Basic Principles
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.
33