Change of Longitude

In this post, we shall understand about the change in longitude when we fly from one place to another. In our earlier lesson, we had discussed about the change in latitude while flying from a point to another point.


The change of Longitude while flying from a certain longitude to another is called as ChLong. To consider a calculation of ChLong, it is best to view the earth looking down onto one of the poles.

Let us calculate the ChLong from a position at E170 to W030. If you measure the change in longitude in an anti-clockwise direction, you would find that the change in Longitude is 160 degrees. If you measure in a clockwise direction, between the same longitudes, the change in Longitude would be 200 degrees. Since we would want to travel in the shortest possible path, the smaller angle would represent the shorter distance between the two Meridians. The correct ChLong is therefore 160 degrees in an Easterly direction.

Remember, Westerly Longitudes increases from the left hand side of the map until it reaches its maximum of 180 degrees. Likewise, the Easterly Longitudes increase from the right hand side until it reaches its maximum of 180 degrees.


It is too easy to get confused between East and West Longitude around the Greenwich Anti-Meridian. When Anti-Meridian is in the centre of view, Westerly Longitudes is on the right hand side and Easterly Longitudes are on the left hand side.

Positions should not be confused with directions. Of course the earth is still spinning East, so the direction of East has not changed. Remember to imagine the spherical image of the earth, while applying ChLat and ChLong, and not just to think about a two dimensional representation like a map.

Till now we have understood about the change in latitude and longitude, while moving from one place to another. In the next lesson, we shall understand about representation of latitude and longitude depending on therequired accuracy of graticule.