It is or was, the Noon Gun in seaports. Noon fixed with a sextant, sun directly overhead . Ships would check their chronometer’s rate against the signal. Reference books like Bowditch have graphs to correct time the sound arrives against distance from the gun.
Some ports still do, Sydney Australia from the observatory beside the harbor bridge and of course Greenwich England. Norfolk Naval Shipyard used to may still. Many years ago working in downtown Norfolk across the river from the shipyard used to look at my watch as noon approached checking it against the gun.
William
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