DIY sundial for soldiers of the Wehrmacht and the Afrika Korps in World War II
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The following construction instructions do not originate from the Wehrmacht for their soldiers, but were published in 1942 by the "German Watchmakers' Week" to support the soldiers on the African front
In North Africa, pocket watches and wristwatches often fail. The fine desert sand penetrates even the most tightly sealed watch and causes the mechanism to stop working. Therefore, we are publishing instructions for building a small sundial, which we hope will serve our soldiers in North Africa well. During periods of heavy deployment, soldiers cannot, of course, concern themselves with such things, but there are also quiet days and weeks in between, and this suggestion is intended for that purpose.
The sundial shown here was calculated for a geographical latitude of 31°, but it can also be used in slightly different locations. Carefully cut out the plan to be glued onto the sundial. The dotted triangle is the so-called pointer triangle, which casts the shadow that tells the time. Using small scissors or a razor blade, cut along the two dotted lines. Then, bend the pointer triangle vertically upwards along the north-south line. This work on the pointer triangle must be carried out very carefully, because the pointer is precisely calculated (the angle of 31° shown corresponds to the geographical latitude!).
Bending the pointer triangle upward naturally creates a triangular opening in the base plate. To still be able to read the times between 9 and 12 o'clock clearly, place or glue the base plate onto a piece of cardboard (like a field post box!) and trace the lines radiating from 9, 10, and 11 with a pencil. The sundial is now finished and ready for use.
The cardinal directions are printed on the sundial. For the clock to show the correct direction, it must be completely horizontal and precisely aligned with a compass. The triangle pointer must be positioned exactly along the north-south line. If this is the case, the pointer will cast a shadow in bright sunlight, indicating 'true solar time'.
Unlike a wheel clock, every sundial displays 'true solar time'. Therefore, it shouldn't be surprising if the time read from a sundial doesn't quite match the time displayed by a mechanical clock, because mechanical clocks display a time artificially adjusted for practical use. The solar day is not the same from noon to noon throughout the year. Differences can be as much as 16 minutes. Therefore, anyone who wants to calculate precisely must first convert the true solar time displayed by the sundial into mean local time using the equation of time, which is easy to do using the following table.
However, since it would be very confusing if every place kept its own local time, the earth has been divided into 15° wide zones, each with the same time – the so-called zone time. Each zone has a time difference of one hour. Germany and the countries in the same zone use Central European Time, i.e. the time of the 15th longitude. When the clocks in Germany show summer time, i.e. are one hour ahead, they correspond to Eastern European Time based on the 30th longitude, which also applies to Egypt.
Depending on where in North Africa you use the sundial, you may need to convert the local time into the time zone. Locations located on the 30th longitude (e.g., Alexandria) have the same local time and time zone. For all other locations, 4 minutes must be added or subtracted for each longitude difference. All locations west of the 30th longitude add the difference, while all locations east of it subtract it.
Example: On December 1st, a sundial in Benghazi shows 11:15 a.m. What time is it according to the local time zone? As can be seen from the equation of time, the difference between true solar time and local time on December 1st is -11 minutes. The true solar time of 11:15 a.m. therefore corresponds to a local time of 11:15 a.m. minus 11 minutes, or 11:40 a.m. A glance at the map shows that Benghazi is on the 20th meridian. The difference from the 30th meridian is therefore 10 degrees, or 10 times 4 = 40 minutes. Benghazi is west of the 30th meridian, so the difference must be added: 11:04 a.m. local time + 40 minutes = 11:44 a.m. local time.
If you want to simplify things, compare the time displayed by the sundial with the time displayed by a mechanical clock or radio and note the difference. If you then take this difference into account for all subsequent readings, you will always get the approximate time zone. It's also important to remember that the small pocket sundial is obviously not a precision instrument. However, it will display time to within a quarter of an hour. If you like, you can also draw the sundial larger, based on the blueprint, thereby increasing its accuracy.
Equation of time for the 1st and 15th of each month in minutes: Day: Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Dec. 1. +3 +14 +13 +4 -3 -2 +3 +6 0 -10 -16 -11 15. +9 +14 + 9 0 -4 0 +6 +4 -4 -14 -15 - 5
