Mechanical clocks of the 19th century and earlier were not very accurate timekeepers and tended to drift over the course of a single day. After a few days, these clocks could no longer reliably tell time. Then, the clocks had to be calibrated, usually by clockmakers.
One source of accurate time were observatories. An astronomer with a telescope could tell the precise time by looking at the sun and the stars in the sky. This time was then relayed to the public and to ships using audio-visual cues, such as firing a gun or raising a flag.
The time ball atop Flamsteed House at Greenwich, London. Photo credit: Carmen Seaby/Flickr
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The Family That Sold Time
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