Figure 2-4.-Latitude is measured from the equator;longitude from the prime meridian.A position on the surface of the earth is expressedin terms of latitude and longitude. Latitude is thedistance, either north or south, from the equator.Longitude is the distance, either east or west, from theprime meridian.DistanceDistance as previously defined is measured by thelength of a line joining two points. In navigation, themost common unit for measuring distance is thenautical mile. For most practical navigation, all of thefollowing units are used interchangeably as theequivalent of 1 nautical mile:. 6,076.10 feet (nautical mile)l One minute of arc of a great circle on a spherehaving an area equal to that of the earthl 6,087.08 feet. One minute of arc on the earth’sequator (geographic mile)l One minute of arc on a meridian (1 minute oflatitude). Two thousand yards (for short distances)It is sometimes necessary to convert nautical milesinto statute miles or statute miles into nautical miles.This conversion is made with the following ratioThis means that 1 nautical mile equals 1.15 statutemiles.The rate of change of position is determined byspeed. Speed is expressed in miles per hours, eitherstatute miles or nautical miles. If the measure ofdistance is nautical miles, it is customary to use theterm knots. A speed of 200 nautical miles per hourand a speed of 200 knots are the same. The phrase“200 knots per hour” is incorrect unless you arereferring to acceleration.DirectionDirection is the position of one point in spacerelative to another without reference to the distancebetween them. The time-honored system forspecifying direction as north, northwest, west, etc.,does not meet the needs of modern navigation. Anumerical system meets the needs better for mostpurposes. The numerical system (fig. 2-5) divides thehorizon into 360 degrees, starting with north as 000degrees. Going clockwise, east is 090 degrees, south180 degrees, west 270 degrees, and back to north.The circle, called a compass rose, represents thehorizon divided into 360 degrees. The nearly verticallines represent the meridians, with the meridian ofposition A passing through 000 degrees and180 degrees. Position B lies at a true direction of 062degrees from A, and position C is at a true direction of295 degrees from A.Determination of direction is one of the mostimportant parts of the navigator’s job. In order for thenavigator to accomplish this task, the various termsinvolved must be clearly understood. Unlessotherwise stated, all directions are called true (T)directions.Figure 2-5.-Numerical system used in air navigation.2-5