# Measurement of Directions

Understanding directions on the surface of earth is of paramount importance for Air navigation. In this lesson we shall define the directions on the surface of earth and the conventions used in determining directions. Earlier, we had learnt about the Solar System and Obliquity of Ecliptic.

In order to define directions, we need to select a datum from which directions can be measured. As we had seen earlier, the earth spins around itself as well as revolves around the sun. The end points of the Axis of Rotation are called the Poles of the Earth. Therefore, the Axis of Rotation is also called the Polar Axis.

Measurement of directions starts with an understanding of Cardinal and Quadrantal Directions. The directions of North, South, East and West are called as Cardinal Directions. Similarly, the directions of NE, NW, SE and SW are called as Quadrantal Directions.

### NORTH AND SOUTH

Let us define North and South Poles without reference to other directions. We can say that North Pole is that pole or the end point of axis of rotation of earth, from over which an observer would notice the earth rotating in an anti-clockwise direction. Similarly, South Pole is that pole over which an observer would notice that the earth rotating in a clockwise direction.

The direction in which we have to travel to reach the North Pole is called the direction of True North. I am intentionally specifying True North, because you would understand there are other types of North like Magnetic or Compass North in our later lessons. Similarly, the direction in which we have to travel to reach the South Pole is called the direction of True South.

It is common knowledge that while facing East, our Left Hand points to the North Pole and our Right Hand points to the South Pole. Therefore, when we stand facing east, the direction of North is to our Left and the direction of south is to our Right.

### EAST AND WEST

Let us understand about the directions of East and West. In your younger days, you would have learnt that the direction of sunrise is called East. However we are now aware that the sun is stationary and it is the earth which moves around the sun.

Therefore, we say that the direction into which the earth is rotating or spinning is called East. All other directions are defined in relation to East. In similar terms, we can define West as the direction from which earth rotates. Westerly direction can also be defined as the direction which is exactly opposite to East.