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On Earth’s surface, adjacent lines of latitude and longitude may be several miles apart, and that creates a potential problem if you wish to state the absolute location of a spot that is “between the lines.” For this reason, the graticule contains a couple of levels of refinement (see Figure 3-4).

(© John Wiley & Sons Inc.)

FIGURE 3-4: Degrees, minutes, and seconds.

First, the space between successive degree lines may be subdivided into 60 equidistant units called minutes ('). Second, the space between successive minute lines may be subdivided into 60 equidistant units called seconds ("). And if more exactitude is needed, then seconds may be carried out to as many decimal points as may be necessary.

Doesn’t this sound familiar? Sixty seconds in a minute? And for good reason. The system that you use to tell time goes back to the same Sumerian base-6 arithmetic that Hipparchus used to divide up a circle and also the world. Hmmm … there are 24 hours in a day. Think 24 being evenly divisible by 6 is just a coincidence? No way.