Kew Observatory, also known as the King's Observatory, was an astronomical and terrestrial magnetic observatory founded by King George III (1760-1820), located within the Old Deer Park of the former Richmond Palace in Richmond, Surrey, now within Greater London.
In the past, the Kew Observatory participated in assessing and rating Swiss timepiece movements for accuracy. As marine navigation adopted the usage of mechanical timepieces for navigational aid, the accuracy of such timepieces became more critical. From this need developed an accuracy testing regime involving various astronomical observatories. In Europe, the Neuchatel Observatory, Geneva Observatory, Besancon Observatory and Kew Observatory were examples of prominent observatories that tested timepiece movements for accuracy. The testing process lasted for many days, typically 45 days. Each movement was tested in 5 positions and 2 temperatures, in 10 series of 4 or 5 days each. The tolerances for error were much finer than any other standard, including the modern COSC standard. Movements that passed the stringent tests were issued a certification from the observatory called a Bulletin de Marche, signed by the Directeur of the Observatory. The Bulletin de Marche stated the testing criteria, and the actual performance of the movement. A movement with a Bulletin de Marche from an observatory became known as an Observatory Chronometer, and such were issued a chronometer reference number by the Observatory.
The role of the observatories in assessing the accuracy of mechanical timepieces was instrumental in driving the mechanical watchmaking industry toward higher and higher levels of accuracy. As a result, today high quality mechanical watch movements have an extremely high degree of accuracy. However, no mechanical movement could ultimately compare to the accuracy of the quartz movements being developed. Accordingly, such chronometer certification ceased in the late 1960s and early 1970s with the advent of the quartz watch movement.