But I was actually wondering how to calculate the point at which the sun would rise/set as seen from an aircraft travelling from A to B...
As before, I've also pondered the same thing. A moving map program showing the terminator would do the trick. It's certainly possible using maths alone, but this goes beyond basic algebra and into calculus - ESA/NASA territory.
You have once again sparked my interest. Will have a look around.
edit: Almost positive dispatch has something that may be useful.