The Shadow Stick

The movement of the sun through the sky has been used to determine direction probably since humans first existed. There are several ways that the sun can be utilised to give a general indication of direction, one of which is to track its movement through the sky by marking the movement of a shadow created by placing a straight stick into the ground. This method, known as the shadow stick, can be used to very accurately determine direction but it’s use is often described inaccurately leading potentially to a large degree of inaccuracy if not properly understood.

 

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The method often described is to place a straight stick into the ground and mark with a stone or peg the end of the shadow cast by the stick, you then wait a period of time and place a second marker where the shadow now reaches. By placing your left foot by at the first marker and your right foot by the second marker you should now be facing north

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………or are you?  At certain times of the year, and during the middle part of the day using this approach, you are likely to be not too far out.  As you can see from the diagram below, as you approach both the summer and winter solstice the path described by the shadow makes a distinct curve, which is more pronounced early morning and late evening. Just using the two marker method early in the morning and late afternoon in the middle of both summer and winter could mean you are a considerable way out on your bearings.

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Close to the equinoxes the path described by the shadow’s path is close to a straight line, so this method would be OK in late March and late September.

 

To use a shadow stick to accurately determine direction there are two methods that can be used.  The first requires you to mark the shadow fairly frequently and ensure that you capture the middle part of the day and then determine where the shortest shadow will fall.  The sun in the northern hemisphere will be south when it is at its zenith, this will give the shortest shadow which will therefore be pointing exactly north (see below).

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An alternative method, which doesn’t require you to mark the shadow quite as frequently, is to mark the shadow at some point before the middle of the day and then to scribe an arc with the radius of the length of that shadow, on the ground using string or rope attached to the shadow stick.  As the day progresses the shadow will shorten and move away from the arc as the sun reaches its zenith. As the sun then moves past its highest point and starts to head west, the shadow will begin to lengthen again, and at the equivalent time after the sun’s apex the shadow will re-intersect the drawn arc. A line between the two points will give you an exact West-East line.

 

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Remember that the direction determined using the shadow stick will be different from the magnetic bearing from a compass and also from the grid north on a map, but using a shadow stick properly will give you the true rotational axis north. Also if the moon is bright enough you can use the shadow created by the moon in exactly the same way.

 

Happy navigating.

 

Kev