Both are WWII sun compasses, but they differ on several points:
| Howard Compass | Bagnold Compass |
Preparation | Build paper discs for time and place | Read the sun’s azimuth from tables |
Time reference | Can work either on standard time, or solar time | Designed to work in solar time (see note) |
Utilisation
| Hand held or secured to vehicle | Always secured to vehicle |
Resetting | Not necessary: If the direction is followed, the gnomon’s shadow moves along the hour lines throughout the day (reminder: it is a sundial) | Turn the knob to set the time on the instrument (see note below). |
Reading
| Direction indicated by the red arrow | Direction given by the shadow of the gnomon |
The two key points are preparation and reading.
Preparation
Creating discs is very easy today using a computer (see our Geogebra application). They can be constructed just as easily in standard time as in solar time. However, in standard time, the instrument is undoubtedly easier to use. This is an advantage for the Howard compass today (it’s hard to change the time on our phones!), especially since the Howard compass requires no adjustments throughout the day. But this was likely not the case during World War II, and even less so for the LRDG, which operated under extreme conditions.
Furthermore, the solar azimuth values provided in the Bagnold compass manual apply to the midpoint of the interval. Thus, the error at the beginning is offset by the error at the end. One can therefore navigate without readjustment for the entire duration of the interval specified in the manual.
In my opinion, the Bagnold compass has the edge.
Furthermore, the tables in the instruction manual are very easy to recalculate using modern methods. It is therefore entirely possible to use the Bagnold compass in standard time. The only thing lost is the elegance of the morning/afternoon symmetry. In that case, the entire outer disc should be graduated from 0 to 360.
Reading
The Bagnold compass provides the bearing directly via the gnomon’s shadow. This is an advantage if the driver needs to change course to avoid an obstacle on the terrain (such as a ravine that must be bypassed for a few hundred meters). The logbook will simply note the duration during which the vehicle was driven on a different course. In the evening, the position can be determined precisely (and confirmed by a sextant observation).
This is how the LRDG was able to find a rendezvous point, even after weeks of traveling through the desert.
These two advantages should undoubtedly have made the Bagnold compass the standard in the British Army. However, the Howard compass, which was in use before World War II (although I have not been able to establish with certainty the date of its first use), was not supplanted and remained in use well into the 1990s. Was this due to the controversial reputation of the LRDG and the SAS within the British General Staff?