Astrocompass

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

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.

In its most basic form, the astrocompass consists of a base plate marked with the points of the compass, with a mechanism known as an equatorial drum mounted on it.

Once all these settings have been made, the astrocompass is simply turned until the astronomical body is visible in the sights: it will then be precisely aligned to the points of the compass.

Because of this procedure, an astrocompass requires its user to be in possession of a nautical almanac or similar astronomical tables, one of its chief disadvantages.