sphere itself. It is the largest circle that can be drawnon the sphere; it is the intersection with the surface ofthe earth of any plane passing through the earth’scenter.The arc of a great circle is the shortestdistance between two points on a sphere, just as astraight line is the shortest distance between twopoints on a plane.On any sphere, an infinitenumber of great circles may be drawn throughany point, though only one great circle may bedrawn through any two points that are notdiametrically opposite (fig. 2-2).Circles on the surface of the sphere other thangreat circles may be defined as small circles. A smallcircle is a circle on the surface of the earth whosecenter and/or radius are not that of the sphere. Aspecial set of small circles, called latitude is discussedlater.The intersection of a sphere and a plane is acircle—a great circle if the plane passes throughthe center of the sphere, and a small circle if itdoes not.Latitude and LongitudeThe nature of a sphere is such that any point onit is exactly like any other point. There is neitherbeginning nor ending as far as differentiation ofpoints is concerned. So that points may be locatedon the earth, some points or lines of reference arenecessary so that other points may be located inregard to them. The location of New York City withreference to Washington, D. C., is stated as a numberof miles in a certain direction from Washington,D.C. Any point on the earth can be located thesame way.This system does not work well in navigation.A point could not be precisely located inm i d - P a c i f i c O c e a n w i t h o u t a n y n e a r bygeographic features to use as a reference. Asystem of imaginary reference lines is used tolocate any point on earth. These reference linesare the parallels of latitude and the meridians oflongitude.Figure 2-2.-A great circle is the largest circle in a sphere.LATITUDE.— Each day the earth rotates once onits north-south axis. This axis terminates at the twopoles. The equator is constructed at the midpoint of2-3
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