Let us talk about the basic directions on the surface of earth. In order to define directions, we need a datum or reference point. The easiest and most obvious one is the direction of sunrise. The sun does not rise, but appears to rise in the east due to the earth’s rotation about its axis. Therefore, we can define Easterly direction as the direction into which earth rotates. In the similar manner, Westerly direction is the direction from which earth rotates.
If we look east, with arms wide spread, our Left-hand points to the North Pole and hence called the direction of North. Similarly, our Right-hand points to the south pole called Southerly Direction. In our earlier lessons we had discussed about North and South Poles. North pole is the pole over which earth appears to rotate anti-clockwise direction and South Pole is the one over which earth appears to rotate in a Clockwise direction.
Cardinal and Quadrantal Directions
The directions of East, West, North and South are the basic directions based on which other directions are derived, hence they are called Cardinal Directions or Cardinal Points. The directions between the four cardinal directions are North East, South East, North West, and South West refer to the four quadrants and are therefore referred as Quadrantal points or Quadrantal directions. The Cardinal and Quadrantal directions are further subdivided and are widely used in Maritime Navigation.
However for the purpose of Air Navigation, Cardinal directions are retained as reference but a more precise method of indicating directions is used in the form of Sexagesimal System. Cardinal points are retained as reference for Sexagesimal system. Direction of the geographic north pole, defined as North forms the datum reference for Sexagesimal system. All other directions are measured in degrees, clockwise from north which is indicated as 000. At times North is also referred to as 360.
East is at 90 degrees angular displacement from north and hence referred as 090. Similarly, South has a numerical value of 180 and West has a value of 270. When we use the direction of Geographical North pole or actual north pole as North it is called as True North. When True North is used as reference for Sexagesimal system, the system provides for True directions
Reciprocal directions are in simple terms Opposite directions. For example, the reciprocal of North is South and reciprocal of East is West. When the same logic is applied to Sexagesimal system of reference, reciprocal directions are 180 degrees displaced from any given direction. In order to obtain reciprocal directions, we need to add 180 to directions less than 180 and subtract 180 from directions greater than 180.
Numerical on this Topic
For example, the reciprocal direction of 050 is 050 + 180 = 230 and reciprocal of 230 is 230 – 180 = 050
In Air navigation you would be frequently required to calculate the reciprocal of directions. Hence, as student pilots you need to practice mental calculation of reciprocals accurately and quickly