Half of this circular outline, connecting one pole to the other, is known as a meridian of longitude, or, more simply, a meridian. The outlines of the cuts form circles on the globe, each of which passes through both poles. Now imagine slicing the Earth through the axis of rotation instead of across it, just as you would cut up a lemon to produce wedges (Figure 1.4 B). We use the Equator as a fundamental reference line for measuring position. This circle is known as a parallel of latitude, or a parallel. The Earth's longest parallel of latitude is the Equator , which lies midway between the two poles. Each cut creates a circular outline that passes around the surface of the globe. Lay the globe on its side, so that the axis joining the North and South Poles runs parallel to your imaginary chopping board and begin to slice. Imagine cutting the globe just as you might slice an onion to make onion rings (Figure 1.4 A). This is made up of a system of imaginary circles, called parallels and meridians, which are shown in Figure 1.4. Instead, we divide the Earth into what is known as the geographic grid. Because the Earth's surface is curved, we cannot divide it into a rectangular grid anymore than we could smoothly wrap a globe in a sheet of graph paper. This simple fact has caused mapmakers problems for centuries. It is impossible to lay a flat sheet of paper over a sphere without creasing, folding, or cutting it-as you know if you have tried to gift-wrap a ball.
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