On E-bay recently, you can buy a Howard Sun Compass at reasonable prices . They are British MOD surplus from the First Gulf War, used to navigate in difficult featureless desert terrain where the metallic vehicles made a magnetic compass unsuitable. Similar sun compasses were used during the WWII in North Africa in the 1940's ( see here) and apparently earlier in the 1830's in North America where large amounts of metallic ores in the ground made magnetic compasses not accurate for surveying. The principle of the Howard Sun Compass works and how it is set up to navigate on bearings are well described in detail in Francois Pineau's website. In general, the set-up is straight-forward, once the shadow angle lines are drawn on the circular white plastic time plate. This, however, is not easy. The shadow lines have to be drawn to show the sun's azimuth angle at the current, say, quarter-hour , for the date and the location (latitude /longitude).

The British MOD instrument manual describes how to look up the necessary shadow angles in simplified tables which themselves are based on the Davies and Birdwood "Sun's True Bearing" of 1913. A detail from the table shows,on the right, the shadow angle from noon, for each latitude degree, for each half hour and for the day/month period related to the sun's declination. The manual then describes how the user can draw the necessary shadow angle lines on the white circular plastic disc, as the part of the set-up procedure. The shadow angles drawn on the disc represent shadows actually cast by the sun and so it is necessary to use the real time indicated by the Sun, known as Local Apparent Time, (L.A.T.) and so the user also has to set his watch to Local Apparent Time. That in itself can be quite complicated and for this reason and also because this method uses the averaged data for latitude and dates, with my Howard Sun Compass which I bought in 2009, the seller provides a second and different approach to the drawing of the shadow angles, using sun azimuth angles from the United States Navy Observatory. This has more accurate data and also uses Universal Time rather than Local Apparent Time.

Even so, the instructions for making the circular discs with the shadow angles are still quite complex and fortunately we now have the answer .In collaboration with Cordt Machens, an ex-Lufthansa pilot and interested in sundials and solar compasses, Juergen Giesen has created a Java applet to print the shadow angle discs for the Howard Sun Compass.

As a preliminary, to check Juergen's applet, the shadow angles were drawn onto paper disc, by hand, from the applet's tables. See the photos on the left and a part of one of the applet table above. The checks having been made successfully, they moved on to making printed discs.

For a particular date, latitude, longitude and time zone, the applet prints the shadow lines on the disc. It can be drawn either at Local Apparent Time, where the sun is at its zenith at noon, or Standard Time , the civil time of the particular time zone. In the example on the left, the location is Cordt Machen's home town of Dietzenbach in the state of Hesse, not far from Frankfurt, latitude 50.0167 degrees North, longitude 8.7833 degrees East. The photo was taken by Cordt at 14.00 hr .in February 11th 2009 Local Apparent Time. The sun compass was positioned on a "testbench" on which where the cardinal points True North, E, S and W, were separately verified as accurate. Then the compass was aligned so that the shadow of the gnomon cast by the sun fell on the 14.00 hr line. The "Direction of Advance" red arrow now points directly at True North, confirming the accuracy of the Sun Compass applet's paper disc. For Germany, the zone time is CET (Central European Time) (UTC +1) at longitude 15 degrees East, so the Standard Time is 39 minutes ahead of Local Apparent Time, a combination of 6.22 degrees West from the CET meridian , or approx. 25 minutes, and the Equation of Time for February 11 at approx.14 minutes.

Here is a photo of Cordt's "testbench" showing clearly the vertical gnomon casting its shadow onto the 1400 hrs line on the disc We are intending to photograph our Howard Sun Compasses at early and late hours of the day and also in spring and summer time. Juergen has an exceptional website devoted mainly to physics and astronomy as well as sundials and sun compasses and you can visit his Howard Sun Compass applet at http://www.GeoAstro.de/SunCompass There is also an Applet for the Cole Universal Sun Compass on his website: http://www.geoastro.de/SunCompassCole/