<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> Horology ("the study of time", related to Latin horologium from Greek ὡρολόγιον, "instrument for telling the hour", from ὥρα hṓra "hour; time" and - o - interfix and suffix - logy) is the study of the measurement of time . Clocks, watches, clockwork, sundials, hourglasses, clepsydras, timers, time recorders, marine chronometers and atomic clocks are all examples of instruments used to measure time . In current usage, horology refers mainly to the study of mechanical time - keeping devices, while chronometry more broadly includes electronic devices that have largely supplanted mechanical clocks for the best accuracy and precision in time - keeping . </P> <P> People interested in horology are called horologists . That term is used both by people who deal professionally with timekeeping apparatus (watchmakers, clockmakers), as well as aficionados and scholars of horology . Horology and horologists have numerous organizations, both professional associations and more scholarly societies . The largest horological membership organisation globally is the NAWCC, the National Association of Watch and Clock Collectors, which is USA based, but also has local chapters elsewhere . </P> <P> There are many horology museums and several specialized libraries devoted to the subject . One example is the Royal Greenwich Observatory, which is also the source of the Prime Meridian (longitude 0 ° 0' 0 "), and the home of the first marine timekeepers accurate enough to determine longitude (made by John Harrison). Other horological museums in the London area include the Clockmakers' Museum, which re-opened at the Science Museum in October 2015, the horological collections at the British Museum, the Science Museum (London), and the Wallace Collection . </P>

What is a person who works on clocks called