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Figure 1.10 Sunlight intensity is diminished as one travels toward the Earth's poles.
Source: Based on an image in Kolars, John F. and Nystuen, John D., Human
Geography: Spatial Design in World Society, Englewood Cliffs: McGraw-Hill, 1974.
Original drawings by Derwin Bell.
the position of the Sun's rays. The ray of the Sun is directly overhead at 23.5
degrees north on June 21 (as far north as it ever will be); at 23.5 degrees south
on December 21 (as far south as it ever will be); and at zero degrees on the
equinoxes.
In addition to this basic geometry, travelers often note, when journeying
closer to the Earth's Equator than they are accustomed to doing, that the Sun
seems “hotter” than they were expecting. That observation is not surprising
and it is based upon simple geometry. Sunlight intensity at the surface of the
Earth is a function of latitude. Figure 1.10 shows why: At higher latitudes,
sunlight is spread out over a wider area than it is closer to the Equator and is
thus less intense. The Sun is most intense when the direct ray is overhead or
orthogonal to a tangent plane to the Earth. The even poleward spreading is a
function of the relatively constant curvature of the Earth's surface.
1.4 Theory: Precision of latitude and longitude values
As we have seen, latitude and longitude values can be subdivided into units
smaller than a degree. This subdivision is useful to denote locations on the
Earth. As the number of significant digits to the right of the decimal point
increases, one can pinpoint locations with increasing precision. One way to
conceptualize this idea is that at the Equator, the length of one degree of
longitude is 111.2 kilometers (km), or 69 miles (mi). In Chapter 2, we will
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