Mapmaking - The Science of Navigation - page 93

Parallels and Meridians
We have seen how latitude and longitude have been used from the time
of Hipparchus to describe a location on the surface of the earth. We have
seen that the earth is not exactly a sphere, as was assumed by the ancient
Greeks, and we will see how the location of the prime meridian has been a
bone of contention. Despite these hiccups, latitude and longitude are so
useful that they persist to this day. The earth is very nearly a sphere—we
will treat it as such in this chapter—and the prime meridian is now uni-
versally recognized to pass through the Royal Observatory in Greenwich,
London. 4
Now it is time for us to investigate how the globe is mapped, so we need
to be more precise than earlier about specifying positions on its surface. In
figure 4.3 you can see how a point on the surface of a sphere is uniquely
determined by two angles: latitude (how far north or south of the equator)
and longitude (how far east or west of the prime meridian, which by
definition is located at 0\ longitude). Lines of constant latitude are called
parallels because they form circles which lie in planes that are parallel to
each other, as you can see from figure 4.3b. Circles of latitude decrease in
radius as we move away from the equator. Lines of constant longitude are
called meridians , from the Latin for ''midday,'' because the sun is directly
over a given meridian at midday. Meridians describe great semicircles
around the earth, and they all meet at the poles (fig. 4.3c).
To make a map, we need to find a way to translate these three-dimensional
coordinates onto a two-dimensional piece of paper.
Globes and Maps
Or do we? Why bother? Why not make 3-D representations of the world?
There are a number of advantages of globes, after all. We will see soon
enough that 2-D representations of the earth all su√er from distortions of
one kind or another. This is, in fact, unavoidable. It is simply impossible to
depict a 3-D surface accurately on a flat map. You can see that this is the
case with the following simple, but messy, thought experiment. (Feel free
4. When specifying position on the surface with great precision, we must bear in mind
the eccentricity of the earth, as we saw in chapter 2. The prime meridian is not quite
universally located at the Royal Observatory: GPS places it 100 m to the east.