Using a sextant, the angle measured from the pole star (pivot point of star movement) to the horizon gives one, his or her exact latitude.

The North Star or Polaris is not exactly a pole star or exactly at at the pivot point of the entire sky star movement, but was one degree from it (1966) and in the seventeenth century was slightly over two degrees from the same exact star movement pivot point.

This error - in navigation - was almost entirely eliminated - in the seventeenth century - by taking the readings of the North Star when certain constellations were directly above or below the star, and then respectively adding or subtracting the two+ degrees from the observed angle measurement.