Basic Principles
IN THE STANDARD REFERENCE on the subject , Emergency Navigation , the author , Dr . David Burch , explains a variety of techniques for instrument free ( or improvised instrument ) based navigation . The following is a distillation of some of the techniques described in the book .
Determination of Latitude In the Northern Hemisphere , the altitude ( i . e ., angle ) of Polaris , above the horizon , is equal to the latitude of the observer . Although this is a bit of an approximation , for emergency navigation , the accuracy is more than adequate ( well under a degree of imprecision ). As long as Polaris can be identified , and as long as the horizon can be seen , then the altitude of Polaris - and thus your latitude - can be measured using your hands ( see below ).
Polaris is not visible in the Southern Hemisphere nor is there an equivalent southern pole star . However , if you have memorized the declination of appropriate zenith stars ( stars which will pass near or directly overhead ), and you know how to identify them , then you can determine your latitude . The basic idea is that if a star passes directly overhead ( i . e ., at an altitude of 90 o ), then its declination is your latitude . Even if a star does not pass directly overheard ( the vast majority won ' t ), it is still possible to use it to calculate latitude : e . g ., if a star passes 2 o south of your zenith , then your latitude is 2 o north of the declination of the star .
The altitude of Polaris - and thus your latitude - can be measured using your hands .
Measurement of Altitude of Polaris or Zenith Stars The gold standard for measuring altitude is , of course , the sextant . In an emergency , however , your hand can be used to measure angles within a reasonable degree of precision . Typical angles are as follows :
• Thickness of finger ≈ 2 o
• The distance between each of your knuckles ≈ 3 o
• If you wink , your finger will move by approximately 6 o
• A fist ( with your thumb hidden ) subtends an angle of approximately
10 o
• For an outstretched hand , the angle between your thumb and index finger ≈ 15 o
• For an outstretched hand , the angle between your thumb and pinkie ≈ 20 o
NOTE : For each of these measurements , it is critical that your arm be fully extended .
In order to increase precision , it is possible to " calibrate " your body parts . There are several ways to do this . One technique is to compare your subtended visual angles with the same angles determined by a sextant . Another option is to measure with a tape measure and use simple right angle trigonometry . Of course , this calibration would need to be done in advance of an emergency situation and the numbers would need to be memorized .
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