Responsa for Shabbat 68:2
ת"ר ביה"ש ספק מן היום ומן הלילה ספק כולו מן היום ספק כולו מן הלילה מטילין אותו לחומר שני ימים
Our Rabbis taught: As to twilight [period] it is doubtful whether it is partly day and partly night, or the whole of it [belongs to the] day, or the whole of it night: [therefore] it is cast upon the stringencies of both days.<span class="x" onmousemove="('comment',' This is explained infra. ');"><sup>3</sup></span> And what is twilight? From sunset as long as the face of the east has a reddish glow: when the lower [horizon] is pale<span class="x" onmousemove="('comment',' I.e., dark, no longer red. ');"><sup>4</sup></span>
Shut min haShamayim
They responded: "It shall be neither yours nor mine; cut it in two!" (I Kings 3:26), and within the vision they explained that it shall follow neither the stringent ones nor the lenient. Rather, the time between the setting of the sun and the appearance of the stars should be divided in two, and this point is the midpoint of twilight referred to by Rabbi Yehuda5cf. Shabbat 34b:3.
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