There probably existed in the Temple at Jerusalem a system of sound signals marking the rhythm of the main religious activities of the day. Several biblical, historical and rabbinic clues point in this direction.
The Law already prescribed the use of silver trumpets to summon the assembly and to mark certain acts of worship (Numbers 10.1-10). The priests used them at the sacrifices and the great feasts (2 Chronicles 29.26-28).
The Jewish historian Flavius Josephus reports that a priest stood at the top of the Temple to announce, by blasts of the trumpet, the beginning and the end of the . Archaeological excavations have moreover uncovered a stone bearing the inscription “place of the trumpeting,” which seems to correspond to the spot where this priest stood.
Rabbinic literature, notably the Mishnah (Sukkah 5.5), also mentions several series of blasts accompanying the opening of the Temple gates, the libations and certain moments of the daily worship. A devout Jew who regularly attended the Temple could thus know the main moments of the day thanks to these signals.
The hours of prayer were closely associated with the daily sacrifices: the morning sacrifice took place around the third hour, that is, approximately nine o’clock (Acts 2.15), while the evening sacrifice took place around the ninth hour, that is, around three in the afternoon (Acts 3.1). The faithful naturally gathered around these important moments of worship (Luke 1.10).
It is therefore very likely that Peter and John, going up to the Temple “at the hour of prayer” (Acts 3.1), were guided not only by their knowledge of the liturgical rhythm, but also by the sound signals given at the Temple. When Luke specifies that they were going to the Temple “at the ninth hour,” it is probably not a mere estimate, but an hour known to all and regularly marked by the course of worship and by the sounding of the priestly trumpets.
These liturgical and time markers also explain why the evangelists were able to specify the various hours at which the main events of the day of the crucifixion took place, on April 1, AD 33.