A time signal is a visible, audible, mechanical, or electronic
signal used as a reference to determine the time of day. Church
bells or voices announcing hours of prayer gave way to
automatically operated chimes on public clocks; however, audible
signals (even signal guns) have limited range. Busy ports used a
visual signal, the dropping of a ball, to allow mariners to
check the chronometers used for navigation. The advent of
electrical telegraphs allowed widespread and precise
distribution of time signals from central observatories.
Railways were among the first customers for time signals, which
allowed synchronization of their operations over wide geographic
areas. Special purpose radio time signal stations transmit a
signal that allows automatic synchronization of clocks, and
commercial broadcasters still include time signals in their
programming. Today, GPS navigation radio signals are used to
precisely distribute time signals over much of the world.
Audible and visible time signals  One sort of public ti