You probably know already that the basic coordinate system that’s used to describe the position of a point on the Earth's surface is latitude and longitude. In this system (Figure 3-33), the Earth is imagined to be cut by a series of planes that pass through the north–south axis of rotation. The intersection of such a plane with the Earth’s surface is called a line (really a curve) of longitude, or a meridian. Longitude is measured in degrees, from zero to 360. One meridian (the one that passes through Greenwich, England) is called the prime meridian, and longitude is measured 180 degrees to the west of that and 180 degrees to the east of that. The opposite meridian, 180 degrees around the world from the prime meridian (and the intersection of the longitude plane with the other side of the world) lies about in the middle of the Pacific Ocean.
One consequence of this definition of longitude is that the spacing between two meridians gets smaller as you go north or south from the equator. Think about this the next time you fly west in a jetliner: You would have to move awfully fast to keep up with the sun, and land at the same time of day you took off, if you're flying along the equator, but if you're flying east to west in the far north or far south on the earth, you could easily arrive at your destination a lot earlier in the day than you took off!
The other element of the coordinate system is a series of latitude circles (Figure 3-33). These latitude circles are small circles that are perpendicular to the earth's north-south axis. (A small circle is the intersection between the surface of a sphere and a plane that cut through the sphere but does not pass through the center of the sphere. That’s in contrast to a great circle, which is formed by the intersection between the surface of a sphere and a plane that cuts through the sphere and passes through the center of the sphere.) These small circles are formed by passing planes parallel to the equatorial plane through the earth. By convention, the equator (the curve on the earth’s surface that’s formed by passing a planet through the center of the earth and perpendicular to the north–south axis) is at zero degrees latitude, and the north pole and south pole are at 90° latitude.
Big maps of the earth's surface that are bounded by latitude and longitude lines (as most maps are) are not rectangular: they narrow to the north in the northern hemisphere, and they narrow to the south in the southern hemisphere. Maps of smaller areas, of the kind geologists use when they are mapping in an area that’s a small part of a state, are so close to being rectangular that one can usually ignore the effect of narrowing of longitude lines.