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Figure 1.2 The shortest distance between two points on a sphere is measured along an
arc of a great circle. Two different views of the same paths, on the sphere and in the
plane. Source of base maps: Based on Google Earth™ mapping service (top) © 2012
Google and Image © 2012 TerraMetrics, Data SIO NOAA US Navy NGA GEBCO,
©2012 Cnes/Spot Image; and, Great Circle Mapping Tool (bottom).
• Another set of reference lines is produced using a half of a great
circle, joining one pole to another, that has a unique position: The
half of a great circle that passes through the Royal Observatory
in Greenwich, England (three points determine a circle—both
poles and Greenwich). The historical decision at the Meridian
Conference in 1884 produced the uniqueness in selection. Choose
a set of evenly spaced halves of great circles obtained by rotating
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