(Beraisa): As long as the eastern horizon is reddish, this is day. When it darkens, this is Bein ha'Shemashos. When it blackens, and the top (middle of the sky) is equal [in blackness to] the bottom (the horizon), this is night.

Rebbi says, when the moon is in its period (the middle of the month, when it is full), when the ball of the sun begins to set, and the ball of the moon begins to rise, this is Bein ha'Shemashos.

(R. Chanina): [The text should say] when the ball of the sun finishes to set, and the ball of the moon begins to rise, [this is Bein ha'Shemashos].

Support (Shmuel - Beraisa): The moon does not shine [immediately] when the sun is setting (only towards the end of Shki'ah), and it does not set when the sun [begins] shining (rather, after this. We explained this like R. SHLOMO SIRILIYO, CHAREDIM).

(R. Shmuel bar Chiya bar Yehudah citing R. Chanina): When the ball of the sun begins to set, one standing on top of Mount Karmel, he descends to immerse in the great (Mediterranean) Sea and ascends, and eats Terumah. The Chazakah is that he immersed during the day.

Limitation: This is only if he took the short path, but if he went on the [longer] trodden path, no (perhaps he immersed at night).