The Prime Meridian is the line at which the world’s longitude is set at 0˚. Both longitude on the one hand, and time’s measurement on the other, are based on the Prime Meridian. Since 1884, the Prime Meridian has been fixed at the Royal Observatory, at Greenwich, in the United Kingdom. The fact that Greenwich is the ground zero of planetary measurement is the result of recommendations made at the 1884 International Prime Meridian Conference, held in Washington DC, in 1884. Before 1884, however, there were many different prime meridians at work in the world. This was a source of considerable geographical and astronomical confusion.

In this paper, I examine the relationship between two different prime meridians, those of Paris and of Greenwich in the ‘long’ eighteenth century and do so through the lens of the historical geographies of science. The paper will explore, firstly, the debates between international politics, national mapping, and the local siting of the Paris prime meridian in the period c.1634 to c.1787,
and, secondly, the workings of joint Franco-British attempts to ‘fix’ the prime meridians of Paris and of Greenwich by triangulation in the period c.1787 to c.1830. The paper is about rockets, maps, metrology, and trust in instruments, about the politics and authority of science and about the reliance placed upon questions of accuracy and precision and the tolerance of error as nations delimit themselves.