תלמוד על שבת 68:5

It was stated: All the time that the Eastern sky is red it is certainly daylight. If it became silver colored then it is dusk. If it became black, equally from zenith to horizon, then it is night³³The Babli (Šabbat 34b) has a tannaitic statement: “What is dusk? From sundown all the time that the Eastern sky is red. If the lower part turns silver colored (pale grey) but the zenith is not yet silver colored, that is dusk. If the zenith is silver colored equal to the horizon then it is night; these are the words of R. Yehudah (bar Ilaï).” This statement looks somewhat garbled and it is explained in the name of Samuel as: “From sundown all the time that the Eastern sky is red it is daytime. If the lower part turns silver colored (pale grey) but the zenith is not yet silver colored, that is dusk. If the zenith is silver colored equal to the horizon, then it is night.” The text of the Yerushalmi is from a tannaitic source and supports Samuel’s reading. However, the Yerushalmi version is clearer since when the color of the sky at the horizon is equal to that at the zenith and is darker than pale grey, it is close to being dark..