(See DAY.) Ahaz' sundial implies the Jews' acquaintance with hours before the Babylonian captivity. During it, they would certainly meet with that division of time which prevailed for ages at Babylon. The Egyptians too in early times knew it, Lepsius says as far back as the 5th dynasty. Astronomers knew in ancient times the "hour," that is the 24th part of a civil day; its use in common life is said not to have begun until the fourth century A.D. The hour which is the 12th part of the natural day, between sunrise and sunset, is of the same length as the astronomical hour only at the equinoxes. In our Lord's days the Jews must have had dials, and clepsydrae or water hourglasses, as these were long known to the Persians with whom they had been so closely connected. Christ alludes to the day hours, John 11:9, "are there not twelve hours in the day?" The 3rd, 6th, and 9th hours are mentioned often as the regular hours of prayer (Acts 2:15; Acts 3:1; Acts 10:9).