The title of this chapter is similar to that of a well-known soap opera. It seems appropriate because the turning earth is the stage on which the human drama is played. Like the ongoing soap-opera drama, the spherical earth's surface is continuous, with no obvious
beginning or end. How we view this stage and the relation of the turning globe to its display on world maps is the subject of this introduction to the more specific and detailed treatments in this booklet.
Meridians and Parallels
First, a few basics. The ball-shaped earth spins around an axis that
extends between the North and South poles. The poles are the anchor
points for the geographical coordinate system of latitude and
longitude (Figure 1-1).
Longitude designates distance E or W from
some starting point. It is shown on world maps by a selection of
great-circle-arc meridians that extend from pole to pole and
converge to each. Latitude designates distance N or S from the
equator, a line around the earth halfway between the poles. It is
shown by a series of smaller circles called parallels, that are
concentric to the axis. On the round earth all parallels and
meridians are perpendicular to one another. The selection of
parallels and meridians shown on maps is called the graticule.
The turning earth is converted to something like a stage set on a flat world map that normally displays a graticule, coastlines, and a selection of boundaries. A spherical surface cannot be flattened without distorting it in several ways, so all maps must select and arrange the distortions by means of a systematic transformation called a map projection. There are scores of map projections, some more suitable than others for particular purposes. They display the earth in a variety of shapes (and, of course, sizes), often as ovals and rectangles. Showing the entire earth within a circle is not rare, but it is usually done only to show distances and directions from one point. A principle of stage design is simplicity, in the sense of not diverting attention from the drama being presented. It has its equivalent in the design of world maps.
Because the earth is a ball, we can tilt its axis from the vertical in any way desired without affecting the inherent character or distortion pattern of the chosen map projection. But the way the axis is tilted will greatly affect the appearance of the graticule (Figure 1-2). Since the parallels and meridians are simple shapes, it is usually not desirable to display them as complex curves. Accordingly, except when special needs require otherwise, the preferred design is as A in Figure 1-2. The axis is perpendicular to the viewer, and the equator becomes a horizontal straight line dividing the earth neatly into northern and southern hemispheres. Eastern and western halves are separated by a straight central meridian.
The Earth Viewed from Different Directions
Some stage sets are constructed on a large circular platform that
can be rotated to present different views to the audience. Similarly,
we may rotate the earth around its polar axis so as to bring any
desired region "front and center." For example,
in Figure 1-3, on an
Eckert IV equal-area projection, we turn
the earth as it might be desired by an American, an African,
or an Asian.
The display of the earth with north at the top of the page (or so that "north is up," as many people say) is a fairly recent convention. There is no "up" or "down" in space; those terms are earthbound, "down" being toward the center of the earth and "up," the opposite. The convention stems partly from the beliefs that consistency is a virtue and that it is easier to recognize map shapes in familiar orientations, and from the fact that more mapmakers have lived in the northern hemisphere. Whatever the reasons, the convention rankles some who dwell in the southern hemisphere where some maps are made "upside down" as illustrated in Figure 1-4.
Time Zones and Meridians
As the earth turns it alternately sweeps the surface through
a period of sunlight and a like period of darkness. Several thousand
years ago the Babylonians, using a sexagesimal number system,
divided the circle into 360° and the calendar day into 24 hours.
Consequently, the earth turns through 15° of longitude each hour.
Until well into the 19th century each locality had its own "sun time," noon being when the sun crossed the local meridian. With the development of railroad transportation, the myriad of local times became bothersome. In 1884 an international conference in Washington, D.C., agreed on a system of "standard time" in which, ideally, everyone in each 15° longitude segment, from pole to pole, would set all clocks to the time of the central meridian of that zone. The conference also agreed to start counting longitude east or west from the prime meridian of Greenwich in England, and to reckon standard time as + or - the time in that time zone in units of one hour.
In practice, the boundaries of standard time zones rarely coincide with meridians, being modified for economic and administrative convenience. The standard time concept is, however, practiced almost everywhere, in that time is uniform throughout each region.
Since the meridians converge to the poles, the width of a 15° time zone is very narrow in the polar latitudes. For example, the U.S.S.R. has 11 time zones while Africa, which is wider, actually observes only four. But an hour is an hour; therefore, to portray the concept with a uniform time scale and provide better detail in regions with crowded time zones, it is desirable to show the standard time system on a cylindrical map projection, as in Figure 1-5, where the meridians are shown as parallel, thus having the same width (one hour) from pole to pole.