Measurement of Terrestrial Latitude/Pole Star

Theorem
An observer's (terrestrial) latitude can be identified by measuring the (celestial) altitude of the Pole Star.

Proof
From Altitude of North Celestial Pole equals Latitude of Observer, all the observer needs to do is to measure the (celestial) altitude of the north celestial pole.

The star Polaris is situated at practically exactly that point.

Hence the result.