Figure 2-3.-Planes of the earth.

this axis at right angles to it (fig. 2-3). A great circle drawn through the poles is called a meridian, and an infinite number of great circles maybe constructed in this manna. Each meridian is divided into four quadrants by the equator and the poles. Since a circle is divided into 360 degrees, each quadrant contains 90 degrees. Take a point on one of these meridians 30 degrees north of the equator. Through this point passes a plane perpendicular to the north-south axis. This plane will be parallel to the plane of the equator, as shown in figure 2-3, and will intersect the earth in a small circle called a parallel or parallel of latitude. This particular parallel of latitude is called 30°N, and every point on this parallel will be at 30°N. Parallels can be constructed at any desired latitude. The equator is the great circle midway between the poles. The parallels of latitude are small circles constructed with reference to the equator. The angular distance measured on a meridian north or south of the equator is known as latitude and forms one component of the coordinate system. LONGITUDE.— The latitude of a point can be shown as 20°N or 20°S of the equator, but there is no way of telling whether one point is east or west of another. This is resolved by the use of the other component of the coordinate system—longitude. Longitude is the measurement of this east-west distance. There is not a natural starting point for numbering longitude. With latitude, the starting point is the equator. This problem was solved by selecting an arbitrary starting point. Many places had been used, but when the English speaking people began to make charts, they chose the meridian through their principal observatory in Greenwich, England. This meridian has now been adopted by most other countries as the starting point. This Greenwich meridian is sometimes called the prime meridian or first meridian, though actually it is the zero meridian. Longitude is counted east or west from this meridian through 180 degrees. The Greenwich meridian is the 0-degree meridian on one side of the earth and the 180th meridian after crossing the poles (180 degrees east or west of the 0-degree meridian). If a globe has the circles of latitude and longitude drawn on it according to the principles described, and the latitude and longitude of a certain place have been determined, this point can be located on the globe in its proper position (fig. 2-4). In this way, a globe can be formed that resembles a small-scale copy of the earth. Latitude is measured in degrees up to 90, and longitude is expressed in degrees up to 180. The total number of degrees in any one circle cannot exceed 360. A degree (°) of arc may be subdivided into smaller units by dividing each degree into 60 minutes (’) of arc. Each minute can be divided into 60 seconds (“) of arc. Measurement may also be made in degrees, minutes, and tenths of minutes. Figure 2-3.-Planes of the earth. 2-4