this axis at right angles to it (fig. 2-3). A great circledrawn through the poles is called a meridian, and aninfinite number of great circles maybe constructed in thismanna. Each meridian is divided into four quadrants bythe equator and the poles. Since a circle is divided into360 degrees, each quadrant contains 90 degrees.Take a point on one of these meridians 30 degreesnorth of the equator. Through this point passes a planeperpendicular to the north-south axis. This plane willbe parallel to the plane of the equator, as shown infigure 2-3, and will intersect the earth in a small circlecalled a parallel or parallel of latitude. This particularparallel of latitude is called 30°N, and every point onthis parallel will be at 30°N. Parallels can beconstructed at any desired latitude.The equator is the great circle midway betweenthe poles. The parallels of latitude are small circlesconstructed with reference to the equator. Theangular distance measured on a meridian north orsouth of the equator is known as latitude and formsone component of the coordinate system.LONGITUDE.— The latitude of a point can beshown as 20°N or 20°S of the equator, but there is noway of telling whether one point is east or west ofanother. This is resolved by the use of the othercomponent of the coordinate system—longitude.Longitude is the measurement of this east-westdistance.There is not a natural starting point for numberinglongitude. With latitude, the starting point is theequator. This problem was solved by selecting anarbitrary starting point. Many places had been used,but when the English speaking people began to makecharts, they chose the meridian through their principalobservatory in Greenwich, England. This meridianhas now been adopted by most other countries as thestarting point. This Greenwich meridian is sometimescalled the prime meridian or first meridian, thoughactually it is the zero meridian. Longitude is countedeast or west from this meridian through 180 degrees.The Greenwich meridian is the 0-degree meridian onone side of the earth and the 180th meridian aftercrossing the poles (180 degrees east or west of the0-degree meridian).If a globe has the circles of latitude and longitudedrawn on it according to the principles described, andthe latitude and longitude of a certain place have beendetermined, this point can be located on the globe in itsproper position (fig. 2-4). In this way, a globe can beformed that resembles a small-scale copy of the earth.Latitude is measured in degrees up to 90, andlongitude is expressed in degrees up to 180. The totalnumber of degrees in any one circle cannot exceed360. A degree (°) of arc may be subdivided intosmaller 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 indegrees, minutes, and tenths of minutes.Figure 2-3.-Planes of the earth.2-4

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