How was the precession of the earth north pole calculated in years?

Answer 1

The Greek astronomer Hipparchus noticed precession in the 2nd century BCE by comparing his measurements of star positions with others from the previous couple of centuries. It wasn't until Ptolemy in the 2nd century that anyone got around to really measuring the effect. Ptolemy was able to use position data from Hipparchus and others to calculate the rate of precession. He was about 30% off, due to the inaccuracy of the measurements.

The value of the rate of precession was gradually refined over the centuries, though many astronomers thought the rate varied over time. The Danish astronomer Tycho Brahe, in the 16th century, was the first to recognize that the rate of precession was constant. The key to determining the precession was the ability to accurately measure the position of the sun relative to the fixed stars.

Answer 2

The Earth's north celestial pole points very close to a star known as Polaris. Through a frame of time, such as 50 or 100+ years later, the north celestial pole will point slightly away from Polaris. This difference in these positions can be used trace out a circular path which takes approx. 26,700 years to complete one cycle.