NotesClocks are essential tools for astronomical measurement and their precision defines the accuracy with which the time of a transit can be recorded. In their construction the astronomical clock differs from others mainly in the care with which the parts are produced. For this reason many people regard these clocks as occupying a pre-eminent position in the art of clock making.
There are two main kinds of astronomical clock a common feature of which is their separate hour, minute and second hands. One measures the true mean solar time and the second measures sidereal time which has no direct connection with solar time and instead keeps pace with the apparent movement of the stars and indicates the position of the stars in the sky.
The tilt of the earth on its axis means the earth's orbit is not exactly circular and as a result solar days vary in length throughout the year. To avoid this complication astronomers calculate the time of days by the passage of stars. To do this they measure the time between two successive transits of a star across the meridian. This is known as sidereal time.
Astronomical clocks were made to beat exact seconds of time which depended on the length of the pendulum. The exact length of a pendulum which calculates true seconds was formulated by Huygens at an amount approximating to 39 inches. However this length is slightly different at different parts of the globe and the pendulums themselves can expand or contract depending on the temperature. To overcome this compensated pendulums such as the one invented by the clockmaker Graham contained mercury which expanded and contracted to keep the clock beating exact time.
In 1675 the first Astronomer Royal at the Greenwich Observatory, John Flamsteed, commissioned Thomas Tompion to build two clocks. These were wound once a year and had 13 foot pendulums and are thought to be the first year movements ever made. The pursuit of greater accuracy led to many improvements in astronomical clocks and in some cases, such as Clement's anchor escapement, their innovations were incorporated into other timekeeping devices.
In the early nineteenth century there was a great increase demand for astronomical clocks among private and government institutions in Britain. The maker of this clock Abraham Louis Bréguet was born in 1747 and died in 1823. He founded his watch and clock making company in Paris in 1775 and sold many of his works to European Royalty and the leading scientists of the age. He is acknowledged as one of the primary inventors of many of the technical innovations which lead to the development of modern clock and watches and for over two centuries the business has continued to maintain the reputation he established.
William Hardy, along with Robert Pennington and John Roger Arnold, was one of best known and respected makers of astronomical clocks in this period. His major innovations included the detached clock escapement and a clock balance.
Howse, D., Greenwich Time; the discovery of the longitude, Oxford University Press, 1980
Lloyd, H.A., 'Timekeeping Mechanisms and Clocks for Scientific Purposes', in the catalogue of the British Clockmakers Heritage Exhibition, Science Museum, London, 1952
Peck, W., A Popular Handbook and Atlas of Astronomy, Gall and Inglis, London, 1890
Wood, C., 'Robert Molyneux's Astronomical Clocks and Chronometers', in Antiquarian Horology, Number 4, Volume 9, September 1975
Wood, C., 'What's Wrong with Hardy's Escapement?', in Antiquarian Horology, Number 8, Volume 9, September 1976