Great circle navigation in a nutshell:
Imagine you're flying from one point of the equator to the opposite side of the equator, over the North Pole. In the straightest path possible. First your true course will be 360, then, as you cross the pole, it will flip and be 180 for the second half of the trip.
Now imagine a similar flight where you pass kinda near the pole, but not directly over it. Your true course will start out something like 020, then as you pass near the pole quickly change to the south, (even though you're still flying a straight line, it is crossing each meridian at a different angle). Finishing the trip at 160.
A trip between pretty much any 2 points on the planet, is a less extreme example of the same thing, and the course angle relative to the meridians will always be changing.
Last edited by Vessbot; 30th Sep 2022 at 12:46.