Bein ha-shemashot is the transition period between day and night, called in the Bible bein ha-arbayim (Exodus 12:6), and in rabbinic literature bein ha-shemashot ( Berakhot 2b; Avot 5:9). Whether twilight forms part of day or the night is a moot question in the Talmud (Shab. 34b). Its exact duration was also a matter of dispute. According to R. Yose, the transition from day to night is instantaneous, whereas R. Nehemiah said twilight lasted for nine minutes after sunset (i.e., the length of a walk of half a mile= 1000 ells, approx. 560 meters). The amora Samuel said it lasts for 13 1/2 minutes and according to another opinion 12 minutes (Shab. 34b). The codifiers established the duration of twilight at 18 minutes, i.e., when the sun is about 3 1/2 degrees below the horizon (Tur, OH 293). Actual night begins only with the appearance of three stars in the sky (called: zet ha-kokhavim, Ber. 2b; see also Neh. 4:15). This traditional calculation of the duration of twilight deviates only slightly from the exact astronomical twilight. Twilight on Friday is reckoned as Sabbath eve and consequently no work may be performed then. The Sabbath candles must be lit before twilight (Shab. 2:7). The twilight at the end of the Sabbath is calculated as still belonging to he Sabbath day which concludes with the appearance of three stars in the sky. This rule applies also to the beginning and conclusion of the holidays. Before the beginning of the Day of Atonement, twilight is reckoned from approximately one hour before the stars would become visible. All religious ceremonies which ought to be performed only at night, e.g., the recital of the evening service, the kindling of Hanukkah lights, the reading of the Megillah, should be observed only after twilight; but if they are performed during twilight they are valid and do not have to be repeated.