Article ID: CBB001202239

“A thorn in the side of European geodesy”: Measuring Paris-Greenwich Longitude by Electric Telegraph (2014)


The difference in longitude between the observatories of Paris and Greenwich was long of fundamental importance to geodesy, navigation and timekeeping. Measured many times and by many different means since the seventeenth century, the preferred method of the later nineteenth and early twentieth centuries made use of the electric telegraph. I describe here for the first time the four Paris-Greenwich telegraphic longitude determinations made between 1854 and 1902. Despite contemporary faith in the new technique, the first was soon found to be inaccurate; the second was a failure, ending in Anglo-French dispute over whose result was to be trusted; the third failed in exactly the same way; and when eventually the fourth was presented as a success, the evidence for that success was far from clear-cut. I use this as a case study in precision measurement, showing how mutual grounding between different measurement techniques, in the search for agreement between them, was an important force for change and improvement. I also show that better precision had more to do with the gradually improving methods of astronomical time determination than with the singular innovation of the telegraph, thus emphasizing the importance of what have been described as 'observatory techniques' to nineteenth-century practices of precision measurement.

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Authors & Contributors
Martin, Jean-Pierre
Débarbat, Suzanne
Boistel, Guy
Kershaw, Michael
Carter, Bill
Carter, Merri Sue
Journal for the History of Astronomy
Histoire & Mesure
Studies in History and Philosophy of Science
Rittenhouse: Journal of the American Scientific Instrument Enterprise
Journal for the History of Arabic Science
Naval Institute Press
Presses Universitaires de Rennes
Johns Hopkins University Press
Editions Isoète
Longitude and latitude
Exactness; precision; accuracy
Chandler, Seth Carlo
Huygens, Christiaan
Morse, Samuel Finley Breese
al-Bīrūnī, Muḥammad ibn Aḥmad
Poincaré, Jules Henri
Lacaille, Nicolas Louis de
Time Periods
19th century
18th century
20th century, early
17th century
20th century
14th century
Paris (France)
United States
Greenwich (England)
South America
Convention du mètre (1875)
Royal Observatory Greenwich
Observatoire de Neuchâtel
Johns Hopkins University (Baltimore, Md.)

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