Commentary for Shevuot 6:1
הרי זה בעולה ויורד יש בה ידיעה בתחילה ואין בה ידיעה בסוף שעיר הנעשה בפנים ויום הכיפורים תולה עד שיתוודע לו ויביא בעולה ויורד:
But if the doubt exists, that the sight of the impurity dates partly from to-day and partly from the morrow, the impurity is certain, but the sacrifice uncertain. On account of this, R. Hiya bar-Joseph answered in the presence of B. Yohanan: Who is it who taught that one of these occasions of impurity can be divided into two? It was R. Yosse. He answered: You thus refute your own opinion; for you say that each twinkling of an eye of the time accomplished in a half-mile, according to R. Nehemiah, is doubtful, and not only the end of it. No contradiction can be offered to this; when the Prophet Elijah shall return to this world, and will explain to us what this twilight means, no one will contest him. R. Hanina argued against the disciples of the Rabbis: Since, said he, it is night as soon as three stars are visible, be the sun still high in the heavens, the same must apply (before the day) in the morning. R. Abba said: It is written (in Gen. xix. 23) : "The sun was risen upon the earth when Lot entered into Zoar;" and is written (in Lev. xxii. 7): "And when the sun is down he shall be clean." The sunrise is compared with the sunset: As sunset corresponds to the disappearance of the sun from the sight of man, so also sunrise is manifested by the appearance of the sun to the eye of man.