FINDING THE LONGITUDE
The Old Royal Observatory Founded 1675 by Charles II to find "the longitude of places for perfecting navigation and astronomy"
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All navigators, whether sail ing the Earth 's oceans, cross ing the desert sands or en ro ute to the moon, need to know where they are. We take this fo r granted today: at the flick ofa sw itc h electro nic eq uipment can te ll our coordinates and pinpo int our position. But thi s is onl y poss ibl e because of centuri es of diligent work by pas t and present-day sc ienti sts. Of central im portance we re the Royal Observato ry astronomers of the 17th- and I 8 th centuri es at G reenwich, who pl ayed the leadi ng ro le in he lping the nav igator fi nd hi s bearings. T he seventeenth-century seaman was able to dete rmine onl y one of the co-ordinates essential fo r fin d ing h is pos ition. By tak ing noon-sights, measuring the max imum angle of the sun above the horizon, he knew hi s latitude , how fa r north or south he was. But w ithout hi s long itude-hi s east-west pos iti on- he could not be certain of w here he was along that line of latitude. T hi s was a peril ous situati on fo r men <;i nd ships, and to a sea-going nati o n li ke seventee nth-century G reat B ri ta in it was also a costl y problem . A solution to the " longitude problem" had to be fo und. There were many theoretica l answe rs and one of these, the " lunar dista nce method" of findin g long itude, was reported to King Charles II by hi s princ ipa l mi stress, Loui se de Keroua lle , on beha lf of a Frenchman. The King was s ufficientl y impressed to have the cl aim examined . But fo r the lunar di stance method to wo rk at least three new problems had to be so lved. Both the movement of the moon and the pos itions of the stars had to be acc urate ly catalogued, and , thi rd ly, the nav igator needed an instrument to carry out his observations once at sea. Charles was advised that it was possible to tum thi s theoreti cal solution into a practica l one and so he founded the Royal Observatory on 22 June 1675 " in order to the fi nding out of the longitude of pl aces fo r perfecting nav igation and SEA HISTORY 66, SUMM ER 1993
astronomy." A site in Greenwich Park was chosen and Chri stopher Wren, astronomer and architect, started work on the building. John Flamsteed was appointed firs t Astronomer Royal and set upon the long tas k of cataloguing the stars. T he lunar di stance method was based on the fac t that the moon moves fa irl y quick ly against the bac kgro und of the stars, rather like a hand mov ing over the d ial of a cl ock. If a seaman could acc urately measure the moon 's pos iti on on thi s " di al," he could then determine a standard time at which this configuration wo uld occur. He wo uld need an accurate sighting instrument and an almamac cataloguing the times of the va ri ous positions of the moon and stars. Lastl y, he would
need the knowledge and sk ill to perform elaborate calcul ations beyond the abilities of most seamen. The aim of the earl y Greenwich astronomers was to produce the almanac with the moon and star pos itions. John Flamsteed spent all hi s wo rking life, 1675 to 17 19, cataloguing the stars. He was paid ÂŁ 100 a year and from this had to provide hi s own instruments. Speciali zed equipment fo r thi s unique work was not readil y available and so he spent much time on the design and construction of instruments. Once the ste ll ar and lunar observations for the lunar di stance method of so lving longitude had been completed they had to be made availa ble to seaman. It was the fi fth As tronomer Royal, Nev il Mas kelyne, who saw them through to publication in the Nautical Almanac fo r the year 1767 ; and such tabl es have been publi shed annuall y ever since. The Royal O bservatory had completed the wo rk it set out to do ninety years earlier. Once the nav igator had the necessary tables and an instrume11t to make hi s observati ons-an octant deve loped by John Hadley in the early 1730s-the lunar di stance solution to longitude moved from theory into practice. But there were great prac tica l difficulties in the method, and fe w navi gators mas tered the art. Simultaneously, an alternate method was be ing independentl y pursued. It req uired an excepti ona ll y accurate timepiece telling the nav igator w hat time of day it was at a known long itude, i.e. Greenwich. By comparing his noon-sight, which gave him local time, with the exact time at the known longitude, he could detennine the longitude where he was, sinee each hour Above, the fa cade of the fi rst Royal Observat0ty building, F/amsteed House, built 1675 by Sir Christopher Wren. Below, an aerial view of the Royal Observatory showing the South Building and the Altazimuth Pavilion, built in the 1890s.
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