FINDING THE LONGITUDE
of di ffere nce represented one twentyfo urth of the earth 's circumfe rence. John Harrison broke through the technology barrier with the development of a timekeeper (Harri son No. 1) for carrying standard time to sea. A pocket-watch version (Harri son No. 4), produced in 1759, showed that his technology could come in smaller and more workable packages. In the fo llow ing yea rs marine chronometers were produced on a commerc ial bas is, enabling all navi gators to find long itude witho ut resorting to the com pli cated lunar di stance method . The Observatory's later achievements included introduction of Greenwich Mean Time and international acceptance of the Greenwich Meridi an as 0° longitude. As the fa me of the Observ atory grew in the nineteenth century, people flocked to visit it. Today the Observatory is known the wo rld over and welcomes all visitors. The as tronomers have moved to a new site leav ing the earl y buildings and instruments for the public. On 24 March this year, the Observatory emerged from a 15-month transformation. The exteriorremains as it was, but inside things have been radically altered. W ith the help of sixteen new galleries, the Observatory explores the history of man's attempts to map space and measure time. The Observatory features one of the world's fi nest collections of telescopes, precision clocks and scientific instruments. Now, interacti ve science stations explain the principles of longitude and latitude, time di fference between countries and nav igation by the stars. In the new Time Gall ery are sundia ls, clocks, watches and other timepieces from all over-including Harri son' s firstfourmarinechronometers! The Observatory highlights two intriguing stories in the canon of navigation. The th ree hundredth anni versary this year of John Harri son ' s birth marks his lonely but ultimate ly successful struggle to build an acc urate timekeeper-a case of new ideas from unlike ly sources challenging those old and established. The newly illuminated P rime Meridian line , which streaks out of the Observatory courtyard and is lost to the gaze in the labyrinth of London, is a reminder of a different struggle-the long, contentious debate between nations which ended onl y I 09 years ago, giving the seafaring world a universal meridian from which to measure longitude.
This short histOJy has been producedfrom materials provided by the Na tiona l Maritime Museum, Greenwich, London .
18
THE SEARCH FOR LONGITUDE & THE PRIME MERIDIAN by Donald S. Johnson
The hi story of sc ientific nav igation goes science las ting for two millenni a. At the back virtuall y to the beginnings of West- time of C hri stopher Columbus, Ptolemy e rn c ivilization . That the earth was a was still the standard authority. sphere and not a flat plane was recogProgress in nav igation required new nized as early as 525BC by the Greek knowledge in many fi elds. Advances in philosopher Pythagoras . It was also ap- astronomy , along with instruments caparent that in order to properl y represent pable of producing more accurate data, the position of any point of land on a map were needed. Celesti al observations by or globe, a grid system of ve rti cal and Tycho Brahe and other scientists in the horizontal lines was necessary. From the court of John II of Po1tugal, were gathered earliest printing of maps, notation of and tabulated. Quadrant, astrolabe and these lines, if not pl aced directl y on the cross-staff readings, when used with these tabulated observations and apsurface, were at least preSamuel de plied to navigation, aided the sented in the margin . While the solution for de- Champlain was mariner in determining his position of latitude at sea. termining latitude had been Ce1tain reference points for resolved quite earl y, there re- of the conviction latitude were obvious and canmained the problem of acc uthat "God did ratel y and reliabl y determinnot intend that sistentl y used by mariners of all nations. The equator, polar ing longitude. Even after the accompli shment of ca lcul at- man should ever points, and the swing of the ing longitude, there was no be able to deter- sun fro m its most northerl y decl ination of23°30 ' through con s iste nt s ta rtin g po int to itsopposite,and equal ,dec(prime meridi an) for the ver- mine longitude at sea. " Ii nation in the southern hemitica l lines circumscribing the sphere, were un varied. earth and pass ing through the Manuscript tables of declination were poles. Today, the meridian of 0° long itude is universall y accepted by con ve n- known and in use as earl y as the 13th tion to be through Greenwich , Eng land . centu ry, enabling mariners to find their However, we ll into the 17th century an latitude position. But the ability to estabitem as elemental and bas ic to nav iga- lish long itude at sea eluded navigators for tion and cartograph y as the pl acement of a no th e r five centuri es . S a mu e l de thi s one critical line , vari ed from nation Cham pl ain was of the conviction that to nation a nd fr o m m a pm a ke r to "God d id not intend that man should ever mapmaker. What is even more s urp ris- be able to determ ine longitude at sea." ing, is that acceptance by the nati ons of And Sebastian Cabot, on his deathbed, the world of Greenw ich as the datum confi ded that by divine revelation he could point for measurement of longitude oc- calcul ate longitude, but that he was not allowed to share thi s knowledge. Although curred only I 09 years ago. Of pivotal importance in the hi sto ry the theory fo r working out longitude, based of navi gation and cartography were the on time, was dev ised as earl y as 1530 by maps and manu sc ripts o f C la udiu s the mathemati cian Gemma Fri sius, it Ptolemy, a great geographer of cl ass ical wasn' t until an accurate timepiece, the antiquity (2nd century AD) who wo rked chronome ter, was constructed that the in the Alexandri a Library. The Geo- theory could be put into practice. Di viding the g lobe into circumferengraphia Atlases come to us as 15th century tran sc ripti o ns fro m hi s wo rk s. ti al, ve rti cal lines, equally spaced was a Ptolemy created a body of know ledge, logica l co nstruct. Hipparchus of Rhodes incorporated into hi s maps, of unri va led (c. 167- 127BC), one of the greatest of ex tent, hi s influence on cartography and G reek as tronomers and founder of trigoSEA HISTORY 66, SUMMER 1993