Signalman 1 & C - Historic Naval Ships Association

ParallelsFigure 9-4.—The terrestrial sphere.Parallels also require a reference: the Equator.The Equator (0°) is located halfway between the poles,and divides the world into Northern and SouthernHemispheres. The letter designator (N or S) must alsobe used in referencing a position. The principle of thenumbering system is similar to that used for meridiansexcept that parallels range from 0° at the Equator to90° N or S at the poles.Latitude and LongitudeEvery spot on Earth is located at the point ofintersection between a meridian and a parallel. Everypoint's location is describable in terms of latitude andlongitude.Latitude (parallel) is the angular position indegrees, minutes, and seconds of arc north or south ofthe Equator, measured along the meridian of the point.(See fig. 9-5.)Longitude (meridians) is the angular position indegrees, minutes, and seconds of arc east or west ofthe 0° meridian, measured along the parallel of thepoint. (See fig. 9-5.)For navigational purposes, accuracy demands arerigid. The exact position must be designated.Consequently, in giving navigational position, 1° isdivided into 60 minutes, and 1 minute is divided into60 seconds. Thus, a latitude may be 45° 12 minutes 22seconds N (or S). The same system is used for east orwest longitude. In all reports concerning navigationhazards and positions of lightships, buoys, and the likereceived on radio nets or read in Notices to Mariners,positions are given in latitude and longitude.Nautical DistanceOn Earth's surface, 1° of latitude may beconsidered 60 nautical miles in length; whereas thelength 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 ofdistance at the same latitude. Therefore, in measuringFigure 9-5.—Earth on the Mercator projection.9-4