Longitude: the exact location of an object on the earth in terms of north, south, east and west. This is especially important when sailing, so ships are aware of their location and avoid being wrecked. In the early 18th century this was a bit of a problem as no precise way to determine Longitude had been found. In the past, crews had relied upon star positions for guidance but this was by no means accurate.
In October 1707 the fleet of Admiral Sir Clowdisley Shovell was wrecked off the Scilly isles due to longitudinal error. Over two thousand men lost their lives. To stop this from happening again, in 1714 Parliament passed the Longitude act and set a prize of £20,000 for the person who could invent a way of finding longitude to an accuracy of 30 miles after a six week voyage to the West Indies.
Harrison set out to solve the problem.
Harrison’s first sea clock- H1
In 1730 Harrison designed his first sea clock. He presented his ideas to Astronomer Royal Edmund Halley and was referred to major clockmaker George Graham who loaned him the money to make a model version. It took Harrison five years to make the clock which was an adaptation of his earlier precision clocks except with two balances linked together in place of a pendulum to compensate for being at sea.
The clock was trialled on a route to Lisbon and due to its successful performance, on his return John Harrison was granted £500 for further development.
Harrison’s H1 clock- Image source
Mr George's Museum Of Time, Haltwhistle 