Latitude and Longitude

Figure 9-4.—The terrestrial sphere. Parallels Parallels also require a reference: the Equator. The Equator (0°) is located halfway between the poles, and divides the world into Northern and Southern Hemispheres. The letter designator (N or S) must also be used in referencing a position. The principle of the numbering system is similar to that used for meridians except that parallels range from 0° at the Equator to 90° N or S at the poles. Latitude and Longitude Every spot on Earth is located at the point of intersection between a meridian and a parallel. Every point's location is describable in terms of latitude and longitude. Latitude (parallel) is the angular position in degrees, minutes, and seconds of arc north or south of the Equator, measured along the meridian of the point. (See fig. 9-5.) Longitude (meridians) is the angular position in degrees, minutes, and seconds of arc east or west of the 0° meridian, measured along the parallel of the point. (See fig. 9-5.) For navigational purposes, accuracy demands are rigid. The exact position must be designated. Consequently, in giving navigational position, 1° is divided into 60 minutes, and 1 minute is divided into 60 seconds. Thus, a latitude may be 45° 12 minutes 22 seconds N (or S). The same system is used for east or west longitude. In all reports concerning navigation hazards and positions of lightships, buoys, and the like received on radio nets or read in Notices to Mariners, positions are given in latitude and longitude. Nautical Distance On Earth's surface, 1° of latitude may be considered 60 nautical miles in length; whereas the length of 1° of longitude varies with latitude. Hence, the latitude scale must be used for measuring distance. Although this scale is expanded on a Mercator chart, the expansion is exactly equal to the expansion of distance at the same latitude. Therefore, in measuring Figure 9-5.—Earth on the Mercator projection. 9-4