Astrocompass

An astrocompass is a navigational tool for determining the direction of true north through the positions of various astronomical bodies.

There are certain circumstances when magnetic compasses and gyrocompasses are unreliable. The most obvious is in polar regions, where the force exerted on the needle of a magnetic compass is nearly vertical and gyrocompasses become unstable due to the rotation of the Earth. Magnetic compasses are also susceptible to disruption from magnetic fields other than the Earth's, such as those produced by the hulls of some metal vehicles or craft. Before the advent of electronic navigational aids such as GPS the most reliable way to ascertain north in such circumstances was through the use of an astrocompass.