Published online by Cambridge University Press: 01 July 1976
The least spherical distance between two points on the surface of a sphere is that along the minor arc of the great circle on which the two points lie. It is an easy matter to demonstrate this using a cord stretched between two points on a small globe; noting that the circle whose plane coincides with that of the cord contains the centre of the sphere and that it is, therefore, a great circle. It is a little more difficult to prove the fact analytically. The practice of sailing along the shortest route on an ocean passage—sometimes known as ‘orthodromic’ as opposed to rhumbline or ‘loxodromic’ sailing—would require the course to be continually changed (except, of course, in cases in which the rhumb-line and great circle arc coincide). This would clearly be impossible; the term ‘approximate great circle sailing’ is sometimes used to describe the techniques of following a great circle route as closely as practicable.
1 Gerard Mercator (1512–1594) constructed his world map (Nova et aucta oibis terrae descriptio ad usum navigantium emervidate accommodatd) in 1569. This remarkable map, based on what is now known as a conventional cylindrical orthomorphic projection, was published in atlas form comprising 24 sheets. We have no direct evidence of how Mercator constructed this map: careful measurements of the spacing of the projected parallels of latitude manifest errors of a magnitude which indicate that he could not have used trigonometrical tables (although they were available at the time) as the basis of construction.
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