The Bible does not clearly define day and night or their divisions, such as "evening, morning, and noonday" (Ps. 55:18), the watches of the night (Ex. 14:24; Judg. 7:19), midnight or half the night (Ex. 11:4; 12:29), and the notion of "hour" is not mentioned at all. The duration of a "halakhic" day is from dawn until the appearance of the stars, i.e., a full solar day; it is divided into secondary periods, according to three systems:
(a) Every day (and every night) is divided into 12 "variable" ("זמניות") hours, no matter what season it occurs in; the duration of the hour is therefore dependent on the yearly season (Sanh. 38b). Ultra-Orthodox Jews in Israel follow this system to the present day. (b) The entire day (day and night) is taken as one unit and is divided into 24 standard and fixed hours of 60 minutes each, as in the commonly accepted time system. The division of day and night into fixed hours, with a specific duration, is mentioned for the first time in the literature of the tannaim (see, e.g., Mekh. SbY to 12:29: "He is seated on the sundial [a time device probably introduced into Ereẓ Israel during the Hellenistic period] and shows the hours with an accuracy that is within a hair's breadth"). The notion of an "hour" as an undefined and not standardized lapse of time has, however, been maintained in the Mishnah ("The early pietists waited an hour.…"; Ber. 5:1). Though it was only theoretical, there was also more detailed division; an onah ("term") is 1/24 of an hour, an et ("period") is 1/24 of an onah, and a rega ("moment") is 1/24 of an et (Tosef., Ber. 1:3). In this classification the rega is approximately ¼ of a second. The rabbis, therefore, said that "a human being … does not know his ittim [plural of et], rega'im [plural of rega], and hours … but God … entered into it by a hair's breadth" (Gen. R. 10:9). A different, more precise calculation existed in Erez Israel: "How much is a rega – 1/58,888 of an hour" (Ber. 7a). A wide literature, notably the Baraita de-Shemuel, deals with such time calculations within the framework of astrological research. Another division of the hour is into 1,080 parts; this is also very ancient and is based on the lunar month. (c) The solar day (alone) is divided according to the changes in the brightness of the sunlight. In this system, the day is divided as follows: dawn, the appearance of the first morning twilight, is the starting point when all precepts to be fulfilled during the day become obligatory. Halakhah, however, prefers sunrise to dawn because the commencement of the day presents problems of definition; haneḥ ha-ḥammah ("first appearance of the sun") occurs after dawn and precedes zeriḥah by the period of time it takes to walk a mil ("mile"). At that time, the pious read the Shema. Zeriḥah – full sunrise – is the moment when the entire sun appears over the horizon. Sunset is the moment when the entire sun disappears below the horizon. Evening twilight is the light after sunset and it is doubtful whether this period may be called day or night, and diverse opinions have been given by the tannaim as to its exact nature and time (Shab. 34b). According to Maimonides (Yad, Shabbat 5:4), the evening twilight begins with sunset and lasts until the appearance of three medium-sized stars, and from then on it is night. R. Tam argues that evening twilight begins from the period it takes to walk three and a quarter mil after sunset to the appearance of the stars. Until then, it is still day. In the Shulḥan Arukh (OḤ 261), this second opinion is accepted as binding. According to a third opinion, held by some of the early commentators, night begins immediately with sunset and the evening twilight is a period prior to sunset, lasting the time it takes to walk three and a quarter mil.