There are two types of circles used in making this set of intersecting lines - great and small circles. A great circle is the largest possible circle that can be drawn on a given sphere. Another way of looking at a great circle is to imagine a large ball of clay and cutting it so that the cut passes directly through the center of this ball. If you now lay the two halves back together, the line marking where the cut was made will be a great circle. If this same ball of clay were cut in any way such that the cut does not go through the center of the ball, then the line that results is a small circle . (See Figure 1)
Now, what criteria would you use for establishing the placement of this network of imaginary lines? This was certainly a problem until people understood that the Earth is rotating on its axis once every 24 hours. This spinning of the Earth establishes two key reference points - the North and South Poles of rotation, or the "true" poles, or the geographic poles. The poles are the end points of the axis about which the Earth rotates. A great circle that can be drawn around the Earth halfway between the two poles is called the equator. With this line we can begin to describe the location of points in terms of the angular distance the points are north or south of the Equator by using angles whose centers are at the center of the Earth. This angular measurement north or south of the equator is called latitude. The lines of latitude are parallel to each other and to the equator, and they run in an east-west direction around the globe. All parallels of latitude are small circles except for 0 degrees latitude which is the great circle called the equator. Latitude measurements can go from 0 degrees to 90 degrees. (See Figure 2)
Latitude by itself is not enough to accurately describe location. By saying that something has a latitude of 35 degrees only describes the small circle that is 35 degrees north or south of the equator. We can clarify which of the two circles we mean by calling them "north" or "south," but we still have not pinpointed where the feature is located. A second set of lines must be drawn to limit the possibilities to one unique position. This second set of lines are great circles which run north-south connecting the North and South Poles of the Earth. Measurements involving these lines are east or west of some mutually agreed to reference line called the Prime Meridian. The angular distance east or west of the Prime Meridian is longitude.
Longitude measurements can range from zero to 180 degrees to the east or to the west (that is from the Prime Meridian to the 180th meridian in each direction). The Prime Meridian is an imaginary north-south line that passes through the Royal Observatory at Greenwich, England. This was selected as the line of zero degrees longitude by an International Conference in 1884. (See Figure 3)
In traveling on the Earth, it is important that we consider its spherical nature. The meridians are half-circles (composed of 180 degrees) that intersect at the North and South Poles. The Greenwich or Prime Meridian and the International Date Line comprise a meridian circle of 360 degrees. The distance between meridians is shorter the farther we travel north or south of the equator. The parallels of latitude are within 0.7 miles (1.1 kilometers) of being equidistant. They are not equally spaced because the Earth is not a true sphere. If we travel north from the equator along any meridian, we can go north only as far as the North Pole, or 90 degrees North latitude. Should we continue to travel on this great circle path after reaching the North Pole, we must travel south towards the South Pole.
When we travel east or west from any meridian, our direction of travel never changes. We continue in one direction until eventually we return to our point of origin. This should not be too difficult to understand if we remember that the parallels are circles which go completely around the Earth.
