The fact that people often create hemisphere maps is undoubtedly related to a common perceptual phenomenon: Looking at a spherical object such as a globe, we can see only half. Actually we see slightly less than half, but at any reasonable viewing distance "seeing the globe" inevitably suggests seeing half of it.
The term hemisphere derives from French, Latin, and Greek words of the same meaning, suggesting that human beings have long recognized the need for a term to designate half a sphere. There are no terms in the dictionary for a quarter of a sphere or for an eighth of a sphere, even though they would be just as easily derived etymologically. Clearly they lack perceptual justification.
Hemispheres by the Dozen
The earth can be divided into an infinite number of hemisphere pairs. Any plane passed through the center of the earth will intersect the surface along a great circle (a line of maximum circumference) that bounds two equal portions of the earth.
Only a few of the hemisphere pairs are of sufficiently wide interest or utility to have been given names. The northern and southern hemispheres are centered on the North and South poles and are bounded by the equator (Figure 4-1). They are the most "natural" pair of hemispheres and we encounter them primarily in connection with such natural phenomena as star patterns and atmospheric and oceanic circulation systems.
The western and eastern hemispheres are bounded far more arbitrarily and are not well-named. One might think they would be bounded by the prime meridian and the International Dateline, thus separating the portions of the earth that are labeled with east and west longitude. They generally are not (Figure 4-2). They are usually bounded by approximately 20° W and 160° E longitude, more conveniently separating North and South America from Europe, Africa, Asia, and Australia.
The lack of connection to west longitude and the close identification of the term western hemisphere with North and South America has led some to refer to the American hemisphere, rather than western hemisphere. Physically, the eastern and western hemispheres have little, if any, significance, but politically and historically they are the Old World and the New World to Asian and European-based culture. The western hemisphere is a particularly common reference in political rhetoric.
Among other halves of the globe that have interested us are the so-called land and water hemispheres (Figure 4-3). The earth is divided such that maximum land area is contained within one of the divisions and maximum water area in the other.
Another named half of the earth is the so-called Pacific hemisphere. This term is used in referring to the economic and diplomatic matters of the Pacific Rim and the intervening islands, many being of strategic interest in the modern world.
Centering a Hemisphere
When we pick up a globe and look at it, seldom will any of the variously named hemispheres be the ones at which we are looking. We generally turn the globe until the particular point or area of interest is right in the middle. The location in the world of that place is best seen in that position, i.e., we can see it within a hemisphere whose edge is equidistant from it (Figure 4-4). Positioning the globe in that way is so fundamental that we would find it incredible if someone tried to use a globe by positioning it with an arbitrary or preselected center, leaving a place of interest at the periphery of the visible hemisphere. Many globes are therefore available in cradles, rather than on fixed stands, because they are more useful when they can be turned to view any hemisphere.
The necessity of hemispheric viewing seems to be removed when we turn from the globe to projected maps. But one whole class of map projections is usually limited to showing a hemisphere in order to avoid excessive amounts of distortion. These are the planar or azimuthal that are projected onto a plane touching or intersecting the globe and on which the directions (azimuths) from the center to all other points on the map are correct (Figure 4-5). In addition, there are projections referred to as "globular" that project the hemisphere into a circle but are not azimuthal, i.e., central angles are not correct (Figure 4-6). These projections were generally developed before 1700 and were commonly used in pairs to represent the whole earth.
If we want to show more than a hemisphere, there are many non-planar, non-globular projections from which to choose. Yet some of the planar projections have properties that encourage us to put up with hemispheric representations, and, just as important, the representation of the hemisphere is often desirable in its own right. As to reasons for tolerating hemisphere representations, the azimuthal property (correct central angles) of all planar projections is often desirable. The Azimuthal Equidistant planar projection also shows distances from the center point (or, with a modification, two points) correctly. In addition, most azimuthal projections are generally good for showing any compact area (hemisphere or smaller) with relatively small amounts of distortion.
Turning to reasons for wanting to show a hemisphere, rather than merely tolerating it as a limitation, we often want to simulate the experience of seeing a globe, the globe itself being an undistorted representation of the earth. The Orthographic projection is readily recognized as globe-like and is even perceived as having no distortion (Figure 4-4). Just as we recognize that a person several feet away is just as tall as one closer to us, we mentally compensate for foreshortening in the Orthographic projection, and it "looks right." The Orthographic projection is used to show large regional layouts, and it is often employed as a locational reference map for larger-scale depictions on other projections (Figure 4-7).
In summary, depiction of hemispheres on flat maps is of special interest in the employment of projections. A hemisphere is what we see when looking at a globe; it is the amount of area that can be represented reasonably on most planar projections; historically it was the unit of representation on the globular projections; and, when depicted on the Orthographic projection, it is the amount of earth area that most readily simulates looking at the globe.
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