Thomas Earnshaw, born 4 February 1749 in Ashton-under-Lyne, was an English watchmaker who further simplified the process of producing the marine chronometer, making them affordable to the general public.
In 1780, Thomas modified the detached detent escapement which later became the standard form in marine chronometers. The same was patented by Thomas Wright in 1783 for whom he worked under.
In June 1801, on the first voyage and mapping of Australia's coastline, Matthew Flinders' ship, HMS Investigator, carried two Earnshaw E520 and E543 boxed Timekeepers and two Arnold's timekeepers. To compensate for the ship's motion, the chronometer, E520, was mounted in a wooden box with gimbals. Flinders went to shore on a regular basis to check the chronometers' settings against the stars. Thomas' chronometer was the only one that worked at the end of the expedition, prompting Flinders to refer to it as "This excellent timekeeper" in his book A Voyage to Terra Australis.
In 1805, Thomas was granted awards by the Board of Longitude along with John Arnold for their respective improvements made to chronometers following which he received £2500 while Arnold’s son John Roger Arnold received £1672 on his fathers’ behalf.
From 1831 to 1836, chronometer no. 509 was aboard HMS Beagle commanded by Captain Robert FitzRoy, a future Vice-Admiral and creator of the Meteorological Office on a trip to round the globe and construct a chain of properly recorded longitude locations throughout the planet. Considerable accuracy was necessary for this mission hence the Beagle carried a number of chronometers.
Thomas’ bimetallic compensatory balance and spring detent escapement has been employed almost routinely in marine chronometers since then, and as a result he is widely considered as one of the pioneers of chronometer development.
He built a clock for the Armagh Observatory when asked by Nevil Maskelyne. This clock included his new escapement design as well as a number of unique features, including as an airtight case (designed to reduce dust and draughts).
The same was commended by John Thomas Romney Robinson in the nineteenth century, who thought it was the most accurate clock in the world at the time. Following this, Thomas’ second clock, which was operated at sidereal rate with Edward Troughton's Equatorial Telescope, was also purchased by the Observatory.
A George III mahogany and brass-mounted two-day marine chronometer
THOMAS EARNSHAW, NO.719/3190. CIRCA 1805 (Image courtesy of Christies)