After playing Black Flag (not the most accurate source of info, but regardless), I had some thoughts about naval trade after going around the West Indies. After looking on Wikipedia, I found that a good amount of trade routes in the Atlantic and Pacific Oceans are curved. I'm just wondering, why, after the innovations of motor-based sailing and GPS, do we not just beeline straight to our destination? I'm sorry if I'm posting this in the wrong subreddit.
Because the earth isn't flat. It seems counter-intuitive, but on a globe the shortest distance between two points is not a straight line. For this same reason a flight from Europe to the US will take you over Greenland, instead of straight across the Atlantic, even though that seems the long way round on a map.
For the mathematics on this you probably want something like http://www.reddit.com/r/askscience/