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See under sidereal time, solar day.
The Jews reckoned the day from sunset to sunset (Lev. 23:32). It was originally divided into three parts (Ps. 55:17). "The heat of the day" (1 Sam. 11:11; Neh. 7:3) was at our nine o'clock, and "the cool of the day" just before sunset (Gen. 3:8). Before the Captivity the Jews divided the night into three watches, (1) from sunset to midnight (Lam. 2:19); (2) from midnight till the cock-crowing (Judg. 7:19); and (3) from the cock-crowing till sunrise (Ex. 14:24). In the New Testament the division of the Greeks and Romans into four watches was adopted (Mark 13:35). (See WATCHES.) The division of the day by hours is first mentioned in Dan. 3:6, 15; 4:19; 5:5. This mode of reckoning was borrowed from the Chaldeans. The reckoning of twelve hours was from sunrise to sunset, and accordingly the hours were of variable length (John 11:9). The word "day" sometimes signifies an indefinite time (Gen. 2:4; Isa. 22:5; Heb. 3:8, etc.). In Job 3:1 it denotes a birthday, and in Isa. 2:12, Acts 17:31, and 2 Tim. 1:18, the great day of final judgment.