Longitude Calculated
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Longitude on Land |
The calculations to determine longitude on land were based on the work of Giovanni Domenico Cassini. As professor of astronomy at Bologna, he published in 1668, ephemerides that documented the times of immersion (eclipse) and emersion of each of Jupiter's principal satellites. Drawings which accompanied these calculations illustrated the planet and the configuration of the satellites around it (Williams, 1992:79). It had been discovered that once an accurate table, timing each eclipse was calculated for a standard meridian then the difference between this and the local time of the eclipse would be the longitude (Taylor, 1956:248). This finding would prove to be a major break-through, and progress was then so rapid that by 1700 the longitude of many land locations in the world had been calculated relative to the position of the Paris Observatory (Williams, 1992:79). (The time meridian thus had been placed through Paris. It was later to be moved to Greenwich). This advance allowed land maps at least to be corrected for longitude, but ships were another matter.
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Longitude at Sea |
Being able to calculate one’s longitudinal position by using Jupiter’s satellites as time-keepers provided no assistance to ocean going sailors. The task of taking accurate measurements of Jupiter’s satellites proved to be a near impossibility at sea because instruments were unable to be kept steady - a key requirement. Creative solutions were suggested such as seating the observer in a heavy swinging chair that would remain steady as the ship moved beneath it. Unfortunately this still did not work as the chair simply moved irregularly and the problem of in-stability persisted. Moreover, Jupiter is not always visible at night, and is invisible during the day, so it is at best an infrequent time-keeper and hence unreliable (Berthon and Robinson, 1991:118 and Taylor, 1956:249).
The English, like all the ocean going peoples were also keen to resolve the problem of calculating longitude at sea. In 1674, Charles II of England appointed a Royal Commission to investigate the vexed question of longitude. John Flamsteed, an English astronomer, assisting the Commission reported that the best charts and tables available at the time could cause errors of longitude greater than five hundred miles in equatorial latitudes. His findings directly led to the foundation of the Greenwich Observatory and his own appointment as the first Astronomer Royal (Williams, 1992:80).
The Royal Warrant of 4 March 1675 directed Flamsteed "forthwith to apply himself with the most exact care and diligence to rectifying the tables of motions of the heavens , and the places of the fixed stars, so as to find out the so-much desired longitude of places for the perfecting the art of navigations." (Williams, 1992:80).
The Observatory was itself established by a Royal warrant dated 22 June 1675 that stated "...in order to the finding out of the longitude of places and for perfecting navigation and astronomy, we have resolved to build a small observatory within our park at Greenwich..." (Williams, 1992:80).
