Why did the Sages say that one may not insulate hot water for Shabbat in something that adds heat, even while it is still day? It is a decree lest one come to cover it in hot ashes that contain a glowing ember. People may not differentiate between addition of heat by means of hot ashes and other additions of heat. Abaye said to him: Let him insulate it with hot ashes, what is the problem? Rava answered him: It is a decree lest one come to stoke the coals in order to make them burn on Shabbat and thereby violate a Torah prohibition.

The Sages taught a baraita which discusses the range of problems that arise with regard to the twilight period. Twilight is a period of uncertainty. It is uncertain whether it consists of both day and night, it is uncertain whether it is completely day, and it is uncertain whether it is completely night. Therefore, the Sages impose the stringencies of both days upon it. If there is a stringency that applies on either of the days, one is obligated to adhere to it during the twilight period.

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.

It was taught in the baraita that the Master said: The Sages impose the stringencies of both days upon twilight. The Gemara asks: With regard to what halakha was this stated? Rav Huna, son of Rav Yehoshua, said: With regard to the matter of ritual impurity, as we learned in a mishna: With regard to a zav who saw an emission for two consecutive days during twilight, it is unclear whether it should be considered as if he only saw the emission for a single day, as perhaps twilight of the first day was part of the following day, and twilight of the second day was part of the previous day; or, whether it should be considered as two days, attributing each twilight to either the previous or the following day; or, whether it should be considered three days, as it is possible to view the twilight period as two days. By Torah law, a zav who saw two emissions is ritually impure, and all of the stringencies of a zav apply to him. If he sees a third emission, he is liable to bring an offering as part of his purification ritual. Therefore, this zav, with regard to whom there is uncertainty whether he saw emissions for one day, two days, or three days, has uncertain status with regard to both ritual impurity and to sacrifice. If he saw an emission one day during twilight, he has uncertain status with regard to ritual impurity because it may be considered two days.

The Gemara comments on the baraita cited by the Gemara. This baraita is itself difficult, self-contradictory. Initially you said, 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. By inference, if the bottom segment lost its color, and the upper one has not lost its color, it is night. And then the baraita taught: 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. There is an apparent internal contradiction in the baraita. Rabba said that Rav Yehuda said that Shmuel said: In order to resolve the contradiction, unify the two statements and teach it as follows: 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 also the twilight period. Only if the upper segment lost its color, and it equals that of the lower one, is it night. And Rav Yosef said that Rav Yehuda said that Shmuel said otherwise: From when the sun sets, as long as the eastern face of the sky is reddened by the light of the sun, it is day. 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 lost its color and it equals that of the lower one, it is night.

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,