Celestial Navigation works by measuring the distance of Polaris to the horizon in Degrees. The measure you get corresponds with your latitude. If you measure 90 degrees you are at the north pole, if you measure 59 degrees you are at 59 degrees north. When done correctly celestial navigation can be extremely accurate, even more then so compasses. Celestial Navigation has been used since Ancient Greece with a huge degree of accuracy. Also as a side note you also have to take axial tilt into account but this was a extreme of simplification. The tools used, such as a sextant, for celestial navigation use the geometry of the round earth to work. When this is modeled on a flat earth, it gets a bit iffy. If you take a look at the attached image you will notice that 2 acute angles don't intersect the 90 degree angle at the same spot. In reality three people standing at 3 diffrent parts of the earth there line's of sight would intersect the same location.