the distance that the sun travels during the day. Know that this is true because how far can an average person walk in a day between dawn and the emergence of the stars? One can walk ten parasangs, which are forty mil, and from dawn until sunrise one can walk only four mil, which are one parasang. Therefore, it is found that the thickness of the firmament is only one-tenth of the distance that the sun travels during the day. This is a refutation of the opinion of Rava. Similarly, it is a refutation of the opinion of Ulla, who holds that an average person can walk fifteen mil in half a day, whereas Rabbi Yehuda holds that the average person can walk sixteen mil in half a day. The Gemara concludes: It is indeed a conclusive refutation.