system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did sun boards or sun compasses work for course correction when sailing?

Having just watching the first episode of the tv show Vikings, the character Ragnar produces a 'sun board' and explains that the board can help them navigate west by measuring the length of the shadow cast by a stick in the middle of the board. The next day, if they again measured at noon and the shadow was shorter or longer than the previous day, it would mean that they had drifted off course north or south and were no longer travelling straight west, but rather northwest or southwest, but I do not understand how they came to this conclusion. 

I tried looking it up online but either complex mathematics seemed to be involved or I just found pages that repeated what the characters had said without explaining to me WHY the length of the shadow corresponds to north or south? I do not understand the relationship. :(
gpt
It has to do with the angle between and sun, earth, and ship. Let's assume that the sun is in the same plane as the equator. At noon, if you are at the equator also the sun will be directly overhead and cast no shadow. Now, if you move north any significant distance the sun will no longer be overhead and will start to cast a shadow. The farther north you go, the longer the shadow. So, if the shadow's length changes from day to day they could tell if the had moved north or south from the previous day.