Latitude and the Magnetic Earth, by Stephen Pumfrey

The Earth location scheme based upon spherical coordinates, known as latitude and longitude, was developed by the Greeks over two thousand years ago (National Geographic Society 1999). This system was applied and expanded to include arc minutes and seconds to the gazetteer and maps generated by Ptolemy (ca. 90-168 AD) (Wilford 2001). Accurately determining latitude and longitude has been a focus of geographic inquiry ever since, spanning the technological realm from protractors to the Global Positioning System. Reasonably accurate latitude determination was mastered by the ancient Greeks from solar angle measurements, but the determination of longitude took much longer. Longitude, which has no convenient reference points or lines, required the development of accurate clocks known as chronometers in the eighteenth century. John Harrison’s technical and political tribulations in developing the chronometer are described in Longitude by Dava Sobel (1995) and are portrayed in the History Channel movie by that title which first aired in 1998.