At high latitudes, where magnetic variation can be erratic and the angle of magnetic dip can be steep, the magnetic compass is essentially useless. This instrument, on the other hand, indicates true north when pointed at a known star. It was made for military use during World War II, and surplussed soon thereafter. Ref.: Sperti, Inc., Instructions for Use of the Astro Compass MK. II (Cincinnati, n.d.).
