Thomas Earnshaw (4 February 1749 – 1 March 1829) was an English watchmaker who, following John Arnold's earlier work, further simplified the process of marine chronometer production, making them available to the general public. He is also known for his improvements to the transit clock at the Royal Greenwich Observatory in London and his invention of a chronometer escapement and a form of bimetallic compensation balance.
thumb|left|Diagram of Earnshaw's standard chronometer detent escapement.
In 1780, he devised a modification to the detached chronometer escapement, the detent being mounted on a spring instead of on pivots. This spring detent escapement was patented by Thomas Wright (for whom he worked) in 1783. Whilst initially the design was crude and unsuccessful, with modifications it later became the standard form in marine chronometers, following the invention of the detent escapement by Pierre Le Roy in 1748. John Arnold also invented a similar escapement in 1782.
In 1805, Earnshaw and Arnold were granted awards by the Board of Longitude for their improvements to chronometers; Earnshaw received £2500 and John Arnold's son John Roger Arnold received £1672. The bimetallic compensation balance and the spring detent escapement in the forms designed by Earnshaw have been used essentially universally in marine chronometers since then, and for this reason Earnshaw is generally regarded as one of the pioneers of chronometer development.
Although he was principally a watchmaker, he did not shy away from building clocks. When asked by Nevil Maskelyne, he produced a clock for the Armagh Observatory. This clock incorporated Earnshaw's new design of escapement and had a number of novel features, including an airtight case (designed to reduce dust and draughts). It was highly praised by John Thomas Romney Robinson in the 19th century, who at that time believed it to be the most accurate clock in the world. In 1794, its purchase price was £100 and Earnshaw charged £100 to travel with it to Armagh and set it up in the new Observatory.
The Observatory also purchased Earnshaw's second clock which was operated at sidereal rate with Edward Troughton's Equatorial Telescope.
Chronometers on notable voyages
thumb|Earnshaw chronometer No. 506
In July, 1791, Captain William Bligh purchased on behalf of the Admiralty Earnshaw's chronometer no. 1503 at a price of for an expedition he was to command in to transport breadfruit plants from Tahiti to the West Indies. This was the second expedition that Bligh had undertaken with this mission. The first expedition was with HMS Bounty which had ended in the infamous mutiny led by Fletcher Christian and from which Bligh returned to England only with the greatest of difficulties. The second expedition, however, was entirely successful.
alt=Site of the business premises of Thomas Earnshaw 1749-1829 Noted watch and chronometer maker |thumb|Blue plaque on "Site of the business premises of Thomas Earnshaw 1749-1829 Noted watch and chronometer maker" opposite Holborn tube station entrance on Holborn, London
Between 1831 and 1836, chronometer no. 509 was carried on HMS Beagle on a voyage to circumnavigate the globe and establish, for the first time, a chain of points around the world of accurately-known longitude. The ship was commanded by Captain Robert FitzRoy, a future Vice-Admiral and founder of the Meteorological Office. This was only one of many chronometers carried by the Beagle since great accuracy was required for this task. This was also the voyage that carried Charles Darwin who was afterwards inspired to write his book about the theory of evolution, On the Origin of Species. The chronometer is object 91 in the BBC Radio 4 series A History of the World in 100 Objects.