Longitude and latitude are used to describe the position of any location on the surface of Earth. The determination of latitude presented no problem; the latitude north of the equator is, by definition, equal to the altitude of the celestial North Pole. Longitude, the angle east or west of a reference location, was however much more difficult to determine.
Eratosthenes was the first to attempt to produce a map of the world based on a system of lines of latitude and longitude. Hipparchus was the first to specify the positions of places on Earth using latitude and longitude as coordinates. Ptolemy wrote his Guide to Geography that listed places in Europe, Africa and Asia giving their latitude and longitude. Abu Rayhan al-Biruni (973-1048) believed that Earth rotated on its axis and made accurate calculations of latitude and longitude.
The Age of Exploration (early 15th century- early 17th century) quickly revealed the problems of navigation. Firstly there were no maps by which to navigate and a major task of the early explorers was to map the lands which they discovered. However, if maps were to be produced it was necessary to be able to specify position on Earth.
Just as the hour hand of a clock shows the time as it moves along the numbers engraved on its clockface so, it was thought, it might be possible to get an indication of time as the Moon apparently moved at 15°/hour along its journey among the stars. Charles II (55; r.1660-1685) told the Royal Society (est.1660) to determine whether the Moon could serve as a universal clock. John Flamsteed soon reported that lunar and stellar positions were not known sufficiently well to make the method reliable. Nevertheless, in 1675 Charles II founded the Royal Observatory, and installed Flamsteed as the first Astronomer Royal (1675-1719).
Local time, which we use to regulate our lives locally, is measured by the rising and setting of the Sun at our locality. Longitude is related to the differences in local times: one hour corresponding to 360/24=15 degrees. Determining longitude therefore requires a universal clock – such as Jupiter’s moons were for Cassini and Richer – that will allow comparison of different local times to obtain the horizontal angular distance (=longitude) between them.
The most direct solution to the problem was to produce a reliable clock, set at the local time at a reference location (Greenwich, 1884) and to compare this with local times at other locations. Accurate pendulum clocks existed in the seventeenth century but the motions of a ship and changes in humidity and temperature would have prevented it from keeping accurate time at sea. In 1714 the Board of Longitude (1714-1828) made an offer of £20,000 for a solution which could provide longitude to within half-a-degree. In 1735 John Harrison (1693-1776) produced his first marine timepiece (H1) and this was taken in trials to Lisbon and back.
The Board was impressed with the clock’s performance and funded Harrison for its further development. In 1764 Harrison’s son William Harrison (1728-1815) with the fourth chronometer H4, set sail for Barbados aboard the Tartar. The watch’s error over a voyage of 47 days was found to be only 39.2 seconds. Chronometers soon became the preferred solution to the problem of longitude.
Observatories were built alongside major ports. Observations were made at noon (or one p.m.) and timeballs dropped as a signal by which ships’ chronometers could be checked. On land the roughness of the roads caused disturbances in the chronometers during their journeys from one location to the next, a problem later solved by observing transits and comparing chronometers by electric telegraph (invented 1844).
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