Prior to the invention of clocks, people around the world kept time using some type of instrument to observe the Sun’s zenith at noon. The earliest time measuring devices used either the Sun’s shadow or the rate in which water runs out of a vessel. The pendulum clock was developed during the 17th century – these clocks were sufficiently accurate to be used at sea to determine longitude and for scientific time measurement in the 18th century.
In 1764, English horologist John Harrison, discovered that a clock could be used to locate a ship’s position at sea with extraordinary accuracy. A new Longitude Act, known as the Act 5 George III, followed in 1765. Chronometers, which measure time accurately in spite of motion or varying conditions, became popular instruments among many merchant mariners during the 19th century.
Still, even after developments regarding longitude, many towns and cities set clocks based on sunsets and sunrises. Dawn and dusk occur at different times but time differences between distant locations were barely noticeable prior to the 19th century because of long travel times and the (lack of) long-distance communications. The use of local solar time became increasingly awkward as railways and telecommunications improved. Time zones were therefore a compromise, relaxing the complex geographic dependence while still allowing local time to be approximate with mean solar time.
