Paradox that the fastest way to travel southwest is to go Northwest (degree, sun)

[EricS39]

For great earth distances west and south, you can observe this by just knowing the summer solstice sun moves along 23.44 degrees north "tropic of cancer line"

But at say latitude 55 "Northern Ireland" when sun sets, that sun is in the NW sky...

Clearly following the linear direction of sunset will bring you to where the sun is shining more and more directly...and clearly the end is on the Tropic of Cancer line, which is NOT actually Northwest of you

Therefore what happens is you are heading toward a southwest location by taking the shortest distance route along a great circle that starts out bearing NW, the bearing changes to WNW then W then WSW and eventually SW and then SSW..

That bearing changes because the lines of East and West are curved, not straight (except on the equator line)

An inch south of the North Pole you can draw an east-west line around the entire globe by drawing a little circle around the Pole. The more removed you are from the Pole and closer you are to equator, the more East-West resembles a straight line and not a round circular sketch.

So I wonder how early sailors who didn't know the earth was round, were able to explain why when they sailed west, they ended up gearing south of west ...assuming they believed the earth was actually flat

[EricS39]

Flight path example

What it looks like if you drew the earth as flat but actually traveled in a straight direction (truly the shortest surface distance path) from Belfast, Northern Ireland to San Francisco, California U.S.

Great Circle Mapper

This second photo should explain the reason for the change of directions that east isn't a straight direction (this one is shortest distance path to Hawaii from Northern Ireland but when seeing that earth shaped as round and not flat

Great Circle Mapper