A meridian is a half-circle drawn on the globe from the North Pole to the South Pole. The prime meridian is the meridian chosen as the starting line: the one that passes through the old Royal Observatory at Greenwich, in London. The longitude of a place is the angle between the prime meridian and the meridian of that place, seen from the axis of the Earth. It goes from at the prime meridian to , and we say whether the place is east (E) or west (W) of the prime meridian.
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Example 14.5 (Reading the coordinates of Sydney)
On the map, Sydney lies below the equator: it is in the southern hemisphere, a little above the parallel S, closer to than to ; its latitude is about S. It lies to the right of the prime meridian, a little beyond the meridian E; its longitude is about E. We write: Sydney, S, E.