Astronomical compasses were used by both sides during World War II, when other alternatives could not be used.
- For example, a magnetic compass can be severely disrupted near a large mass of metal (aboard a bomber, or at the head of a column of tanks, for example).
- Or you can’t signal yourself by radio when you fly over enemy territory, for fear of revealing your position.
In these cases, finding the North without a magnetic compass is essential, hence the astronomical compasses. On the Allied side, the Astro Compass Mk II was arguably the most widely used, equipping American and British bombers.
The “TRUE HEADING” indication shows that the primary function of the instrument is to find the “True Heading”. But we understand, by noting the complexity of the instrument, that it does more than that by reproducing the celestial sphere, thanks to its three moving disks. It requires the use of astronomical tables.
Note that the indications are in white for the northern hemisphere and in red for the southern hemisphere.
The reconstruction below, drawn on GeoGebra, highlights the three circles: declination, hour angle (LHA) and latitude. Any celestial object whose coordinates are known can be pointed at and give the heading.
