Navigation Equipment and Methods - Field-expedient Methods
9-5. FIELD-EXPEDIENT METHODS
When a compass is not available, different techniques should be used to determine the four cardinal directions.
a. ShadowTip Method.
(1) This simple and accurate method of finding direction by the sun consists of four basic steps (Figure 97).
Step 1. Place a stick or branch into the ground at a level spot where a distinctive shadow will be cast. Mark the shadow tip with a stone, twig, or other means. This first shadow mark is always the west direction.
Step 2. Wait 10 to 15 minutes until the shadow tip moves a few inches. Mark the new position of the shadow tip in the same way as the first.
Step 3. Draw a straight line through the two marks to obtain an approximate eastwest line.
Step 4. Standing with the first mark (west) to your left, the other directions are simple; north is to the front, east is to the right, and south is behind you.
(2) A line drawn perpendicular to the eastwest line at any point is the approximate northsouth line. If you are uncertain which direction is east and which is west, observe this simple rule--the first shadowtip mark is always in the west direction, everywhere on earth.
(3) The shadowtip method can also be used as a shadow clock to find the approximate time of day (Figure 97).
(a) To find the time of day, move the stick to the intersection of the eastwest line and the northsouth line, and set it vertically in the ground. The west part of the eastwest line indicates 0600 hours, and the east part is 1800 hours, anywhere on earth, because the basic rule always applies.
(b) The northsouth line now becomes the noon line. The shadow of the stick is an hour hand in the shadow clock, and with it you can estimate the time using the noon line and the 6 o'clock line as your guides. Depending on your location and the season, the shadow may move either clockwise or counterclockwise, but this does not alter your manner of reading the shadow clock.
(c) The shadow clock is not a timepiece in the ordinary sense. It makes every day 12 unequal hours long, and always reads 0600 hours at sunrise and 1800 hours at sunset. The shadow clock time is closest to conventional clock time at midday, but the spacing of the other hours compared to conventional time varies somewhat with the locality and the date. However, it does provide a satisfactory means of telling time in the absence of properly set watches.
(d) The shadowtip system is not intended for use in polar regions, which the Department of Defense defines as, being above 60° latitude in either hemisphere. Distressed persons in these areas are advised to stay in one place so that search/rescue teams may easily find them. The presence and location of all aircraft and ground parties in polar regions are reported to and checked regularly by governmental or other agencies, and any need for help becomes quickly known.
b. Watch Method.
(1) A watch can be used to determine the approximate true north and true south. In the north temperate zone only, the hour hand is pointed toward the sun. A south line can be found midway between the hour hand and 1200 hours, standard time. If on daylight saving time, the northsouth line is found between the hour hand and 1300 hours. If there is any doubt as to which end of the line is north, remember that the sun is in the east before noon and in the west after noon.
(2) The watch may also be used to determine direction in the south temperate zone; however, the method is different. The 1200hour dial is pointed toward the sun, and halfway between 1200 hours and the hour hand will be a north line. If on daylight saving time, the north line lies midway between the hour hand and 1300 hours (Figure 98).
(3)The watch method can be an error, especially in the lower latitudes, and may cause circling. To avoid this, make a shadow clock; and set your watch to the time indicated. After traveling for an hour, take another shadowclock reading. Reset your watch if necessary.
c. Star Method.
(1) Less than 60 of approximately 5,000 stars visible to the eye are used by navigators. The stars seen as we look up at the sky at night are not evenly scattered across the whole sky. Instead they are in groups called constellations.
(2) The constellations that we see depend partly on where we are located on the earth, the time of the year, and the time of the night. The night changes with the seasons because of the journey of the earth around the sun, and it also changes from hour to hour because the turning of the earth makes some constellations seem to travel in a circle. But there is one star that is in almost exactly the same place in the sky all night long every night. It is the North Star, also known as the Polar Star or Polaris.
(3) The North Star is less than 1° off true north and does not move from its place because the axis of the earth is pointed toward it. The North Star is in the group of stars called the Little Dipper. It is the last star in the handle of the dipper. Two stars in the Big Dipper are a help in finding the North Star. They are called the Pointers, and an imaginary line drawn through them five times their distance points to the North Star. There are many stars brighter than the North Star, but none is more important because of its location. However, the North Star can only be seen in the northern hemisphere so it cannot serve as a guide south of the equator. The farther one goes north, the higher the North Star is in the sky, and above latitude 70°, it is too high in the sky to be useful (Figure 99).
(4) Depending on the star selected for navigation, azimuth checks are necessary. A star near the north horizon serves for about a half hour. When moving south, azimuth checks should be made every 15 minutes. When traveling east or west, the difficulty of staying on azimuth is caused more by the likelihood of the star climbing too high in the sky or losing itself behind the western horizon than it is by the star changing direction angle. When this happens, it is necessary to change to another guide star. The Southern Cross is the main constellation used as a guide south of the equator and the above general directions for using north and south stars are reversed. When navigating using the stars as guides, the user must know the different constellation shapes and their locations throughout the world.