The First Time Balls

We have uncovered evidence in the National Archives of the United States that the world's first time ball was dropped at Portsmouth, England late in 1829, more than three years before a time ball was erected at the Royal Observatory at Greenwich. The same evidence makes possible further investigations that shed new light on the events leading to the establishment of the Greenwich time-ball service and other early time balls. Now largely an historical curiosity, the maintenance of this service was considered an important function of many astronomical observatories during much of the nineteenth century. The service was superseded by radio time signals. Although dropping a time ball at a preconcerted instant provided very accurate time signals to horologists and the general public, its primary purpose was to act as a visual signal for rating chronometers, the all-important timepieces that began to come into widespread use at the beginning of the nineteenth century by which ships at sea could determine their longitude.' For determining longitude accurately, the problem of 'rating' chronometers-determining their exact error so that it could be applied to the longitude calculation-was cruciaJ.2 This rating process, ideally carried out at least every seven to ten days, required an accurate determination of time independently of the chronometer being rated. A navigator could in principle accomplish this through astronomical observations with a sextant and artificial horizon, but the results depended on the accuracy of the equipment and the experience of the observer." A better method was to observe the Sun or stars with a transit instrument, a delicate apparatus seldom carried on a ship, but usually found in astronomical observatories. Thus, the need for an improved rating service as chronometers came into more general use led to the assignment of this responsibility to the Royal Greenwich Observatory in England,' and the creation ofthe Depot of Charts and Instruments, the forerunner of the United States Naval Observatory." There were many drawbacks to this method if the chronometers were actually removed from the ships as they sometimes were: the risk of damage during transport, the time expended, and the fact that there was a slight difference between the rate determined on land and the rate determined at sea." While another timepiece could be used as an intermediary to transfer time from the observatory to the ship without removing the chronometer, the simple method of finding time through observation of an instantaneous signal such as a rocket or falling object -visible from the harbour and regulated by a transit instrument-was convenient and efficient.