Convergence
1. Go to a good toystore.
2. Buy a reasonable globe (of the Earth that is).
3. Sellotape a piece of string over London.
4. Use the string to find the shortest route to your destination - over the pole to China/Japan is good
5. This is the Great Circle Route. Look how the 'string' is a straight line on the Earth's surface but its bearing changes with regard to the lines of longitute it crosses.
It helped me understand it...