72 years per degree, hold your arm out with your thumb and finger about two inches apart, that is about how much the stars appear to move along the horizon because of precession every 72 years. The monitored this with the "Celtic Cross."
If you're on a ship there is no way you are going to be able to measure precession on a boat bobbing around. 1 angle every 72 years? In a space of a minute it would have moved 0.0001369863013698 degrees.
They could tell where they were in relationship to the predictable positions of of where the constellations would be in the future with simple triangulation.
Yeh, they were presumed to have been of bison or reindeer for decades until some paleontologists came along.
Knowing the future position of the stars is no good for longitudinal navigation, if you dont know your current time. http://en.wikipedia.org/wiki/Celestial_navigation
Precession is a rate, time/distance, that is the time they kept for ancient mapping, not Earth spin time, as they didn't have watches back then you know.
Dear God how the hell did you become so stupid???? Without time or distance you HAVE no rate. PS you read that link quick.......