The difference in longitude between the observatories of Paris and Greenwich was an important standard – long of relevance to astronomy, geodesy, navigation and timekeeping. Measured many times and by many different means since the 17th century, the preferred method of the later 19th and early 20th centuries used the electric telegraph to compare traditional astronomical time determinations at the two observatories. I describe here for the first time, from both published and archival sources, the four Paris-Greenwich telegraphic longitude determinations made between 1854 and 1902.

The first gave a result materially different from the value that had been trusted for decades, yet was adopted in large part because of assertions of the excellence of the new telegraphic technique. It was soon found to be inaccurate. A second determination showed a modest but irreconcilable difference between the French and British results. There followed an ill-tempered dispute over which was to be relied upon, and the exercise ended in failure. A third failed in exactly the same way. So by the 1890s the uncertainty over Paris-Greenwich longitude was described as a ‘thorn in the side of European geodesy’, and there was international pressure for a definitive and fourth determination. This final telegraphic measurement was said to have completely settled the matter of the correct value; there was, however, far from clear-cut evidence for the claimed success.

I use this as a case study in precision measurement, showing how comparison and dispute between different measurements, in the search for ‘reasonable agreement’ between them, was an important force for change and improvement. What constituted reasonable agreement is shown to be only tenuously related to numerical consistency; rather, it was highly contingent on the contemporary interpretation of inconsistencies and on external circumstances. I also show that the resulting improvements in precision had more to do with the gradually improving methods of astronomical time determination than with the singular innovation of the telegraph, thus emphasising the importance of what have been described as ‘observatory techniques’ to 19th century practices of precision measurement.