Time signal
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A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.
These automatic signal clocks were synchronized by telegraphy in 1905 before the widespread use of radio
A RBL 40-pounder Armstrong gun being fired as "the one o'clock gun" on Woolwich Common, circa 1905
The time ball on the roof of Greenwich Observatory, London
A modern LF Radio clock
Marine chronometer
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A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing
A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844–1860.
The marine "Chronometer" of Jeremy Thacker used gimbals and a vacuum in a bell jar.
Henry Sully (1680–1729) presented a first marine chronometer in 1716
John Harrison's H1 marine chronometer of 1735