The measurement and keeping of time was one of the most important functions of early astronomical observatories. Thus it was necessary to ensure that they possessed the most accurate clocks available. Precise timing also allowed observatories to measure the Right Ascension of a star, one of the two celestial coordinates, needed for the compilation of accurate star catalogues.
Armagh Observatory today retains six heritage astronomical clocks (“regulators”) that were used in the 18th and 19th centuries.
Earnshaw Number 1
Built by Thomas Earnshaw of London between 1790 and 1795. Thomas Earnshaw shared in the award of the Longitude Prize by the British Admiralty for the construction of a marine chronometer, alongside clocks built by Harrison and Arnold. Earnshaw’s design ultimately developed into the standard ship’s chronometer. The Earnshaw Number 1 was the most accurate regulator in the world at the time (1/2s week according to Director Romney Robinson). An 8-day regulator, rated to sidereal time, it was extensively tested by the Astronomer Royal Neville Maskelyne in Greenwich Observatory before it was delivered to Armagh. The clock case is exceptionally well sealed, featured several novel methods of sealing screws, joints and the winding mechanism. It featured a 9-bar steel and brass gridiron pendulum, in order to maintain constant length of the pendulum in different temperatures. In the 1820’s this was modified by Christopher Sharp of Dublin to a mercury pendulum.
Earnshaw Number 2
Thomas Earnshaw of London made a second instrument for the Armagh Observatory, the Number 2 regulator. An 8-day regulator with 5-bar steel & zinc compensation pendulum, rated to sidereal time. This regulator featured a much simpler design than the Number 1, and in turn cost significantly less. As of 2024, it is the only regulator in the collectionwhich is kept running, though it has been modified to run in solar time.
Buchanan
Though the clock is marked Buchanan of Dublin, it is heavily influenced by those made by Dutton in England. This is understandable as Archibald Buchanan was the foreman at the Dutton works and married into the family. Though accurate, it spent many years in St Patrick’s Cathedral Armagh (Church of Ireland) where it provided the time for the ecclesiastical day in the cathedral. It was returned to the Armagh Observatory in the 1980’s and restored by Bertie McClure.
Crossthwaite
John Crossthwaite of Dublin mean-time clock, this clock was previously part of the Dunsink Observatory equipment and was bought in the 1790’s to supplement the two Earnshaw instruments.
Recordon
Built by Louis Recordon c1795 in London, and used by King George III in his Kew Observatory in Richmond, London. With an ornate mahogany case intended to grace the Royal Apartments. Believed to be the first pendulum clock in the world compensated for both temperature and atmospheric pressure.
Shelton Mean Time Clock
Built by John Shelton of London for King George III and used for timing the transit of Venus in the King’s Observatory in Richmond on 3 June 1769. Captain Cook used a similar regulator to time the transit from Tahiti in the South Pacific. This was part of a global scientific experiment to measure the distance of the Earth to the Sun (the Astronomical Unit) and so establish the scale of the Solar System. Part of the King George III collection presented to Armagh Observatory by Queen Victoria in 1841.
