
Fig7_1 Prime Meridian
Fig. 7.1 . Great circles through the North and South Poles of the Earth create circles of longitude. They are perpendicular to the equator where they intersect it. The circle of longitude that passes through Greenwich England is called the Prime Meridian. The longitude of any point, P, is the angle, l, measured westward along the equator from the intersection of the Prime Meridian with the equator to the equatorial intersection of the circle of longitude that passes through the point. The latitude is the angle phi, f, measured northward (positive) or southward (negative) along the circle of longitude from the equator to the point. In this figure, the point P corresponds to San Francisco.
Copyright 2010, Professor Kenneth R. Lang, Tufts University

